How to Measure Dew Point in a Compressed Air System: A Comprehensive Guide

What Is Dew Point and Why Does It Matter in Compressed Air Systems?

Dew point is the temperature to which air must be cooled for water vapor to condense into liquid (at a given pressure)​. In a compressed air system, the relevant measure is often the pressure dew point – the dew point at the system’s working pressure. Compressed air can hold much less moisture at high pressure, so its dew point is higher than at atmospheric pressure. For example, air with a -10 °C dew point at 1 atm will have roughly a -1 °C dew point if compressed to 2 atm​. A lower dew point means drier air; the drier the air, the lower the dew point​. Maintaining a low dew point in compressed air is critical for reliability and product quality. When compressed air cools or expands, any moisture above the saturation point will condense as liquid water. This moisture can cause corrosion in pipes, malfunction of pneumatic equipment, and even freeze-ups in cold conditions​. This can contaminate products (e.g. blisters in paint finishes, or microbial growth in food/pharma applications) and reduce lubrication of moving parts​. Therefore, controlling dew point is essential to avoid downtime and defects. In fact, continuous dew point monitoring is considered critical for protecting moisture-sensitive equipment from damage​.

Industry standards reinforce the importance of dew point. The ISO 8573-1 standard for compressed air quality defines strict classes of dryness based on pressure dew point. For instance, Class 1 air must have a dew point of -70 °C or lower, while Class 4 air (typical of refrigerated dryers) allows up to +3 °C​(In Fahrenheit, Class 1 is -94 °F and Class 4 is +37 °F​). These classes correspond to how air is used – e.g. critical instruments or processes may require Class 1 or 2 (-40 °C or lower), whereas general plant air for tools might tolerate Class 5 or 6 dew points around +7 to +10 °C​. Complying with the appropriate dew point specification (e.g. ISO 8573-1 Class 2: -40 °C PDP) ensures no liquid water will form in your system under normal conditions.

Purity Class	Pressure Dew Point °C	Pressure Dew Point °F
0	As specified by equipment supplier	As specified by equipment supplier
1	≤ -70	≤ -94
2	≤ -40	≤ -40
3	≤ -20	≤ -4
4	≤ 3	≤ 37.4
5	≤ 7	≤ 44.6
6	≤ 10	≤ 50

** This standard also has specifications for solid particles and oil, but those were omitted from the chart above for the purposes of this article.

The Impact of Moisture in Compressed Air

Even small amounts of moisture in compressed air can have outsized effects. As compressed air cools in receivers or pipelines, water can condense out if the dew point is above the ambient temperature. Liquid water in the system leads to:

• Corrosion and Rust: Water inside piping and tools causes rust and scaling, damaging distribution lines and end-use equipment. Rust particles can further contaminate products or clog components.

• Equipment Malfunctions: Pneumatic valves, cylinders, and instruments can stick or fail if water washes away lubricants or forms ice. In high-precision applications, even a fine mist can cause quality issues or sensor errors​.

• Product Quality Issues: Processes like painting, powder coating, and food or pharmaceutical production require very dry air. Excess moisture can ruin paint finishes (causing bubbles or poor adhesion) and foster bacterial growth in products​.

• Compliance Problems: Many industries must meet ISO 8573-1 moisture classes or other standards. Excess moisture means the compressed air is out of compliance with purity specs​, potentially invalidating warranties or safety standards.

• Maintenance and Downtime: Water leads to frequent filter replacements, drain issues, and maintenance interventions. In extreme cases, it can cause catastrophic failure of air dryers or compressors (for example, desiccant dryer desiccant can be destroyed by liquid water carryover).

By understanding dew point and maintaining it within specified limits, facility managers ensure reliable, efficient, and safe operation of compressed air systems. The goal is to keep the dew point low enough that condensation never occurs during normal use. In practice, this often means using drying equipment to achieve a dew point well below the coolest ambient temperature or the most sensitive process requirement.

(Formula Spotlight:) Dew point can be calculated from temperature and relative humidity using empirical formulas. One common approximation is the Magnus formula. For example:

where $T$ is temperature (°C), RH is relative humidity (%), and constants $a=17.27,\; b=237.7\,°C$. This yields the dew point $T_{\text{dew}}$ in °C​. Such formulas highlight that higher humidity or lower temperature results in a higher dew point (closer to the ambient temperature). In compressed air, lowering moisture content via dryers is what reduces the dew point.

The IOthrifty Dew Point Calculator article (https://www.iothrifty.com/blogs/news/dew-point-calculator-convert-relative-humidity-to-dew-point-temperature) includes a downloadable Excel spreadsheet which uses this formula to convert between relative humidity and dew point temperature.

