Temperature measurement is one of the most common instruments that is put to use in a variety of industries.  All temperature measurement loops have a sensor as the first component in the loop.  The temperature sensor plays a vital role in the accuracy of the entire temperature measurement loop.  Like all instruments, it especially needs to be accurate, and it needs to be calibrated regularly.

Temperature Sensors

A temperature sensor is a device, typically a thermocouple or RTD, that provides for temperature measurement through an electrical signal.  It has an output signal proportional to the applied temperature.  When the temperature of the sensor changes, the output will change accordingly.

There are a variety of temperature sensor types, all with different output signals.  Some have a resistance output, some have a voltage signal, digital signal, and more.  They make thermocouple from two dissimilar metals that generate an electrical voltage in direct proportion to changes in temperature.

Factors to Consider

When looking at how to calibrate temperature sensors, there are some factors that you must take into consideration.

• Handling the Temperature Sensor – Different sensors have different mechanical structures and different mechanical robustness. Most industrial temperature sensors are tough and can withstand normal handling.  However, Standard Platinum Resistance Thermometers (SPRT) which reference sensors in temperature laboratories, are fragile, so they must take extra care.  Before taking any action, understand if your instrument is sturdy or more fragile.
• Preparation – Start with a visual inspection of the temperature sensor. Check to see if it is bent, the wires are alright, or if there is any other visible damage to the instrument.  External contamination can frequently be an issue. Be aware of where the sensor usage has been and what kind of media it has been measuring.  It may need cleaning before calibration.
• Temperature Source – To calibrate a temperature sensor, you need to have a temperature source. A temperature dry-block is portable and thus accurate enough for instruments.  If they need higher accuracy, a liquid bath will work.  For zero Centigrade point, a stirred ice-bath will work.  The most accurate is fixed-point cells.  They use them the most in accredited temperature calibration laboratories.
• Reference Temperature Sensor – For more accurate results, they insert a separate accuracy reference temperature sensor in the same temperature as the sensor for calibration. That kind of reference sensor will accurately measure the temperature that the sensor for calibration is measuring.  Therefore, the reference sensor must have a valid traceable calibration.
• Measuring the Output Signal – Most temperature sensors have an electrical output that needs temperature measuring and converting. You will need to have a device to be used for the measurement.  If you measure the electrical output, you will need to convert that into temperature using international standards.
• Immersion Depth – How deep you insert the sensor into the temperature source is an important consideration when calibrating temperature sensors. The sensor for calibration and the reference sensor immersion should specifically be the same depth.
• Stabilization – Temperature changes pretty slowly, and because of that, you should always wait long enough to have all parts stabilized to the target temperature.

Other Factors to Consider in Calibration

It’s important to realize, that there are other factors to consider with temperature sensor calibration.  These are the same for calibration with any instrument.  It will need documentation in a calibration certificate, traceability, and uncertainty.  If you are unsure about the instruments, or you need more calibration information, SRP controls works with leading manufacturers to provide the best products and services in the industry.  As an ISO 9001 registered company with a state-of-the-art ISO 17025 accredited laboratory, we can help.  For this reason, contact us today.