How to calibrate the temperature sensor

Temperature measurement is one of the most common measurements in the process industry.
Each temperature measurement loop has a temperature sensor as the first component in the loop. Therefore, it all starts with the temperature sensor. The temperature sensor plays a vital role in the accuracy of the whole temperature measuring circuit.
As any measuring instrument you want to be accurate, the temperature sensor also needs to be calibrated regularly. If you don't care about accuracy, why do you measure temperature?
Veren will introduce how to calibrate the temperature sensor and what are the most common things to consider when calibrating the temperature sensor.

What is a temperature sensor?

As the name implies, a temperature sensor is an instrument that can be used to measure temperature. It has an output signal that is proportional to the application temperature. When the temperature of the sensor changes, the output will also change accordingly.
There are various temperature sensors with different output signals. Some have resistive outputs, some have voltage signals, some have digital signals, and so on.
In fact, in industrial applications, the signal from the temperature sensor is usually connected to the temperature transmitter, which will convert the signal into a format that is more convenient for long-distance transmission, and then transmit it to the control system (DCS, SCADA). The standard 4 to 20 mA signal has been used for decades, because the current signal can be transmitted over a longer distance, and even if there is some resistance along the wire, the current will not change. Transmitters that now use digital signals or even wireless signals.
In short, to measure temperature, the measuring element used is a temperature sensor.

Measuring temperature sensor output
Since most temperature sensors have an electrical output, it is obvious that this output needs to be measured in some way. Having said that, you need a measuring device to measure, for example, output, resistance, or voltage.
Measuring equipment usually displays the quantity of electricity (resistance, voltage), not the temperature. Therefore, it is necessary to know how to convert the electrical signal into a temperature value.
Most standard temperature sensors have international standards that specify how to use tables or formulas to calculate electrical/temperature transitions. If you are using a non-standard sensor, you may need to obtain this information from the sensor manufacturer.
Some measuring devices can directly display the temperature sensor signal as temperature. These devices also measure electrical signals (resistance, voltage) and internally write sensor tables (or polynomials/formulas), so they convert them to temperature. For example, temperature calibrators typically support the most common RTD (resistance temperature detector) and thermocouple (T/C) sensors used in the process industry.

How to calibrate the temperature sensor?
Before we discuss the various considerations when calibrating temperature sensors, let's look at the general principles.
First, because the temperature sensor measures temperature, you need to have a known temperature to immerse the sensor in it for calibration. It is not possible to "simulate" temperatures, but you must use a temperature source to create real temperatures.
You can generate an accurate temperature or use a calibrated reference temperature sensor to measure the generated temperature. For example, you can insert the reference sensor and the sensor to be calibrated into a liquid bath (preferably stirred), and then you can calibrate at that temperature point. Alternatively, a so-called dry block temperature source may be used.
For example, using a stirred ice bath provides very good accuracy for 0 ° C (32 ° F) point calibration.
For industrial and professional calibration, temperature bath or dry block is usually used. These can be programmed to heat or cool the temperature to a set point.
In some industrial applications, it is common practice to replace the temperature sensor regularly rather than calibrate the sensor regularly.

How to calibrate temperature sensors – considerations
Let's begin to delve into the actual calibration of the temperature sensor and the different things to consider

1 - Processing temperature sensor
Different sensors have different mechanical structures and different mechanical robustness.

The most accurate SPRT (Standard Platinum Resistance Thermometer) sensor, used as a reference sensor for temperature laboratories, is very fragile. Our temperature calibration laboratory personnel said that if the SPRT touches something and you can hear any sound, you must check the sensor before further use.

Fortunately, most industrial temperature sensors are rugged and can be handled properly. Some industrial sensors are very robust and can withstand fairly rough handling.

However, if you are not sure about the structure of the sensor that should be calibrated, it is better to be safe than sorry.

It is never wrong to handle any sensor like SPRT.

In addition to mechanical shocks, rapid changes in temperature may obstruct the sensor and damage it or affect accuracy.

Thermocouples are generally less sensitive than RTD probes.

2 - Preparation
Usually there is not so much preparation, but there are some things to consider. First, perform a visual inspection to see if the sensor is OK, and make sure that it is not bent or damaged, and that the wires are OK.
External contamination can be a problem, so it is best to know where the sensor is used and the type of medium being measured. You may need to clean the sensor before calibration, especially if you plan to use a liquid bath for calibration.
The insulation resistance of the RTD sensor can be measured before calibration. This is to ensure that the sensor is not damaged and the insulation between the sensor and the chassis is high enough. A drop in insulation resistance can cause measurement errors and is a sign of sensor damage.

3 - Temperature source
As mentioned earlier, you need a temperature source to calibrate the temperature sensor. It is just unable to simulate temperature.
For industrial use, temperature drying blocks are the most commonly used. It is convenient and portable, and usually accurate enough.
For higher accuracy requirements, a liquid bath can be used. In any case, this is not usually easy to carry, but can be used under laboratory conditions.
For zero degrees Celsius, a stirred ice bath is usually used. It is very simple and reasonably priced, but it provides good accuracy for Zero.
In order to obtain the most accurate temperature, a fixed point battery is being used. These are very accurate, but also very expensive. These are primarily used for accurate (and accredited) temperature calibration laboratories.