Methods of Measuring Dew Point in Compressed Air

Measuring dew point in a compressed air system can be done with several types of instruments and sensors. The choice of technology affects the accuracy, range, response time, and maintenance requirements of the measurement. The most common dew point measurement methods are:

• Chilled mirror hygrometers – an optical condensation method (primary standard).

• Capacitive sensors – thin-film dielectric sensors that absorb moisture. These include metal oxide sensors and polymer sensors as two sub-types.

• Other hygrometric methods (less common) – e.g. resistive sensors or spectroscopic analyzers (beyond the scope of this guide).

Each method has its own strengths and best-use scenarios. Below, we discuss these methods and their principles, along with key considerations for compressed air applications.

Chilled Mirror Hygrometers (High-Precision Primary Method)

A chilled mirror hygrometer directly measures dew point by cooling a reflective mirror surface until condensation (dew or frost) forms. The instrument detects the formation of dew (often by an optical sensor seeing a change in reflectance) and records the mirror’s temperature at that moment – that temperature is the dew point.  This method is based on the fundamental definition of dew point, so it is considered a primary standard for humidity measurement.

Advantages: Chilled mirror hygrometers are extremely accurate and traceable. High-quality chilled mirror instruments can achieve dew point accuracy on the order of ±0.15 °C​, making them ideal for calibration and reference. They have a wide measurement range (from very dry up to high humidity) and are immune to calibration drift because they measure the physical event of condensation. They are robust in terms of operating principle and can be used to calibrate other sensors.

Disadvantages: This method requires precision optics and temperature control, making units expensive and somewhat bulky. They also require maintenance – the mirror must be kept clean and free of contaminants. Oil, dirt, or other impurities on the mirror can interfere with dew detection​. Chilled mirrors are also slower to respond to rapid changes in moisture (as the cooling/heating mechanism stabilizes). Frequent cleaning is needed to maintain accuracy​. Because of cost and upkeep, chilled mirror hygrometers are often used when absolute accuracy is essential (e.g. in a lab or for sensor calibration) and where a trained operator can perform regular maintenance​. They are less commonly used for routine online monitoring in a plant (except in critical installations) due to these practical constraints.

Use Case: Chilled mirror dew point meters shine as calibration standards or in research and development. For instance, a calibration lab might use a chilled mirror to verify the accuracy of other dew point transmitters. In compressed air plants, they might be reserved for periodic audits of air dryer performance or troubleshooting when other sensors’ readings are in question. If used in the field, some advanced chilled mirror instruments include self-cleaning features and robust enclosures, but the general trade-off is ultimate accuracy vs. cost/maintenance.

Polymer Capacitive Dew Point Sensors (Thin-Film Polymer Hygrometers)

Polymer dew point sensors are a type of capacitive sensor, that use a hygroscopic polymer film as the dielectric. These sensors essentially measure relative humidity and temperature, then calculate the dew point internally​. A typical construction is a layered sensor with a polymer dielectric whose capacitance changes with absorbed water, paired with a temperature sensor on the same substrate​. By measuring the relative humidity (RH) and temperature of the gas, the device’s electronics compute the dew point (since dew point is the temperature at which RH would be 100% for that moisture content).

Advantages: Polymer sensors are very versatile and cover a wide range of humidity and dew point values with good accuracy​. Modern polymer dew point transmitters can measure from high humidity conditions down to surprisingly low dew points ( -60 °C) thanks to dynamic auto-calibration techniques. They offer excellent long-term stability – the polymer film does not drift as quickly as metal oxide does in varying conditions​. They also handle condensation well: if the sensor gets wet, it typically isn’t permanently damaged; it will read 100% RH until dried out, and many designs include a heated purge cycle to dry the sensor after a saturation event. Polymer sensors are relatively fast responding across most of their range (especially in moderate humidity conditions), and have reasonable accuracy (often ±1 °C in dew point in typical compressor dryer ranges). They are also cost-effective for most industrial applications – generally less expensive than chilled mirrors, and often comparable or lower in cost than specialized metal oxide sensors. In fact, polymer dew point transmitters have become the standard choice for general-purpose dew point monitoring in compressed air systems.

Disadvantages: Polymer sensors are limited to higher dew points (above -40 °C or so) and lose accuracy at the very dry end. Some technologies have extended this range with auto-calibration, but accuracy does degrade at extremely low humidity. For example, at a dew point of -50 °C, the RH in room-temperature air is below 0.5%, pushing the limits of direct measurement. Advanced devices mitigate this with automatic calibration cycles that allow accurate readings down to around -80 °C dew point​. Another consideration is response time when drying – a polymer film that has absorbed moisture may take some time to equilibrate when the air becomes drier. In practice, this means a slight lag in reporting a dropping dew point (minutes, perhaps) after a wet-to-dry transition. Polymer sensors can also be affected by certain contaminants: oil mist or chemicals can foul the sensing film. However, many industrial designs include protective filters or purges, and polymer elements can often be replaced or cleaned more easily than a poisoned metal oxide sensor. Overall, polymer sensors are best suited for moderate to low dew point ranges (from ambient down to -60 °C or so), covering the needs of most compressed air dryers, and they strike a good balance of accuracy, durability, and cost for day-to-day monitoring.

