Sanitary temperature sensor calibration
Sanitary temperature sensors are commonly used in many industries, but their calibration is different and more difficult than for normal temperature sensors.
Temperature is one of the critical process parameters in many industries and the accuracy of the temperature measurement in processes is a crucial consideration.
The food and beverage, dairy, pharmaceuticals and life science industries have additional requirements for temperature measurement sensors because of their processes. They require temperature sensors that are sanitary, meaning that these sensors need to be suitable to be installed in hygienic and aseptic process environments. These sensors need to be hygienic and designed to be easy to clean, often supporting clean-in-place (CIP) processes. The mechanical design needs to be free from any cavities, dead pockets, gaps or anything that would complicate the hygienic cleaning.
Surface finishes of these sensors are hygienically graded and need to meet the strict standards in these industries, such as the 3-AR (https://www.3-a.org/) or EHEDG (European Hygienic Engineering & Design Group) https://www.ehedg.org/.
The material of the wetted parts in these sensors is often high-grade stainless steel, suitable for these applications, and for easy installation, the sanitary sensors are often provided with the clamp installation.
The temperature ranges typically go up to around 150°C, or in some cases up to 200°C, which in itself is not very challenging.
The role of calibration
In any industry, it is vital that the process measurements measure correctly and as accurately as designed. This can be achieved with the help of suitable process instruments and with a proper calibration program.
Within the food and beverage, pharmaceutical and life science industries, calibration plays an even more important role than most other industries. In these industries, the consequences of a bad or a failed calibration can have a dramatic effect, as we are talking about consumer and patient health and safety. As failed calibration can be very costly in these industries, it has to be avoided by all means possible.
These industries also have dedicated strict regulations concerning calibration.
Why are sanitary sensors difficult to calibrate?
Sanitary sensors are very short
Sanitary temperature sensors are typically very short. Most often less than 100 mm and typically around 50 mm, but can also be as short as 25 mm. The diameter of the sensor typically is 3 mm or 6 mm.
The commonly used practice in temperature calibration (and a Euramet guideline recommendation) is that a temperature sensor should be immersed deep enough in a temperature bath to achieve sufficient accuracy. The recommendation is to immerse to a depth that is 15 times the sensor diameter (plus the length of the sensor element). But with these short sanitary sensors, it is simply impossible to immerse the sensor to sufficient depth during the calibration. For example, a typical sanitary sensor with a diameter of 6 mm should be immersed to at least 90 mm (15 x 6 mm) depth during the calibration to ensure accurate results. But if that 6 mm sensor has a length of only 50 mm, sufficient immersion is simply not possible.
Some rules of thumb for the immersion depth when calibrating in a liquid bath are:
- 1% accuracy: immerse five diameters plus the length of the actual sensing element inside the sensor
- 0.01% accuracy: immerse 10 diameters plus the length of the sensing element
- 0.0001% accuracy: immerse 15 diameters plus the length of the sensing element
The accuracy in the above rule is to be calculated from the temperature difference between the block temperature and the ambient temperature.
For example, if the ambient temperature is 20°C and the block temperature is 120°C, there is a 100°C difference. If you then immerse the probe only five times the dimension (plus the sensing element length) — say you have a 6 mm probe with a 10 mm sensing element inside of it and you immerse it 40 mm (5 x diameter + sensing element) — you can expect about 1°C error due to the low immersion (1% of 100°C).
Figure 1 illustrates the relationship between thermometer immersion depth (in diameters) and the relative error of the temperature difference (of the temperature block and ambient temperatures). So if you don’t immerse at all, you naturally get a 100% error, and if you immerse deep enough the error caused by immersion becomes insignificant.
This rule of thumb can become quite significant at higher temperatures or for extremely short sensor lengths, and so this should be kept in mind with sensors less than 40 mm.
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.
Sanitary sensors often have a clamp connection with a flange
As mentioned previously, sanitary sensors also often have a so-called clamp connection (Tri-clamp, ISO 2852, DIN 11851, DIN 32676, BS 4825, Varivent, etc), so there is a relatively large metallic flange that causes heat to conduct away and leak from the sensor to the flange. In practice this means that the flange causes the sensor to read a slightly lower temperature when calibrating to a temperature higher than ambient temperature.