4 - Reference temperature sensor

The temperature is generated by some heat sources mentioned in the previous chapter. Obviously, you need to know the temperature of the heat source very accurately. Dry blocks and liquid baths provide an internal reference sensor for measuring temperature. However, for more accurate results, you should use a separate accurate reference temperature sensor, which is inserted at the same temperature as the sensor to be calibrated. This reference sensor will more accurately measure the temperature being measured by the sensor to be calibrated.
Naturally, the reference sensor should have an effective traceable calibration. Sending the reference sensor for calibration is easier than sending the entire temperature source (if you always calibrate only the reference sensor and not the temperature block, it is also better to remember the temperature gradient of the temperature block).
As far as thermodynamic characteristics are concerned, the reference sensors should be as similar as possible to the sensors to be calibrated to ensure that they behave the same during temperature changes.
The reference sensor and the sensor to be calibrated shall be immersed to the same depth in the temperature source. Normally, all sensors are immersed in the bottom of the drying block. For very short sensors, it will become more difficult because they will only immerse a limited depth into the temperature source. You should ensure that your reference sensors are immersed at the same depth. In some cases, this requires the use of a dedicated short reference sensor.
With the fixed-point unit, you do not need any reference sensors because the temperature is based on physical phenomena and its properties are very accurate.

5 - Measuring temperature sensor output signal
Most temperature sensors have an electrical output (resistance or voltage) that needs to be measured and converted to temperature. Therefore, you need some measuring equipment. Some temperature sources also provide measurement channels for sensors, including DUT and reference.
If you measure the electrical output, you need to convert it to temperature using international standards. In most industrial situations, you will use measuring equipment that can convert for you, so you can easily view the signals in temperature units (Celsius or Fahrenheit).
No matter what method you use to measure, please ensure that you understand the accuracy and uncertainty of the equipment and ensure that it has an effective traceable calibration.

6 - Immersion depth
The immersion depth (the depth at which the sensor is inserted into the temperature source) is an important consideration when calibrating the temperature sensor.
Our temperature calibration laboratory personnel give the following rules of thumb when using a stirred liquid bath:
1% accuracy - immersion of 5 diameters+length of sensing element
0.01% accuracy - immersion 10 diameters+length of sensing element
0.0001% accuracy - immersion 15 diameters+length of sensing element
The heat conduction in stirred liquid bath is better than that in dry block, and the immersion depth required is smaller.
For dry blocks, Euramet recommends that you immerse the sensor diameter plus 15 times the length of the sensor element. Therefore, if you have a sensor with a diameter of 6 mm and a 40 mm element inside, immerse it (6 mm x 15+40 mm) 130 mm.
Sometimes it is difficult to know the length of the actual element inside the sensor, but it should be mentioned in the sensor specification.
In addition, you should know the location of the sensor element (it is not always at the tip of the sensor).
The sensor to be calibrated and the reference sensor shall be immersed to the same depth so that the midpoint of the actual sensor element is at the same depth.
Of course, for very short sensors, it is impossible to immerse them very deeply. This is one of the reasons for the high uncertainty when calibrating short sensors.

7 - Stable
Remember that the temperature sensor always measures its own temperature!
The temperature changes very slowly and you should always wait long enough for all components to stabilize to the target temperature. When you insert a sensor into a temperature, it always takes some time before the temperature of the sensor reaches that temperature and stabilizes.
Your reference sensor and the sensor to be calibrated (DUT) may have very different thermodynamic characteristics, especially when they are mechanically different.
One of the biggest uncertainties usually associated with temperature calibration may be that the calibration is completed too quickly.
If you calibrate similar types of sensors most often, it is wise to perform some type tests to understand the behavior of these sensors.

8 - Temperature sensor handle

The sensor handle part, or transition junction, usually limits its heat. If the heating is too hot, the sensor may be damaged. Make sure you understand the specifications of the sensor you are calibrating.
If calibrating at high temperatures, it is recommended to protect the sensor handle with a temperature shield.

9 - Calibration temperature range
For temperature sensors, the entire temperature range of the sensor is not normally calibrated.
The top of the range is something you should be careful about when calibrating. For example, if an RTD sensor is calibrated at too high a temperature, it may drift permanently.
In addition, the coldest point of the sensor temperature range can be difficult to calibrate/expensive.
Therefore, it is recommended to calibrate the temperature range that the sensor will use.

10 - Calibration point
In industrial calibration, it is necessary to select enough calibration points to see that the sensor is linear. It is usually sufficient to calibrate 3 to 5 points over the entire range.
Depending on the sensor type, you may need to get more points if you know that the sensor may not be linear.
If you calibrate the platinum sensor and plan to calculate the coefficient based on the calibration results, you need to calibrate at the appropriate temperature point to calculate the coefficient. The most common coefficients for platinum sensors are ITS-90 and Callendar van Dusen coefficients. For thermistors, Steinhart Hart coefficients can be used.
When calibrating the sensor in an approved laboratory, the point can also be selected according to the minimum uncertainty of the laboratory.

11 - Adjust/fine tune the temperature sensor
Unfortunately, most temperature sensors cannot be adjusted or fine tuned. Therefore, if you find calibration errors, you cannot adjust them. Instead, you need to use a factor to correct the sensor reading.
In some cases, you can compensate for sensor errors in other parts of the temperature measurement loop (transmitters or DCS).

Other considerations
file
As with any calibration, the temperature sensor calibration needs to be recorded in the calibration certificate.

Traceability
In calibration, the reference standards used must have effective traceability to national standards or equivalent standards. The traceability of shall each have a continuous chain of specified uncertainty calibration.

uncertain
As with calibration, temperature sensor calibration is the same, and you should understand the total uncertainty of the calibration process. In temperature calibration, the calibration process (the way you calibrate) can easily become the largest uncertainty component in the total uncertainty.

Automatic calibration
Temperature calibration is always a very slow operation, because the temperature changes slowly, you need to wait for stability. If you can perform temperature calibration automatically, you will benefit greatly. Calibration still takes a long time, but if it is automated, you don't need to wait there.
This will naturally save you time and money. Tel./WeChat 18717811268 Huang Gong Email sales@weilianchina.com