Picking a thermocouple comes down to two questions: what are you measuring, and how hot or cold does it get? Answer the first and you know the style of sensor. Answer the second and you know the type (J, K, T and so on).
This guide walks through both, in that order. If you need the basics first (how a thermocouple works, color codes, wiring), start with Thermocouples 101.
Why thermocouples?
Thermocouples are the default industrial temperature sensor because they cover the widest range, survive the worst conditions, and cost the least. RTDs and NTC thermistors are more accurate, but only within narrower limits.
Here is what makes thermocouples hard to beat:
- Range. Base metal types run from about -200°C to 1,260°C. Noble metal types go to roughly 1,700°C. No other contact sensor comes close.
- Toughness. A thermocouple is two wires welded together. There is no fragile element to crack from vibration or thermal shock.
- Speed. The junction can be tiny. A fine-wire or exposed-junction thermocouple responds in milliseconds.
- Size and shape. They can be made as small as a few thousandths of an inch, bent, flattened for surfaces, or sharpened into needles.
- Cost. A good thermocouple often costs a fraction of an equivalent RTD.
- Simplicity. Thermocouples generate their own signal. No excitation current, no self-heating, and the curves are standardized, so a Type K works with any Type K instrument.
| Thermocouple | RTD (Pt100) | NTC Thermistor | |
|---|---|---|---|
| Typical range | -200 to 1,260°C (base metal), up to ~1,700°C (noble metal) | -200 to 600°C | -50 to 150°C |
| Typical accuracy | ±0.5°C (Type T) to ±2.2°C | ±0.15 to ±0.3°C at 0°C | ±0.1 to ±0.2°C in its range |
| Response time | Fastest (small junctions) | Slower (larger element) | Fast to medium |
| Ruggedness | Excellent | Good, element can be damaged by shock | Good |
| Cost | Low | Medium to high | Low |
| Signal | Millivolts, needs cold junction compensation | Resistance, needs excitation | Resistance, non-standard curves |
| Interchangeability | Standardized by type | Standardized (IEC 60751) | Varies by manufacturer |
When an RTD or thermistor is the better call: If you need better than ±0.5°C and your process stays below 600°C, an RTD is usually worth the extra cost. Thermistors make sense for precise measurements in a narrow, near-ambient range, like lab equipment or HVAC. For everything else, especially high temperatures, harsh environments, fast response or tight budgets, use a thermocouple.
The tradeoff is accuracy. Thermocouples also produce a small millivolt signal, so wire them properly and use the correct extension wire.
Step 1: What are you measuring?
Start with the medium. It decides the sensor's construction more than anything else.
Air and gases
Air has low thermal mass, so you want a sensor with low thermal mass too. The less metal around the junction, the faster and more accurate the reading.
- Beaded wire thermocouples are the simplest and fastest choice. The junction sits bare in the air. They work well for ovens, environmental chambers, lab testing and general ambient readings. See our beaded wire thermocouples.
- Exposed-junction probes give you a beaded junction on the end of a rigid sheath, so you can push it through a duct wall or hold it in position.
- Sheathed probes are the right call when the gas is corrosive, dirty or under pressure. Expect a slower response.
Two things to watch for. In a hot oven or furnace, nearby walls and heaters radiate onto the junction, so a bare bead can read high. Shielding the junction helps. In still air, response is slow no matter what sensor you use, so give the reading time to settle.
Surfaces
Surface measurement is harder than it looks. The sensor has to make good thermal contact, and the leads can pull heat away from the junction. The right style depends on how permanent the installation is.
- Adhesive and cement-on thermocouples bond flat to the surface. They are thin, fast and accurate, and they suit permanent or semi-permanent installs on heaters, molds, electronics and test parts. Our adhesive-backed surface thermocouple has a flat junction for full contact and responds in under 100 ms.
- Magnetic thermocouples snap onto steel surfaces. They are great for quick checks and moving between measurement points.
- Pipe clamp and band thermocouples strap around a pipe for process lines, HVAC and hydronic systems.
- Washer and bolt-on thermocouples mount under an existing bolt or screw. They are rugged and simple, and common on motors and machinery.
To get a good surface reading, run the first few inches of lead wire along the surface rather than straight away from it. Use thermal paste where you can, and insulate over the sensor if the surface is much hotter or colder than the surrounding air.
If the surface is moving, too hot to touch, or can't be contacted at all, consider an infrared sensor instead. Our article on infrared vs. contact thermocouples covers that choice. You can browse all of our surface thermocouples.
Liquids
For liquids, use a sheathed probe. The metal sheath protects the thermocouple wires from the fluid. Which probe you need depends on how wet the whole sensor gets.
- Standard sheathed probes work when only the tip goes in the liquid and the transition (where the sheath meets the lead wire) stays dry. Tanks, baths and open vessels are typical uses. Our transition joint probes have a 1/8" stainless sheath, an ungrounded junction and a rugged transition, and come with an adjustable fitting to set insertion depth.
- Hermetically sealed wire thermocouples are the right choice when the entire sensor gets wet, including the leads. Washdown areas, food processing, coolers and outdoor installs all qualify. Moisture that gets into a standard sensor wicks into the insulation and causes erratic readings. Our HSTC has a weld-sealed tip and continuous PFA insulation, rated IP67 from -75 to 260°C. Because it is a flexible wire, not a rigid probe, it bends around obstacles and feeds deep into tanks, pipes or holes that a straight probe can't reach.