Use Case: You will find polymer dew point sensors in a wide variety of dew point meters and transmitters on the market. For example, a wall-mounted dew point transmitter for a refrigeration dryer system might use a polymer sensor to monitor that the dew point stays around +2 °C. Portable handheld dew point meters often use polymer sensors for quick spot-checks of dryer performance. Because of their stability, polymer-based transmitters can be left in service for long intervals (a year or more) before needing recalibration. They are also used in hybrid arrangements: some high-end dew point instruments combine a polymer sensor (for mid to high humidity) with a secondary sensor or algorithm for the very dry range, achieving a wide overall range. In summary, polymer dew point sensors are the workhorses of industrial humidity measurements, well-suited for most facility managers’ needs.

(Note: Both polymer and metal oxide sensors are types of thin-film capacitive sensors. The key difference is the sensing material. Aluminum oxide sensors excel at ultra-low dew points but need more care, whereas polymer sensors handle a broader range with better stability​. Many dew point instruments use one of these or a combination to cover the full required range.)

Metal Oxide Capacitive Dew Point Sensors (Trace Moisture Sensors)

Metal oxide sensors, another type of capacitive sensor, are used for trace moisture measurement in very dry gases. They consist of a thin porous layer of metal, often aluminum oxide on a substrate, sandwiched between electrodes. Water molecules absorbed into the oxide layer change the dielectric constant, causing a measurable change in capacitance proportional to moisture content​.  The sensor output can be calibrated to read directly in dew point temperature.

Advantages: Aluminum oxide (often called metal oxide) sensors can measure extremely low moisture levels — dew points down to -100 °C or even lower​. This makes them suitable for ultra-dry systems (semiconductor fabs, instrument air, etc.). They are relatively small and can be installed in pressurized lines. Response to increasing moisture (dry-to-wet) is fast, and they operate over a wide pressure range. They are also comparatively rugged in terms of withstanding high pressure and have long lifespans in consistently dry conditions. For applications requiring very low dew point monitoring, these sensors are often the go-to choice​.

Disadvantages: The achilles heel of aluminum oxide sensors is exposure to high humidity or liquid water. They can be permanently damaged by condensation or very wet conditions​. Sudden exposure to moisture can cause a shift (drift) in the sensor’s calibration. They also tend to exhibit measurement drift over time even in normal use, especially if the dew point varies into higher ranges periodically​. This means frequent calibration is required to maintain accuracy (often the sensor must be sent back to the manufacturer or a specialized lab for recalibration)​. Additionally, their response when drying out (wet-to-dry) can be slow, as the sensor’s porous layer releases moisture gradually. If not used with care, they may read falsely high dew points for some time after a saturation event. In summary, aluminum oxide sensors excel at very low dew point measurements, but require careful protection from moisture spikes and regular calibration to ensure long-term reliability​. They are also normally much more expensive than polymer capacitive sensors and are therefore most suitablewhen dew point temperature measurement in ultradry environments is required.

Use Case: These sensors are commonly integrated into dew point transmitters for industrial dryers, breathing air systems, or gas production where very dry gas is the norm. For example, a heatless desiccant dryer skid might include an metal oxide dew point transmitter to monitor the -40 °C PDP (Pressure Dew Point) performance. Users of such sensors must ensure they are installed downstream of effective moisture removal and often include filters to prevent oil or liquid water ingress. Many units also include an alarm to warn if the dew point rises too high (which could indicate either a dryer failure or the sensor becoming wet).

Comparing Methods at a Glance

Each dew point measurement technology involves trade-offs. Here’s a quick comparison summary:

• Chilled Mirror: Highest accuracy (primary standard) and no intrinsic drift; slow response; requires maintenance (keep mirror clean); expensive; can measure full range (high to very low dew points); best for calibration or critical measurement​.

• Polymer: Wide range (mid RH to low dew points, with auto-cal down to ~ -80 °C); good accuracy (±2 °C dew point typical); stable long-term; tolerates getting wet (with recovery time); moderate cost; general-purpose use in many compressed air applications​.

• Meal Oxide: Very low dew point capability (-100 °C); fast response to moisture increases; small, inline probes; drifts over time and after getting wet; must be kept dry (can be damaged by high RH); calibration needed often; best for trace moisture in consistently dry systems.

