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Calibration


long enough and reaches the bottom on the insert, the boring is tight enough, and we waited long enough for stabilisation, we can get good calibration with little error.


uncertainty component also when the external reference sensor is used. Otherwise a strict auditor may profoundly question the traceability of your measurements.


The above picture illustrates how the external In the right side picture we can see what


happens if the sensor to be calibrated is too short to reach to the bottom of the insert. In this case, the internal reference sensor and the sensor to be calibrated are located at different heights and are measuring different temperatures, causing a big error to the calibration result .


2. ExtErnal rEfErEncE sEnsor


The other way is to use an external reference sensor. The idea here is that you insert a reference sensor into a suitable hole in the insert, while you enter the sensors to be calibrated in the other holes in the same insert. As the external reference sensor is inserted into the same metal insert with the sensors to be calibrated, it can more precisely measure the same temperature as the sensors to be calibrated are measuring. Ideally, the reference sensor would have similar


thermal characteristics as the sensors to be calibrated (same size and thermal conductance). In that case, as the insert temperature changes, the external reference sensor and the sensor to be calibrated will more accurately follow the same temperature changes. The external reference sensor naturally needs


to be measured somehow. Often a dry block has internal measurement circuitry and a connection for the external reference sensor or you can use an external measurement device. For uncertainty, you need to consider the uncertainty of the reference sensor and the uncertainty of the measurement circuitry. Using an accurate external reference sensor


results in a more accurate calibration with smaller uncertainty (compared to using an internal reference sensor). So, it is highly recommended if you want good accuracy (small uncertainty). An external reference sensor also promotes


reliability. If the internal and external sensor readings differ a lot, it’s a warning signal to the user that something is probably wrong and the measurements may not be trustworthy. For recalibration, in the case of an external


reference sensor, you can send just the reference sensor for recalibration, not the whole dry block. In that case, you naturally will not have the dry block’s functionalities being checked (and adjusted if necessary), like the axial temperature homogeneity, for example. If you do not send the dry block for calibration,


be sure to measure and record the axial gradient regularly yourself, as it is typically the biggest


Instrumentation Monthly January 2019


reference sensor and the DUT (Device Under Test) sensor are both located in the insert. The first picture shows the case when both sensors reach the bottom of the insert, resulting in best calibration results. The second picture shows what happens if


the reference sensor and DUT sensor are at different depth. This will cause a big temperature difference between the two sensors and will result in error in calibration. The third picture shows an example where the


DUT sensor is short, and the reference sensor has been correctly positioned in the same depth as the DUT sensor. With this you can get the best possible calibration result, although the homogeneity of the insert is not very good at the upper part of the insert. So, if the sensors are located at different


heights, that will cause additional error, but using an external reference sensor the error is still anyhow typically smaller than when calibrating a short sensor using the internal reference sensor.


3. axial tEmpEraturE homogEnEity


Axial homogeneity (or axial uniformity) refers to the difference in temperature along the vertical length of the boring in the insert. For example, the temperature may be slightly


different in the very bottom of the boring in the insert, compared to the temperature a little higher in the boring. Typically, the temperature will be different in


the very top of the insert, as the temperature is leaking to the environment, if the block’s temperature is very different than the environmental temperature. Some temperature sensors have the actual


measurement element being shorter and some longer. Also, some have the element closer to the tip than others. To assure that different sensors are in the same temperature, the homogenic zone in the bottom of the block’s insert should be long enough. Typically, the specified area is 40 to 60mm. A dry block should have sufficient area in the


insert bottom within which the temperature homogeneity is specified. During a calibration of the block, this may be calibrated by using two high-accuracy reference sensors at different heights or using a sensor with a short sensing element that is gradually lifted higher from the bottom. This sort of short sensing element sensor needs to be stable but does not necessarily need to be calibrated because it is used just for


The above picture illustrates what the “axial


temperature homogeneity” means. Typically, a dry block has a specified area in the bottom that has a homogenic temperature, but as you start to lift the sensor to be calibrated higher, it will not be in the same temperature anymore.


4. tEmpEraturE diffErEncE bEtwEEn thE borings


As the title hints, the temperature difference between the borings, sometimes referred as “radial uniformity”, is the temperature difference between each boring (hole) in the insert. Although the insert is made of metal compounds and has a good thermal conductivity, there can still be a small difference between the borings, especially the opposite ones. In practice, when you have two sensors in the


insert installed in the different borings, there can be a small temperature difference between them. The difference can be caused by the insert touching the block more on one side or the insert being loaded unequally (more sensors on one side, or thicker sensors in one side than on the other side). Of course, the heaters and Peltier elements, located on different sides, have their tolerances too.


Continued on page 24... 23


measuring temperature difference at different heights. If needed, the axial temperature gradient can typically be adjusted in a dry block. If you have a short (sanitary) temperature sensor that does not reach all the way to the bottom of the boring in the insert, then things will get a bit more complicated. In that case, the internal reference measurement in the dry block cannot really be used, as is typically in the bottom of the block. An external reference sensor should be used and it should have the centre of the measurement zone inserted as deep as the centre of the measurement zone of the short sensor to be calibrated. Often, this means that a dedicated short


reference sensor should be used, and inserted into the same depth as the short sensor to be calibrated. It gets even more difficult if the short sensor to be calibrated has a large flange as that will soak up temperature from the sensor. In summary, during the calibration you should


ensure that your reference sensor is inserted to the same depth as the sensor(s) to be calibrated. If you know the lengths and the locations of the sensing elements, try to align the centres horizontally. If that is not possible, then you need to estimate the error caused by that. You should use an external temperature sensor, if the accuracy requirements of the calibration are higher, or if the sensor to be calibrated is not long enough to reach the bottom on the insert hole.


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