For any application that requires precise temperature measurement over an extended period of time (i.e. months or years), calibration is critically important. It’s a topic that anyone working seriously with temperature sensors needs to have a basic understanding of, or risk the accuracy of your measurements over time. Luckily the team at IOThrifty has decades of experience working in the calibration space and can help you navigate the murky waters.
Why is calibration so important?
Unfortunately, electronic sensors and instruments don’t live forever. This does not mean that they just stop working, but over a period of months, or years your temperature sensors and instruments can drift from their published accuracy specification. In many applications, a drift of just a few degrees can be devastating. Calibration is the process of adjusting your instruments to correct for the accuracy drift. Since temperature sensors generally can't be adjusted calibration for sensors is the process of determining and documenting their degree of error.
The amount of time it takes for sensors and instruments to drift depends on many factors. For temperature sensors, particularly thermocouples, the most common reason for drift is the mechanical stress caused by thermal expansion and contraction. The amount of impact these stressors can have on a sensor varies widely depending on use, the severity of temperature changes, and even the size of the sensor.
Types of Temperature Calibration
There are three different types of calibration related to industrial temperature measurement:
- Sensor Calibration
- Instrument Calibration
- System Calibration
The most common types of industrial temperature sensors are thermocouples, RTD (PT100s), and thermistors. So that’s generally what we’ll be referring to in this section.
Sensor calibration is a bit of a misleading term as temperature sensors don’t have an adjustable output, meaning even if your sensor isn’t reading properly you can’t adjust it. Instead, the goal of temperature sensor calibration is to verify the amount of offset from the true reading and document that offset. When your sensor is put to use in your application, you’ll need to tune your instrumentation to properly read from the sensor's offset output to get an accurate measurement.
Types of Temperature Sensor Calibrators
In a nutshell, temperature sensor calibrators are devices that will produce an accurate temperature that you can compare your sensor reading against. Below are the most common types of temperature sensor calibrators.
Block calibrators consist of a precise heating control system that heats a block of metal to a specific temperature. The block referred to as the insert contains holes in which a temperature probe may be inserted.
- Portable and lightweight
- Clean and easy to use
- Fast heat up and cool down
- Optional cooling for below ambient temperature calibration
Fluidized Bath Calibrators
Fluidized temperature bath calibrators use a fluidized bed to create a very stable heat reference. The fluidized bed is created by passing heated air through a bed of a particulate solids such as aluminum oxide. Under these conditions the particulate bed behaves much like a liquid providing excellent thermal contact but able to reach much higher temperatures than true liquids. Fluidized baths also have a wide range of applications outside of calibration including thermal cleaning of small extrusion tooling, polymer rheometer and melt flow indexers parts, nitinol stent shape setting, reactor heating, and general heat treatment of devices and materials.
- Large volume & heat capacity
- Can calibrate many sensors at a time
- Wide temperature range
- Safer than salt baths, more stable & uniform than ovens
Blackbody calibrators are mainly used for calibrating infrared temperature sensors and work by heating one surface to a specified temperature.
Similar to sensors, instruments, or the devices used to measure outputs from sensors, also drift over time. Instrument calibration is closer to what you’d think traditional calibration would entail, as opposed to sensor calibration where you’re not adjusting the sensor itself. An instrument calibrator will produce a specified output to simulate the output of a sensor. With this information in hand, you can easily see the offset of your instrument and adjust the instrument to meet its published accuracy. Below is an example of an instrument calibrator.
System calibration is the combination of both sensor and instrument calibration to ensure that the system as a whole produces the correct output.
Let’s assume for example that you test your sensor and find that it has a +1 degree drift from the specified output. Let’s also assume that your instrument has a +1 degree drift. This will lead to an overall +2 degree system drift. You could calibrate the sensor and instrument separately, or you could calibrate the system as a whole and adjust the offset accordingly. This approach requires that the same sensor and instrument always be used together.
System calibration is a relatively straightforward process:
- Connect your instrument to the sensor that it will be reading from in your application
- Use a sensor calibrator to produce a defined temperature that your system will read from
- Adjust your instrument to eliminate system drift
How often should I calibrate?
As mentioned earlier, sensors and instruments drift from their specifications due to a number of factors including time, mechanical stressors, and a number of characteristics of the instrument or sensor itself. Due to all of these factors, knowing when to calibrate isn’t an exact science and depends on your application, and how critical accurate temperature readings are. With that in mind, the team at IOThrifty generally recommend that sensors and instruments are calibrated annually.
How should I go about getting my sensors/instruments calibrated?
There are two options for performing calibration on your devices:
- Send your sensors and instruments to a calibration lab
- Buy calibrators and do it yourself
The decision of how to get your sensors/instruments calibrated depends on your application/industry and cost. For some applications, it is required that your calibration is done by a NIST and ISO17025 compliant calibration provider to ensure a higher degree of accuracy and oversight. For applications that fall into this bucket, your best bet is to use a calibration lab that satisfies those credentials. If your application requires high accuracy but isn’t legally required to have NIST compliant calibration, the decision typically comes down to cost.
Most calibration labs charge somewhere around $50/point per sensor and they’ll often calibrate at 3 different points. Points are referring to the different temperature points at which sensors are calibrated. This means the typical sensor costs around $150 to calibrate. If your application requires hundreds or even thousands of sensors, the cost of calibration can become a pretty big line item. Purchasing your own calibrator starts to become a good option if you need to perform roughly 12 to 13 calibrations over the period of a few years, which could be 6 sensors calibrated every six months.