• Others: Less common methods (e.g. lithium chloride dew cells or optical sensors) are generally niche or outdated for compressed air. Modern compressed air dew point monitoring is usually done with one of the above technologies or a combination.

Using Dew Point Meters, Transmitters, and Sensors Effectively

Dew point instrumentation comes in various forms – sensors, transmitters, and portable meters – but all serve to measure the moisture content of your compressed air. It’s important to use these devices correctly to get reliable readings and avoid damage. Before diving into best practices, note the terminology:

• A dew point sensor typically refers to the sensing element or probe itself (which might output a raw signal).

• A dew point transmitter usually means a sensor integrated with electronics that output a standardized signal (e.g. 4–20 mA or Modbus) for monitoring systems​. In other words, any humidity/dew sensor with analog/digital output is called a transmitter. These are designed for fixed installation and continuous monitoring.

• A dew point meter often refers to a device with a local display (and sometimes data logging) for dew point/humidity.  Many handheld or bench-top units are called meters. If it logs data for download, it might be called a dew point data logger.

In practice, the sensor inside a transmitter or meter might be the same; the differences are in packaging and features (display, outputs, etc.). Here are some best practices for installing and using dew point measurement devices in compressed air systems:

• Install at a Representative Location: Measure dew point downstream of the air dryer and after any after-coolers or filters that could affect moisture. A common point is right at the dryer outlet (dry air header). For point-of-use monitoring, place the sensor as close to the critical process as possible. Ensure the sensor is exposed to the air that matters (for example, after a receiver tank if the tank could introduce moisture). Avoid dead-legs or areas with no flow, as they won’t reflect system conditions.

• Use a Sampling Cell for Best Results: The optimal installation is to isolate the sensor from the main line by using a sample cell off a tee​. This cell (often stainless steel) is connected via tubing to the air line, and a small stream of air is bled through it to the sensor​. This setup allows controlled flow and easier maintenance. Include an isolation valve before the sample cell so you can remove the sensor without depressurizing the whole system.

• Control the Sample Flow Rate: Dew point sensors (especially capacitive types) are sensitive to flow and pressure. Use a flow regulator or orifice to maintain about 1 liter per minute of sample flow through the sensor chamber​. Too high a flow can create a pressure drop and cool the sensor, skewing the dew point reading lower than actual​ (mainly a concern for chilled mirrors). If you want to measure the pressure dew point (dew point at line pressure), place the flow control downstream of the sensor so the sensor stays at full line pressure​. Conversely, if you prefer to measure at atmospheric pressure, vent through the sensor (flow control upstream) so the sensor sees ambient pressure​. Always follow the sensor manufacturer’s guidelines on recommended flow.

• Avoid Temperature Extremes: Try to mount the sensor where the compressed air is close to ambient temperature (around 20–25 °C)​. Dew point measurement assumes a known temperature; if the sensor gets very hot (near a compressor) or very cold, it may affect the reading or even exceed the sensor’s operating conditions. Many transmitters specify an operating temperature range – adhere to it. If the air is hot, consider cooling the sample (some sampling cells include a coil or heat exchanger).

• Use Appropriate Tubing and Fittings: Use stainless steel or PTFE tubing for any sample lines​. Avoid rubber or porous plastic tubing which can absorb moisture and cause slow response or measurement errors​. Stainless steel is ideal as it does not affect the moisture content of the sample. Keep tubing lengths short if possible and avoid dead volumes that can trap moisture.

• Prevent Liquid Water and Oil Ingress: Ensure filter separators and drains upstream are working so that no liquid water reaches the sensor. A slug of water can overwhelm and potentially ruin certain sensors. If using a sample cell, you might incorporate a coalescing filter or a simple sintered metal filter as a buffer. Also, oil mist from compressors should be filtered out before the sample reaches the dew point sensor – oil can coat sensor surfaces (especially polymers and mirrors) and interfere with readings. Many dew point transmitters come with disposable filter caps for the sensor – use them if oil or dust is present.

• Allow Stabilization Time: When first installed or after a sensor has been exposed to ambient air, it may need time to equilibrate to the low moisture levels of the compressed air. Capacitive sensors that were at ambient (which might be, say, 30% RH at 25 °C = ~5 °C dew point) and then put in a -40 °C dew point air line need a few minutes to dry down to that level. Follow the device manual for warm-up or stabilization times. Portable dew point meters often have guidelines like "let the reading stabilize for X minutes". Chilled mirrors need time to cool the mirror. Patience ensures you’re reading the true dew point and not a transient.

