What Is a Temperature Sensor and How to Calibrate It?
If you’ve ever left your phone or laptop in your car on a hot day, you may have seen a warning that your device is likely to overheat. These electronics have an embedded temperature sensor that records the interior temperature of the device.
If it reaches a specific temperature, the sensor sends a signal to the device’s computer to restrict its use until it cools to a safe temperature.
The temperature sensor works similarly in other devices and lab equipment, measuring the temperature of the environment and converting it into electronic data to record, monitor, or warn of temperature fluctuations. Temperature sensor calibration is necessary for the accuracy and performance of these devices.
What Is a Temperature Sensor?
A temperature sensor is an instrument that’s used to measure temperature.
There are numerous types of temperature sensors – some require contact with the object being monitored, such as contact temperature sensors, and others can indirectly measure the temperature with accuracy, such as non-contact temperature sensors.
Examples of contact temperature sensors include thermocouples and thermistors. A thermocouple features two conductors, each with metal, that are joined to form a junction. When this junction is exposed to heat, the voltage generated corresponds to the temperature.
A thermistor functions differently – the resistance decreases as the temperature increases. The primary types of thermistors include positive temperature coefficients and negative temperature coefficients. These are more precise than thermocouples, typically within 0.05 and 1.5 degrees Celsius,
Non-contact temperature sensors are typically infrared sensors that remotely detect IR energy coming from an object and send that information to an electronic circuit that calculates temperature.
How to Calibrate Temperature Sensors
Calibration is a comparison between a reference device and a device under test. The device under test is compared multiple times for accuracy according to a predefined tolerance using the calibration process.
If the device is within the reference tolerance range, it passes calibration. With temperature sensors, calibration may be conducted with multiple temperature points for linearity.
Temperature calibration requires a reliable, repeatable, and documented comparison of a temperature sensor being tested against reference equipment. The device used as a reference must be exact and undergoes regular testing at an accredited laboratory.
Here’s the process for temperature sensor calibration of lab equipment:
Inspect the Sensor
Before calibrating, conduct a visual inspection of the sensor to ensure it’s in working order and hasn’t been damaged in any way. Be mindful of external contamination, so you may need to clean the sensor before calibration.
Determine the Temperature Source
You need to have a temperature source to calibrate a temperature sensor. For industrial sensors, a temperature dry block may be used for its portability and accuracy.
If you need more accuracy, a liquid bath is a preferred method. It’s not portable, but it’s ideal for laboratory conditions.
To reach a zero Celsius point, a stirred ice bath is ideal. This is an affordable, simple, and accurate method of calibration.
Of all the methods, however, the most accuracy comes from fixed-point cells. This is a more expensive calibration method, but it’s used for accurate and accredited temperature calibration laboratories.
Determine the Temperature Sensor Reference
The reference temperature for calibration is generated with a heat source. It’s important to know the temperature of the heat source with a high degree of accuracy.
Both dry blocks and liquid baths have an internal reference sensor that records the temperature, but it’s less accurate than using a separate reference temperature sensor. This provides a more direct comparison between the temperature reference and the sensor being tested.
The reference sensor should have a traceable calibration as well. It’s also important that the reference sensor is as similar as possible to the sensor that’s being tested. This ensures that these sensors will behave the same way when the temperatures change, providing a more accurate calibration.
With fixed-point cells, there’s no need for a reference sensor. The temperature is based on physical phenomena and lends itself to more accuracy.
Measure the Temperature Sensor Output Signal
Temperature sensors usually have an electrical output, whether in resistance or voltage, that must be measured and converted to a temperature.
If you choose to measure the electrical output, you need to convert that to a temperature using recognized standards. In industrial settings, there will typically be a measurement device that can handle the conversion. Regardless of what method you use, make sure that it has a valid traceable calibration.
Wait for Stabilization
Temperature changes occur slowly. Wait long enough to have the device stabilized to the target temperature. In addition, your sensor under test and your reference sensor may have different characteristics and may take different times to reach stabilization.
Determine Calibration Points
For industrial calibration, you need to choose calibration points to ensure the sensor is linear. Focus on three to five points through the range. When sensors are calibrated in a laboratory, the points may be chosen according to the lab’s smallest uncertainty.
Calibrate Temperature Sensors for Accuracy
Temperature sensors are often used in lab equipment that measures heat and ensures that a process is staying in a predetermined range, supporting safe use, or meeting a mandatory condition when dealing with extreme heat or hazardous substances. Temperature sensor calibration is essential to ensuring accuracy and proper function.