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Calibration Uncertainty components


of a temperature calibration using a dry block In this article, Heikki Laurila, Beamex, discusses the possible sources of uncertainties that may arise when conducting a temperature calibration using a dry block. He also provides some tips on how to minimise these uncertainties


however there are many possible sources for uncertainty and error that should be considered. Often the biggest uncertainties may come from the procedure on how the calibration is done, not necessarily from the specifications of the components.


M


WHat Is a “dry BLock”? A temperature dry block is sometimes also called a dry-well or a temperature calibrator. It is a device that can be heated and/or cooled


to different temperature values, and as the name hints, it is used dry, without any liquids. A dry block typically has a removable insert


(or sleeve) that has suitable holes/borings for inserting temperature sensors into. The dry block typically has its own internal


measurement for the temperature, or you may use an external reference temperature sensor that you will insert into one of the holes. Commonly a dry block has interchangeable


inserts, so you may have several inserts, each being drilled with different holes, to suit for calibration of different sized temperature sensors. It is very important in a dry block that the


hole for the temperature sensor is sufficiently tight to enable low thermal resistance between the sensor and the insert. In too loose of a boring, the sensor stabilises slowly or may not reach the temperature of the insert at all due to stem conduction. Commonly, you would insert a temperature sensor in the dry block to be calibrated or calibrate a temperature loop where the temperature sensor is the first component in the loop. The main benefits of a dry block are that it is


easy to carry out in the field and there is no hot fluid that would spill when you carry it around. Also, a dry block will not contaminate the temperature sensors being calibrated. Dry blocks are almost always used dry. In some


very rare cases you may use some heat transfer fluids or pastes. In most cases you may damage the dry block if you use liquids.


Using oil or pastes also cause a potential health and fire risk if later used in temperatures higher than for example a flash point of the foreign substance. A 660°C dry


22


block that has silicon oil absorbed into its insulation may look neat outside, but it will blow out a noxious fumes when heated up. Calibration labs everywhere are probably more familiar with this than they would like to be. As drawbacks for dry blocks, we could consider


that they have a lower accuracy/stability than with a liquid bath and are more difficult to calibrate very short and odd shaped sensors.


UncertaInty components Let us get into the actual uncertainty components. When you make a temperature calibration using a dry block, these are the things that cause uncertainty/error to the measurement results.


InternaL or externaL reference sensor? There are two principle ways to measure the true (correct) temperature of a dry block. One is to use the internal measurement using the internal reference sensor that is built in to the dry block, the other is to use an external reference sensor that is inserted into the insert boring/hole. There are some fundamental differences


between these two ways and they have a very different effect on the uncertainty, so let us discuss these two options next:


1. InternaL reference sensor An internal reference sensor is permanently inserted into the metal block inside the dry block, it is typically close to the bottom part of the block and it is located in the metallic block surrounding the interchangeable insert.


So, this internal sensor does not directly


measure the temperature of the insert, where you insert the sensors to be calibrated, but it measures the temperature of the surrounding block. Since there is always some thermal resistance between the block and the insert, this kind of measurement is not the most accurate one. Especially when the temperature is changing,


the block temperature normally changes faster than the insert temperature. If you make the calibration too quickly without waiting sufficient stabilisation time, this will cause an error. An internal reference sensor is anyhow pretty


handy, as it is always readily inside the block, and you do not need to reserve a dedicated hole in the insert for it. The recalibration of the internal measurement


is a bit difficult, as you need to send the whole dry block into recalibration. An internal measurement sensor’s signal is


naturally measured with an internal measurement circuit in the dry block and displayed in the block’s display. The measurement typically has an accuracy specification given. In practice this specification is only valid in stable conditions and does not include the uncertainties caused if the calibration is done too quickly or the sensors to be calibrated are not within the calibration zone at the bottom part of the insert, in a sufficiently tight boring. The left side of the folowing picture illustrates how the internal reference sensor is typically located in the temperature block, while the sensor to be calibrated is inserted into the insert. If the sensor to be calibrated is


January 2019 Instrumentation Monthly


aking a temperature calibration using a dry block seems like a pretty simple and straight forward thing to do,


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