• Monitor Continuously for Trends: If using a fixed dew point transmitter, integrate it with your monitoring system (SCADA, building management system, or even just a local display) to track the dew point continuously. Trends can tell you a lot – if you see the dew point gradually creeping up over days, your dryer may be deteriorating or due for maintenance. Many dew point monitors have alarm outputs; set an alarm to notify if the dew point exceeds a threshold (e.g. 10 °C higher than normal), so you can take corrective action before it becomes critical.

• Periodic Checks with Portable Meters: Even if you have online sensors, it’s smart to do occasional spot checks with a portable dew point meter as a sanity check. A portable dew point hygrometer (preferably recently calibrated) can be connected to various test points to ensure all is well. This can also help verify that fixed sensors are reading correctly, or help diagnose which part of the system is introducing moisture if there's a problem.

• Safety Considerations: Always depressurize or isolate and slowly bleed down sample lines before removing a sensor. Use appropriate pressure rated fittings. If the sensor is installed directly in a pressurized line, follow lockout-tagout procedures as you would with any pressurized equipment maintenance.

By following these practices, you will ensure your dew point readings are accurate and meaningful. Proper installation (using a sample cell and flow control) and vigilant operation (watching trends, keeping the sensor clean) go a long way to getting the most from your dew point measurement devices. Remember that a dew point sensor is only as good as its environment – if it’s installed poorly or mistreated, it may give misleading information.

Calibration and Maintenance Best Practices

Measuring devices require upkeep to stay accurate, and dew point sensors are no exception. Regular calibration and maintenance will guarantee that your dew point readings remain trustworthy over time. Here are key best practices:

• Follow Manufacturer Calibration Schedules: Nearly all manufacturers specify a recommended calibration interval (often annually). As a general rule, plan to recalibrate your dew point sensors every one to two years. Harsh conditions (very high or low temperatures, exposure to contaminants) or critical applications may necessitate more frequent calibration checks​. 

• Use Accredited Calibration Services or Reference Instruments: Calibration of a dew point instrument usually involves comparing it against a known reference standard. This could be a chilled mirror hygrometer in a calibration lab or a transfer standard device. Many vendors offer calibration services (e.g., sending the transmitter back to the factory or an accredited lab for a certified calibration). This is the most accurate route. Alternatively, if you have access to a standard (like a chilled mirror or a calibrated portable dew point meter), you can do a field calibration or at least a validation check. Always ensure the calibration covers the range of interest (for instance, if you care about -40 °C dew point, the reference should be able to generate that).

• Perform Regular Field Checks (Comparison): You don’t need to wait for the lab calibration to know if a sensor is drifting. Implement a routine of field - checking your dew point readings. A convenient method is using a portable dew point hygrometer as a reference to spot-check installed transmitters. For example, if your fixed transmitter says -40 °C but a calibrated handheld reads -33 °C at the same point, the fixed sensor may need servicing or calibration. Some facilities rotate a portable calibrator through various sensors periodically. Field checks can quickly pinpoint sensors that are out of tolerance before they become a big problem​.

• Watch for Sensor Drift or Malfunction Signs: Be on the lookout for any unusual behavior in the sensor readings (covered in the next section on troubleshooting). If a sensor consistently reads much drier or wetter than expected, or responds sluggishly, it might be out of calibration. Any time you have doubts about a dew point sensor’s performance, check its calibration (either by a swap test with another sensor or sending it for recalibration)​. It’s better to verify than to unknowingly operate with bad data.

• Keep Sensors Clean and Dry: Maintenance of the sensor itself is crucial. If you have a chilled mirror, regularly inspect and clean the mirror surface per the manufacturer’s instructions (often with a gentle solvent and lint-free cloth or by using built-in cleaning mechanisms). For polymer or metal oxide sensors, ensure the protective sintered caps or filters are in place to keep dust and oil out. If the filter element gets dirty or oil-soaked, replace it. Never oil or grease any part of a dew point sensor – it can permanently contaminate the sensing element.

• Avoid Saturating the Sensor: Try not to expose a sensor to conditions far above its intended range. For example, don’t unwittingly install a -40 °C dew point transmitter downstream of a wet receiver that sees +20 °C dew points – that might soak the sensor. If a sensor does get saturated with water (or if the compressor dumps oil mist on it), remove it and dry it gently in a clean, dry environment. Some polymer sensors have a built-in purge/heating cycle – use it to drive off moisture and reset the sensor baseline. However, note that repeated or severe condensation exposure can damage sensors (especially metal oxide types)​. Prevention is the best strategy: maintain upstream dryers and filters so the sensor always “sees” air within its calibrated range.

• Firmware and Electronics Checks: If your dew point transmitter has any self-diagnostic features (some smart transmitters can detect sensor faults or drift), pay attention to those alerts. Keep the transmitter’s firmware updated if applicable, as manufacturers sometimes improve the calculation algorithms or stability via updates.