- Probes in a thermowell fit pressurized lines, fast flow, corrosive fluids, or anywhere you need to pull the sensor without draining the system. The thermowell takes the pressure and abuse, and the probe slides in and out. Expect a slower response.
Insert the probe far enough. A rule of thumb is at least 10 times the sheath diameter beyond the wall, so a 1/4" probe needs about 2.5" of immersion. Too shallow and heat conducts along the sheath, giving you a reading that sits between the liquid and the ambient temperature.
Match the sheath material to the fluid. 316 stainless steel covers most water, food and general process liquids. Inconel is the usual choice for higher temperatures.
Something else
Some applications don't fit neatly into air, surface or liquid:
- Inside a solid or semi-solid. Food, plastics, rubber, soil and dough call for needle or penetration probes with a sharpened tip.
- Furnaces and kilns. High-temperature applications need mineral-insulated probes in Inconel, or ceramic-sheathed assemblies. Above about 1,260°C, you will need a noble metal type (R, S or B). See Step 2.
- Tight spaces. Fine-gauge wire thermocouples, or sheathed probes as small as 0.020" diameter, fit where nothing else will.
- Vacuum. Choose insulation that won't outgas, such as ceramic or fiberglass rather than PVC.
Not sure which bucket you are in? Talk to one of our engineers. Describe the application and we will point you to the right sensor.
Choosing a junction: grounded, ungrounded or exposed
Every sheathed probe comes with one of three junction styles. Pick based on speed versus electrical isolation.
| Junction | How it's built | Response | Best for | Watch out for |
|---|---|---|---|---|
| Exposed | Junction sticks out past the sheath | Fastest | Air and non-corrosive gases | Not for liquids, pressure or corrosive media |
| Grounded | Junction welded to the inside tip of the sheath | Fast | Liquids, high pressure, general use | Ground loops and electrical noise |
| Ungrounded | Junction insulated from the sheath | Slower | Noisy environments, multiple instruments, most PLC and controller inputs | Slightly slower response |
When in doubt, choose ungrounded. It costs a little speed but avoids most noise and ground loop problems.
Step 2: Choose the thermocouple type
The type is the pair of metals in the thermocouple. It sets the temperature range, the accuracy and how the sensor holds up in your environment. For most applications, the answer is K, J or T.
| Type | Practical range | Why use it | Applications | Avoid |
|---|---|---|---|---|
| K | -200 to 1,260°C | General purpose, largest range of the base metal types | Ovens, furnaces, kilns, heat treating, most industrial processes | Reducing or sulfur-rich atmospheres; drift with long use at 800 to 1,050°C |
| J | 0 to 760°C | Matches equipment built around J; handles reducing atmospheres and vacuum | Plastics molding and extrusion, older controllers | Moisture (iron leg rusts); oxidizing atmospheres above ~540°C |
| T | -200 to 370°C | Low temperature range, but the most accurate base metal type (±0.5°C) | Food, cold storage, labs, pharma, cryogenics | Anything above ~370°C |
| E | -200 to 870°C | Highest signal output of the standard types | Low-temperature work, detecting small temperature changes | Reducing atmospheres |
| N | -200 to 1,260°C | Same range as K, more stable at high temperature | Long-running high-temperature processes where K drifts | Higher cost, less common than K |
| R and S | 0 to 1,480°C | High temperature with high accuracy and stability | Glass, high-temp labs, heat treating, reference standards | Contamination; expensive |
| B | 870 to 1,700°C | Highest temperature range | Glass, ceramic and metal furnaces at extreme temperatures | Low output below ~600°C |
Ranges are typical working limits for standard construction. Wire gauge and sheath material affect the real limit.
A few practical rules:
- Default to Type K. It covers the widest range of jobs, and nearly every instrument reads it.
- Match what you have. If you're replacing a sensor, use the same type. Check your controller or logger input setting, and match the extension wire and connectors (the color tells you the type).
- Below 0°C, or need tight accuracy under 370°C? Use Type T.
- Running hot for long periods? Consider Type N over K for better stability.
- Above 1,260°C? You need R, S or B with a ceramic sheath.
Accuracy and calibration
Thermocouples come in two accuracy grades. Standard limits of error is the default. Special limits of error (SLE) roughly halves the tolerance, for example ±1.1°C instead of ±2.2°C on Type K. SLE costs a little more and is worth it for most process control work. The full tolerance table is in Thermocouples 101.
If an auditor, customer or quality system requires documented accuracy, order the thermocouple with calibration. IOThrifty offers 3-point NIST-traceable or ISO/IEC 17025 accredited calibration with the sensor, in one order. Our ISO 17025 buyer's guide explains when you need it.
Quick checklist
Before you order, have answers to these:
- What are you measuring: air, a surface, a liquid, or inside a solid?
- What is the minimum and maximum temperature?
- Is the sensor exposed to moisture, washdown, pressure or corrosive media?
- How fast does the reading need to respond?
- What is it connecting to, and which thermocouple type does that input expect?
- What length, diameter and termination (plug, stripped leads, cable length) do you need?
- Do you need a calibration certificate?
With those answers, you can pick from our thermocouple collection in a few minutes. If you don't see exactly what you need, or you want a second opinion, contact our engineers. We will help you spec it and quote a custom build if needed, usually the same day.