No two calibrations are absolutely identical. Take temperature measuring instruments, for example: the test method chosen for thermometers and sensors is influenced by process conditions, quality standards and safety requirements. The device to be calibrated may be attested up to double precision depending on the method. This can be a crucial factor when endeavouring to maximise process efficiency.
Temperature is measured more often than any other parameter in industrial processes. It is therefore unsurprising that a large number of thermometers and sensors are installed in each system. The type and the power range vary according to the control task. The respective requirements and the ambient conditions simultaneously determine which inspections are necessary to check the measuring instruments. Regular calibration is essential to maintain confidence in the measured value or detect any changes in good time. In the latter case, the instrument can be readjusted or exchanged before any harm is caused to the process.
The effort for calibrating measuring instruments is considerable, and many companies are unable – or unwilling – to shoulder it alone. This particularly applies if the process is certified to ISO 9001 or a similar quality assurance system. Quality systems stipulate regular – and traceable – inspections of all measuring instruments. In Germany, these check measurements are undertaken by calibration laboratories which are certified by the German accreditation body (DAkkS). In South Africa, these labs are accredited by the South African National Accreditation System (SANAS). Many instrument manufacturers, including WIKA, have their own facilities for this purpose which are independent of specific products. A wide variety of services may be provided, from verification of working standards to calibrations of customer-specific designs such as complete multi-point thermometers. The customer’s quality management experts can develop differentiated calibration processes for each individual system in cooperation with the laboratory.
Fig. 1: Calibrating gauges in an external lab usually results in longer downtime for the affected process. This downtime can often be minimised by a mobile calibration lab. Calibration in the mobile unit speeds up the calibration process by a factor of five. As a rule, the service is economically worthwhile for ten or more test items.
They must decide in which situations standard checks are adequate, customer-specific solutions are preferable or a mobile laboratory is essential in order to reduce process stoppages to a minimum. The following calibration methods are available, depending on the requirements:
Comparative calibration
This is the method most commonly employed in calibration laboratories: the device to be calibrated is compared with a higher-quality reference thermometer. The stable test temperature which is necessary for this purpose is usually achieved in immersion baths filled with liquids, salts or powders. Special calibration furnaces are used at higher temperatures. A homogeneous temperature distribution is ensured by interior heating pipes or by solid and highly conductive compensation blocks.
The device to be calibrated is exposed to a constant temperature together with a calibrated instrument as a working reference. As soon as thermal equilibrium has been reached, the values can be read and the measuring characteristics, such as any deviations, determined. This method does not simply enable rapid calibration sequences. It also has economic benefits in that several thermometers can be calibrated at once in the baths or furnaces. Unfortunately, multi-piece test equipment goes hand in hand with a larger measurement uncertainty. Ideally, this will be 10 mK in the WIKA laboratory.
Fixed-point calibration
A much smaller measurement uncertainty – as little as ± 2 mK, and hence up to double the instrument precision – can be realised using fixed-point calibration. Cells in which fixed points of high-purity substances such as gallium or zinc can be set as the “reference”. As a function of temperature and pressure, these substances exist in the three classical physical states. Phase transitions, for example from solid to liquid, can be used for calibration since, at constant pressure, the temperature of a substance also remains constant until the phase transition is complete. Apart from the phase transitions, triple points are also employed for some substances. At the triple point, the three phases of a substance are present in a thermal equilibrium. They can be set very accurately and reproduced at any time. Moreover, they can be maintained over long periods.
The International Temperature Scale defines 16 fixed points, from the triple point of hydrogen (-259,3467°C) to the freezing point of copper (1084,62°C). The WIKA laboratory, for instance, calibrates at the triple points of mercury and water, at the melting point of gallium and at the freezing points of tin, zinc and aluminium. A temperature range from -33,8344°C to 660,323°C is covered in this way.
Not every temperature measuring instrument is suitable for fixed-point calibration, however. The sensor platinum of a resistance thermometer, for instance, must satisfy certain purity criteria and the wire winding must be stress-free.
Fig. 2: In comparison calibration, as soon as the thermal equilibrium has settled, the values are read off and statements about metrological properties can be derived.
Customer-specific calibrations
In some cases, calibration laboratories have to design an inspection procedure on the basis of directives which are explicitly adapted to the device to be calibrated. Temperature sensors with a connected transmitter, which are approved under 2014/32/EU (MID = measuring instruments directive), are a good example here. Their quality must be demonstrated by means of a traceable calibration as soon as they have been manufactured. This entails performing a so-called system calibration at three points in the transmitter’s measuring range: 0%, 50% and 100%. To do this, however, the transmitter and the temperature sensor must first be matched to one another with coefficients.
Tolerance limits exist for the analogue and digital signals depending on the measuring range, and these must be complied with. Both signals must therefore be measured and documented. A complete multi-point thermometer which is calibrated in a tube furnace with an integrated reference is another example of a customer-specific inspection method. The installed thermocouples are measured along the length of the thermometer lance – rather than individually prior to assembly, as is usually the case. A tube furnace containing a block with bores, into which the lance is inserted, is required here. Each of its measuring points is checked at three temperature points (e.g. 200°C, 400°C, 600°C). The length and mass of the tube furnace ensure that the temperature in all bores is identical and remains stable over time.
The multi-point thermometers are prepared for calibration on special benches. A special support protects the lance against mechanical deformation as it passes through. The only limitation on the thermometer length with this method is the available space. Standard tube furnaces can also be used for other calibration tasks, e.g. testing thermocouples for high temperature applications (up to 1600°C).
Calibration vans
When measuring instruments are calibrated in an external laboratory, prolonged process stoppages are generally unavoidable. The devices to be calibrated have to be removed, sent to and from the laboratory and reinstalled again after calibration. In many cases, the resulting downtime can be reduced to a minimum by using a calibration van. WIKA operates three mobile calibration laboratories in Germany. On-site calibration in a mobile unit enables the overall calibration process to be completed in as little as a fifth of the usual time. The more devices to be calibrated, the lower the costs for the van. In spite of the high fixed costs, this service tends to be worthwhile from as few as ten instruments.
Conclusion
When it comes to temperature measuring instruments, calibration often involves more than “simply” checking whether the measured value is indicated correctly within the permissible tolerances. It also has a strategic significance in that different methods can be chosen in cooperation with a qualified laboratory. The inspection intervals can be optimised in this way – and the efficiency of the processes improved because higher precision is achievable.
Contact Greg Rusznyak, WIKA Instruments, Tel 011 621-0000, greg.rusznyak@wika.com