• Battery and Storage (for Portable Meters): For handheld dew point meters, follow the maintenance of batteries (charge or replace as needed) and store the device with care. Many portable sensors are shipped with a protective cap or storage chamber, often containing desiccant, to keep the sensor dry when not in use. Use these when storing the device to ensure it starts up in a low-humidity state for the next use.

• Calibration Logs and Tags: Maintain a log of calibrations for each instrument. Tag the transmitters or meters with the last calibration date and next due date. This helps ensure none are overlooked. In audits or ISO compliance checks, having documented calibration records is important to demonstrate that your measurements are under control.

• Consult the User Manual: It may sound obvious, but always consult the manufacturer’s user manual for specific maintenance tips. Different sensors have different do’s and don’ts (e.g., some newer sensors might have a different recommended calibration interval, or specific cleaning methods). The manual will also detail any field adjustment procedures if the device supports them.

By treating dew point sensors as critical measurement devices – cleaning them, protecting them from contaminants, and calibrating on schedule – you will ensure longevity and accuracy. A well-maintained dew point transmitter can reliably serve for years, providing early warnings of moisture issues before they wreak havoc on your facility.

Troubleshooting Common Dew Point Measurement Issues

Even with proper installation and maintenance, you may occasionally run into puzzling readings or sensor behavior. Below are some common issues in dew point measurement and tips for troubleshooting them:

• Sensor Reading Doesn’t Change (Stuck Value): If the dew point display is stuck at one value all the time, as if frozen, the sensor or transmitter may have failed or the measurement range is exceeded​. For example, a display that never budges from -100.0 might indicate the sensor is offline or the value is out of range (bottomed out). First, check the basics: is the sensor connected and powered? Is there flow across it? If those are fine, try exposing the sensor to a known higher moisture level to see if it responds. A lack of response indicates the sensor element or electronics might be faulty. Many transmitters output a fixed high or low mA signal upon failure – check against the manual (e.g., 22 mA might mean sensor error). Solution: Perform a calibration check with a portable meter or swap in a spare sensor to confirm. If confirmed bad, replace or repair the sensor.

• Reading is Pegged at a Very Dry or Very Wet Value: Similar to above, if the instrument is “bottomed out” at its lowest possible dew point or maxed at the top end, it likely means the actual dew point is outside its measurable range or the sensor is saturated​. For instance, a polymer sensor might max out at 20°C dew point if the air is wetter than that (showing “HHHH” or a high value). Or an metal oxide sensor might read its driest limit even if the air isn’t that dry, due to a calibration drift. Solution: If the reading is at the wet end (and you suspect the air is indeed very wet), ensure no liquid water is hitting the sensor; dry it out and recalibrate if needed once conditions are normal. If the reading is at the dry extreme but should not be, the sensor might have drifted – check with another device. It’s also possible for temperature compensation faults to cause bad values (if the temperature sensor in a polymer dew transmitter failed, the dew point calc could be erroneous).

• Erratic or Noisy Dew Point Readings: If the dew point value jumps around rapidly or randomly, far more than the actual conditions should, there’s likely an issue. Possible causes: Insufficient flow or pressure fluctuations at the sensor, electrical interference, or a failing sensor. Ensure the sample flow is steady and at the recommended rate – turbulence or pulsing flow can cause oscillations. Electrical noise on analog signals (if not shielded) can also make readings spiky. A contaminated sensor (e.g. oil-coated) might also behave unpredictably. Solution: Inspect the sensor and sample setup. Add a small orifice or increase damping if the response is too fast. Check cable shielding and grounding for the transmitter. If erratic behavior persists, consider a sensor calibration or replacement.

• Dew Point Reads Much Higher Than Expected: If your dryer is supposed to deliver -40 °C but the sensor reads -20 °C (much wetter), don’t immediately blame the sensor – it could be doing its job by revealing a problem in the system. First, verify with another measurement (e.g., a second sensor or portable meter) to rule out sensor error. If two devices agree on the high dew point, your air really is wet – check the dryer (regen may have failed, or filters are saturated). If only one sensor reads high and others show normal dryness, that sensor may be contaminated or drifted. Solution: Remove the suspect sensor and check it in dry ambient air or nitrogen – if it still reads high dew point even in a known dry environment, it’s likely contaminated or out of calibration. You may attempt a gentle cleaning or drying procedure: for example, flush the sensor with clean, dry gas or use the sensor’s built-in heating purge if available. Then recalibrate. Also consider compressor oil fouling – oil on a sensor will make it read a constant high humidity because the oil retains moisture. In such cases, cleaning with appropriate solvent (if the manufacturer allows) or replacing the sensor may be necessary.