Liquid bath or a dry block?
Generally you can calibrate temperature sensors in a liquid bath or in a dry block.
Pros and cons of a liquid bath
A liquid bath makes it easier to insert any shape of sensors and you can also use a reference probe inserted at the same time. Depending on the size of the liquid bath, you may be able to insert several sensors to be calibrated at the same time. In case the sensor to be calibrated is an odd shape, a 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 via the liquid.
Even though this sounds like a favourable option, a liquid bath also has several drawbacks:
- A liquid bath always includes some sort of liquid, such as silicone oil, and often you don’t want to contaminate the sanitary sensor in such a liquid. Cleaning is necessary 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.
- Liquid baths are very slow. Even fitting 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 several baths are used, each set to a different temperature, but this is 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, silicone oil has a large thermal expansion, which 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.
- Liquid baths are often large, heavy and expensive pieces of equipment.
Pros and cons of a dry block
The main pros of calibrating the sanitary sensor in a dry block are:
- Because it is a dry, it is also clean and does not contaminate the sanitary sensor to be calibrated.
- A dry block is very fast to change temperature.
- When using a dedicated insert with proper drillings, it is easy to insert the sanitary sensor the same way each time, and the calibration is repeatable every time and with different users.
- A dry block is light and easy to carry compared to a liquid bath.
- Typically, a dry block is also cheaper than a liquid bath.
On the downside, a dry block is a less accurate than a liquid bath, which 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.
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 dry block, the flange of the sanitary sensor makes it impossible to use a normal external reference sensor in the same insert.
Comparing the calibration of a normal (long, no flange) temperature sensor using a reference probe with a short sanitary sensor with a flange (Figure 2), we can see that the short sensor flange covers all the holes in the insert, so it is not possible to insert a normal reference temperature probe.
Using an internal reference sensor
The dry block always includes an internal reference sensor. Trying to use the internal reference sensor in the dry block does not work with short sensors, because the internal reference sensor is located close to the bottom of the temperature block, and the short sensor to be calibrated is located in the very top part of the insert (Figure 3). Dry blocks typically control the temperature gradient on a limited range in the bottom of the insert. The top part of the insert typically has a larger temperature gradient, so the top of the insert does not have the same temperature as the bottom of the insert. The size of the gradient depends on the temperature difference between the insert and environment, and how deep you insert the sensor.
Using a dedicated short reference sensor
As the internal reference sensor in the bottom of the dry block is not suitable, we need to use a dedicated external reference temperature sensor. The aim is to use a dedicated reference sensor that is short enough so that it can be immersed to the same depth as the sanitary sensor to be calibrated. Optimally, the middle of the sensor elements should be aligned to the same depth.
Also, the reference sensor needs to have a thin flexible cable so that the cable can fit under the flange of the sanitary sensor. To help that, we can make a groove in the top of the insert where the reference sensor cable fits and the flange of the sanitary sensor still touches the top of the insert.
Naturally the structure of the temperature dry block needs to be such that the sanitary sensor with the flange fits in its place and touches the insert top end. In some dry-blocks the surroundings prevent the flange from going deep enough to touch the top of the insert.
Locating the reference sensor at the same depth as the short sanitary sensor to be calibrated ensures they are measuring the same temperature. Also, the reference sensor cable is in the groove, so it does not prevent the flange of the sanitary sensor from touching the top of the insert (Figure 4).
Short sensor without a clamp connection
There are also short temperature sensors without the clamp connection and without the flange. With these sensors you should also use an external reference sensor that has been immersed to the same depth as the sensor to be calibrated. The reference sensor should be as similar as possible to the sensor to be calibrated, with a similar diameter, similar response time, etc.
Documentation, metrological traceability and calibration uncertainty
As documentation is included in the formal definition of calibration, it is a vital part of every calibration. This is naturally also valid in sanitary temperature sensor calibration — typically in the form of a calibration certificate.
The calibration equipment used should have a valid metrological traceability to the relevant standards, otherwise the calibration does not ensure traceability in the sensor calibration.
The calibration uncertainty is also a vital part in every calibration. If the calibration equipment (and calibration method and process used) is not accurate enough for the sensor calibration, then the calibration does not make much sense.
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