Calibration
element length) – say you have 6mm probe with a 10mm sensing element inside of it - and you immerse it 40mm (five x diameter + sensing element) - you can expect about 1°C error due to the low immersion (one per cent from 100°C). This rule of thumb can become quite significant
at higher temperatures and/or for extremely short sensor lengths. So, keep this in mind with sensors less than 40mm. Also, it may be worth having a conversation with a design engineer to figure out a way to increase the sensor length. Naturally this accuracy limitation is valid also
when the sensor is installed in the process and measuring the process temperature - being too short, the sensor is not able to accurately measure the process temperature! It is not always easy to know the length of
the actual sensing element inside the probe. If that is not mentioned in the datasheet, you can ask the manufacturer.
2. Sensors often have a clamp connection with a flange As mentioned previously, these sanitary sensors are too short compared to their diameter to enable a proper immersion causing temperature leaks, adding error and uncertainty to the calibration. These sensors also often have a so-called
clamp connection (Tri-clamp, ISO 2852, DIN 11851, DIN 32676, BS 4825, Varivent, etc.) configuration, so there is a relatively large metallic flange, that is causing temperature to conduct / leak from the sensor to the flange. In practice, this temperature leak means that the temperature from the sensor is conducting to the large metallic flange, so the flange causes the sensor to read a bit of a lower temperature (when calibrating temperature higher than environment temperature). This kind of flange makes calibration more
difficult in several ways. First, the flange adds temperature leak from the sensor to the flange, the bigger the flange is, the bigger the temperature difference is to the environment temperature. While at the same time the sensor is very
short, this temperature leak causes the sensor to measure erroneous temperature.
A liquid bAth or A dry-block? Generally, you can calibrate temperature sensors in a liquid bath or in a dry-block. This is also the case with the sanitary temperature sensors.
liquid bath As the name suggests, a temperature liquid bath has liquid inside. The liquid is heated / cooled to the required temperature and the temperature sensors to be calibrated are inserted into the liquid. Often the liquid is stirred for even temperature in the liquid.
Liquid bath pros and cons A liquid bath makes it easier to insert any shape of sensors in it and you can also use a reference probe inserted at the same time. Depending on the size of the liquid bath, you may insert several sensors to be calibrated at the same time. In case the sensor to be calibrated is an odd shape, a
Instrumentation Monthly January 2021
benefit is that it will still fit inside the liquid bath. A liquid bath often enables better uniformity
and accuracy than a dry-block due to better heat transfer of liquid. So, this starts to sound like a favourable option? A liquid bath has several drawbacks as to why it is not always the best option:
A liquid bath always includes some sort of liquid, such as silicone oil, and often you do not want to contaminate the sanitary sensor in such a liquid. There is a lot of cleaning after the calibration to ensure that the sensor is clean when installed back into the process.
Handling of hot oil is dangerous and any spills may cause injuries.
Any oil spills make the floor very slippery and can cause accidents.
Liquid baths are very slow. Even if it could fit several sensors in at the same time, it is often several times slower than a dry-block, so overall effectiveness is not really any better. Sometimes people may have several baths, each set to a different temperature, and they move the sensors manually between the baths to skip the waiting time of the bath to change temperature. This may work in a calibration laboratory but is naturally a very expensive way to calibrate.
The sanitary sensor should be placed so that the surface of the liquid touches the bottom of the flange, but in practice this is not always that easy to do. For example silicon oil has pretty large thermal expansion, it means that the surface level is changing slightly as the temperature changes. So, you may need to adjust the height of the sanitary sensor during the calibration. Also, due to the stirring of the liquid, there are small waves on the surface and the liquid level is often deep in the bath, so it is difficult to see that the sensor is at the right depth.
Liquid baths are often large, heavy and expensive equipment.
dry-block A temperature dry-block (or dry-well) is a device that can be heated and / or cooled to different temperature values, and as the name suggests, it is used dry, without any liquids.
Dry-block pros and cons As we have discussed the pros and cons of a liquid bath in this application, let us now consider the pros and cons for the dry-block. The main pros of calibrating the sanitary sensor in a dry-block include:
As it is dry, it is also clean and does not contaminate the sanitary sensor to be calibrated. Sure, the sensor should still be cleaned after calibration, but the cleaning is way easier than with a liquid bath.
A dry-block is also very fast to change temperature.
When using a dedicated insert with proper drillings, it is easy to insert the sanitary sensor always the same way (no adjustments), and the calibration is repeatable every time and with different users.
A dry-block is light and easy to carry compared to liquid bath.
Typically, a dry-block is also cheaper than a liquid bath.
On the downside, a dry-block is less accurate than a liquid bath, it typically only calibrates one sanitary sensor at a time and needs different inserts drilled for different diameter sensors. Despite these downsides, customers often
prefer to make the calibration of their short sanitary sensors in a dry-block. So, let us discuss the different considerations when calibrating in a dry-block.
how to cAlibrAte in A temperAture dry block To calibrate these short sanitary sensors in a temperature dry-block, there are a few considerations to take into account.
using a reference sensor Firstly, when you do the calibration in a temperature dry block, the flange of the sanitary sensor makes it impossible to use a normal external reference sensor in the same insert because it simply does not fit in, the flange covers the insert top and all the holes in the insert.
using an internal reference sensor The dry-block always includes an internal reference sensor. Trying to use the internal
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