• Dew Point Reads Much Lower Than Expected (Too Good to be True): The opposite scenario is a sensor that claims a very low dew point that seems unrealistic (e.g., it says -80 °C but you have only a refrigerant dryer). This often points to sensor drift or failure. A sensor that has dried out or lost calibration might under-report moisture. It could also happen if the sensor’s temperature measurement is off (leading to a miscalculation making the air seem drier than it is). Solution: Again, cross-verify with another device or simply check ambient conditions. If a transmitter reads an impossibly low dew point (like below the ambient dew point when connected to open air), it’s likely faulty. Recalibrate or replace it. Note: Sometimes a sensor stuck at a low value is just one that has not recovered after a dry-out cycle – e.g., certain polymer sensors heated themselves to purge moisture and haven’t yet returned to normal operation. Give it some time or power-cycle if needed.

• Slow Response / Lag in Readings: If your dew point sensor takes too long to reflect changes (e.g., when a dryer starts malfunctioning, you’d expect the dew point to rise quickly, but the sensor only slowly creeps up), there may be an issue with installation or the sensor itself. Large-diameter sampling lines or long tubing can cause delay due to adsorption/desorption of moisture in the lines. A sensor that has a wet porous filter or one that’s slightly contaminated can also respond slower. Solution: Improve the sampling setup (shorter tubing, stainless lines, proper flow). Check the sensor’s filter – if it’s wet, dry or replace it. Some transmitters allow adjusting the integration time or damping; ensure it’s not set excessively high. Keep in mind that very low dew point measurements inherently have some lag because of the physics of moisture equilibration – but on the order of seconds to a few minutes, not hours.

• Sensor Shows Intermittent Spikes: If you observe occasional sudden spikes in dew point readings (especially on a trend graph), it could be due to water slugs or liquid droplets hitting the sensor momentarily. This might happen if there’s condensation upstream and it periodically gets carried into the sample. It could also be due to an electronic glitch or loose connection. Solution: Check for any entrained liquid. If you suspect droplets, install a knockout pot or additional filter upstream of the sensor. For electrical causes, inspect wiring and power supply stability.

• Sensor Error Codes or Self-Diagnostic Warnings: Many modern dew point transmitters have self-diagnostics. If the device is flashing an error code or flag, consult the manual. Common errors include temperature sensor faults, calibration expiration warnings, or internal analog front-end issues. Follow the prescribed remedy which often is to return for service or perform a reset/calibration.

When troubleshooting, it’s helpful to have a spare sensor or portable meter to swap in. This can quickly tell you if the issue is with the sensor or with the actual air. Additionally, remember that dew point sensors are one part of the system – a “bad reading” might actually be a real reading of a bad situation in your compressed air. So always confirm and investigate the air system itself (dryer, filters, leaks) alongside testing the instrument.

Some telltale signs of a malfunctioning dew point sensor include readings that are implausible (e.g., suddenly extremely dry or wet beyond what the system can produce), or readings that don’t change at all over time. Trust your intuition and knowledge of the system: if something seems off, it’s worth investigating promptly.

Considerations for Selecting the Right Dew Point Measurement Device

Choosing the appropriate dew point measurement device for your facility involves evaluating both the instrument’s capabilities and your system’s requirements. Here are key factors and considerations to ensure you select the right dew point sensor, transmitter, or meter for the job:

• Dew Point Range Requirements: Determine the driest and wettest conditions you need to measure. This is critical in choosing sensor technology. For example, if you need to monitor down to -75 °C dew point (e.g., for an ISO Class 1 air system or specialty gas), a standard polymer sensor might not suffice – an metal oxide or high-end polymer transmitter with extended range is needed. Conversely, if your application only needs to go down to, say, -20 °C (Class 3), a polymer sensor is perfectly adequate and you don’t need to pay a premium for an ultra-dry range device. Always check the specified measurement range of the device (e.g., -100 to +20 °C dew point, etc.) and ensure it covers your needs with some safety margin.

• Accuracy and Calibration: Consider the accuracy you require. For general compressed air work, a device with ±2 °C dew point accuracy is usually fine. If you have critical processes or need to meet stringent standards, you might require higher accuracy or at least a calibration certificate traceable to national standards. Chilled mirror hygrometers provide the highest accuracy​, but they are costly; many transmitters offer a balance of good accuracy (±1–2 °C) at a more affordable price. Check whether the manufacturer provides a calibration certificate and what the recommended calibration interval is. Also, some transmitters have field calibration adjustment features – this can be useful if you want to calibrate the device on-site with your own references.

