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Calibration The temperature difference between the


borings is normally relatively small in practice. To summarise, the specification of the


temperature difference between borings should be taken into account.


6. StabIlIty over tIme


5. Influence of loadIng There is always some heat conducted through


the sensors to the environment (stem conductance) if the block’s temperature differs from the environmental temperature. If there are several sensors installed in the


insert, there will be more temperature “leaking” to the environment. Also, the thicker the sensors are, the more temperature leakage there will be. The bigger the temperature difference between


the insert and the environment temperature, the bigger the leakage will be. For example, if you have the dry block at high


temperature, this temperature leakage will cause the insert to cool down because of the loading. The top of the insert will lose more temperature than the bottom of the insert and the top becomes cooler. The deeper the insert is, the less loading effect


there will be. Also, some dry blocks have two or more heating/cooling zones: one in the bottom, one in centre and one in the top of the block. This will help to compensate the loading effect (e.g. the top heating can heat more to compensate the top of the insert to cool down). If you use the internal reference


measurement of the dry block, there will typically be a larger error since the internal reference is not in the insert but is in the bottom of the surrounding block. Therefore, the internal reference sensor does not see this effect of loading very well. An external reference sensor can better see


the effect of loading, as it is in the insert and it will also have the same change in the temperature. The error caused by the loading effect is much smaller when external reference sensors is used (compared to using internal reference sensor), and the results are better. In summary, check out the loading effect of


your dry block in your application (how many sensors, which type of sensor) and use that as one uncertainty component. The next picture illustrates the stem


conductance caused by the sensors leaking the temperature to the environment. In the second picture there are several sensors inserted, so the stem conductance/leakage is larger.


24


Stability over time describes how well the temperature remains stable over a longer period. The temperature needs to be stable for certain time, as the different sensors may have different thermal characteristics and it takes different time for different sensors to stabilize. If the temperature is constantly creeping up and down, the different sensors may read different temperatures. In case there is some fluctuation in the


temperature, an external reference sensor will anyhow result in more accurate results, compared to the use of an internal reference sensor. Often a dry block manufacturer has given a stability specification, for example for a 30 minute period.


7. do not be In a hurry!


It is good to remember the fact that a temperature sensor will always measure only its own temperature. So, it does not measure the temperature where it is installed, but it will measure its own temperature. Also, temperature changes pretty slowly and it


takes some time before all parts of the system have stabilised to the same temperature, i.e. system has reached equilibrium. If you make a temperature calibration with a


dry block too fast, that will be the biggest source of uncertainty. So, get to know your system and the sensors


you calibrate and experiment to see how long time is enough for sufficient stabilisation. Especially if you use the internal reference


sensor, it will reach the set temperature much faster than the sensors to be calibrated located in the insert. That is because the internal sensor is in the block that is heated/cooled, and the sensors to be calibrated are in the insert. Taking the results too soon can cause a big error. In case of an external reference sensor,


the need for stabilisation depends on how different your reference sensor is compared to your sensors to be calibrated. If they have different diameters, they will most likely have different stabilisation time. Anyhow, using an external reference sensor will be much more accurate than internal one, in case you do not wait long enough for stabilisation. Often a dry block will have a stability indicator, but that may be measuring the stability of the internal reference sensors, so do not trust only on that one. In summary, if you do the temperature


calibration too fast, the results will be terrible.


The above picture illustrates an example


where the temperature set point has been first 10°C and at the five minutes mark it has been changed to 150°C (the blue line represents the set point). There have been two sensors in the dry block – a reference sensor and a sensor to be calibrated. We can see that the Sensor 1 (red line) changes


much faster and reaches the final temperature at about 11 minutes point. The Sensor 2 (green line) changes much slower and it reaches the final temperature at around the 18 minutes mark. The Sensor 1 is our reference sensor and


the Sensor 2 is the sensor to be calibrated. We can see that if we read the temperatures too early at 10 minutes mark, we will get a huge error (about 85°C) in our results. Even if we take the readings at the 15 minutes mark, we still have around 20°C difference. So, we should always make sure that we


wait long enough to make sure that all sensors are stabilised to the new temperature, before we read the readings .


Summary


Making a temperature (sensor) calibration using a dry block seems pretty simple and straight forward thing to do. But there are many possible sources for uncertainty and error that should be taken into account. Often the biggest uncertainties may come


from the procedure on how the calibration is done, not necessarily from the specifications of the components. For example, you may have an accurate


dry block that has combined total uncertainty being 0.05°C and a high- quality reference sensor with uncertainty of 0.02°C. But calibrating a temperature sensor with these devices can have an uncertainty of several degrees, if it is not made properly. It is important to remember TAR (Test


Accuracy Ratio) as it does not take into account all the uncertainties caused by the calibration procedure, it only uses the accuracy specifications. I hope these considerations listed in the article


help you to realise the possible sources of uncertainty and also how to minimise them. Find out more at and calibrating with dry


blocks: https://blog.beamex.com/uncertainty- components-temperature-calibration-dry-block


Beamex www.beamex.com January 2019 Instrumentation Monthly


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