• Response Time: Different sensor types have different response characteristics. If it’s important to catch rapid moisture spikes (for example, a quick upset in dryer performance), look at the sensor’s response time spec. Polymer sensors typically respond to humidity increases very quickly (within seconds) but may take a bit longer to dry out; metal oxide sensors respond quickly when going wet-to-dry until they hit very low levels where they stabilize more slowly. Chilled mirrors respond as fast as their cooling system can react. If your process can tolerate a few minutes of lag, most standard transmitters are fine. If you need instantaneous feedback, consider if the sensor has a fast-response mode or if a smaller sensor tip (for lower mass) is available.

• Environmental Conditions (Temperature, Pressure, etc.): Ensure the device can handle the environment you’ll put it in. This includes the pressure of the system (is the sensor rated for direct insertion at line pressure, or does it require atmospheric sampling?), the temperature of the gas (if you’re measuring hot compressed air, the sensor and sampling system must handle it or cool it), and ambient conditions (outdoor installation might need weatherproof enclosures and temperature compensation). If the compressed air is not clean (contains oil vapor or other chemicals), look for sensors that are chemically resistant or have available sampling conditioning systems (like filters, scrubbers).  In dusty environments, consider a sensor with a robust filter.

• Installation and Form Factor: Consider whether you need a portable meter or a fixed installation. Portable dew point meters are great for spot checks and for use at multiple locations, but they typically are not meant for permanent install (they may not have the right fittings for continuous inline use). Fixed transmitters can be installed permanently and wired into your system for 24/7 monitoring. Also, think about space and mounting: do you need a compact probe that fits into a small pipe tap, or is a larger unit with a wall-mounted enclosure acceptable? Some transmitters separate the probe and the electronics (with a cable in between) which can be useful if you need to mount the probe in a hot pipe and keep the electronics in a cooler area.

• Output and Integration: How do you want to use the data? If you plan to integrate the dew point reading into an automation system or PLC, you’ll want a transmitter with the appropriate output signal – common outputs are 4-20 mA analog, 0-10 V analog, or digital protocols like Modbus RTU, Ethernet/IP, or Profibus. Ensure compatibility with your system. If remote monitoring is needed, some devices even offer wireless outputs or IoT connectivity (Bluetooth, WiFi, cellular). On the other hand, if you just need a local reading, a simple meter with a display might suffice. Many devices offer both a display and analog outputs. Also check if the device provides alarm relays or contacts if you want it to directly control or alarm something when dew point is high.

• Data Logging and Analysis: For compliance or troubleshooting, you might need data logs of the dew point over time. Some transmitters come with PC software or built-in logging memory. If not, you can always log the 4-20 mA signal via an external system. But if you don’t have that setup, a dew point meter with logging capability can be very handy to gather a dew point profile over days or weeks. This can help in dryer performance analysis or audit reporting.

• Maintenance Needs: Think about how much maintenance you’re willing and able to do. A chilled mirror requires regular cleaning – do you have the capacity for that? Metal oxide sensors might need more frequent calibration – is there a budget and process for sending them in every 6 months or year? Polymer sensors are generally low maintenance, which is why they’re popular. Some devices have field-replaceable sensor modules, which can simplify maintenance (swap in a new calibrated sensor tip and you’re good to go). If your team prefers low-touch devices, lean toward those known for stability (polymer sensors with auto-cal features, for example, which have longer recommended cal intervals)​.

• Cost vs. Value: Of course, budget is a factor. Simple sensors are generally less expensive than full transmitters with displays and outputs​. Chilled mirrors cost the most, while mainstream dew point transmitters are mid-range, and small dew point sensors (with no transmitter electronics) cost less. However, consider the total cost of ownership. A cheaper sensor that drifts could cost more in downtime or bad air quality if not managed. Sometimes investing in a quality instrument pays off by providing reliable service and avoiding product spoilage or damage. Evaluate the criticality of dry air in your facility – for non-critical uses, a budget sensor may be fine, but for critical applications, justify the cost of a better device by the risk it mitigates.

In summary, match the device to your application. For a facility air system that requires, say, a -40 °C dew point alarm, a robust polymer dew point transmitter with 4-20 mA output and ±2 °C accuracy is typically a great fit. If you are an engineer verifying multiple dryers in a plant, a portable dew point meter might be needed for flexibility. For precision labs or calibration, a chilled mirror or high-end reference might be justified. Often, plants use a combination: fixed transmitters for continuous monitoring and a portable instrument for verification and spot-checks.

IOThrifty provides a variety of humidity and dew point measurement devices (sensors, transmitters, meters) to choose from, along with technical guidance​. If you're interested in dew point measurement solutions or products, check out our dew point measurement solutions here or reach out to service@iothrifty.com. We'd love to hear from you.