Temperature measurement looks back on a long tradition and will continue to play an important role in metrology in the future as well. Relentless pressure on production departments to optimise the economic efficiency of manufacturing processes normally signifies a higher level of plant automation. Optimisation of the manufacturing process requires an even more exact control of process parameters. For this reason there is likely to be a steadily rising need for industrial temperature sensors.
The demand for industrial thermometers is also being fuelled by tougher environmental laws and stricter product standards in critical manufacturing processes. Compulsory documentation as required, for example, in the heating and ventilation field and ISO 9000, is also resulting in more and more measurements having to be taken of temperature as well as other process variables.
A certain reverse trend, on the other hand, is arising from the pressure on costs: measurements are only being taken where they are absolutely essential. In the case of bearing sleeves, for example, an exact knowledge of the system enables the entire temperature distribution in the bearing to be calculated from a single temperature measurement. The thermometers previously required are now superfluous.
On the whole the number of temperature sensors is rising, albeit the requirements needing to be met by industrial temperature metrology are changing and shifting:
The increasing rationalisation and integration of manufacturing process control and planning systems calls for appropriate measuring systems. For these measuring systems it is necessary in turn to provide uniform, standardised and producer-independent electrical, mechanical and electronic interfaces and user environments. The plant industry is unable to organise separate electronic and user training courses for each measuring sensor. Basically, the uniform adjustment interface for analog measurement transducers was formed by the potentiometer and screwdriver. If an intelligent pressure measuring transducer is used in a plant with the bus protocol from company A, it ought to be possible to operate this measuring transducer with an identical bus protocol from company B. The continuing debate about fieldbuses and about the standardisation of software user environments indicates that this requirement is not to be taken for granted and that its implementation is possible only very slowly. This incompatibility of measuring system interfaces is one of the reasons for the hesitant introduction of fieldbuses. The tendency of large companies to opt for a few, standardised thermometers and sensor types is a further reflection of this. The forces of harmonisation will prevail in temperature metrology as they have elsewhere.
New and optimised manufacturing processes are making greater use of the temperature scale in the upper and lower range. Exhaust gas catalysts work at temperatures of 800 to 1000°C. High-temperature superconductors, which can be used at -250 to -180°C and higher, are also being employed increasingly in industry. The temperature probes used in these applications have to work reliably and steadily under the higher loads and stresses. This calls for the development of new temperature sensor principles or the further development of existing temperature sensor principles into industrial temperature probes. A basis for such developments can be provided by the discoveries made in physics over the past few years. An example of such technological innovations are superconducting quantum interference devices (SQUIDs). The use of SQUIDs in medicine, geophysics and material testing has already begun to change metrology fundamentally.
New materials, material combinations and processes, pose new challenges for temperature metrology and the technical equipment of temperature probes. Two examples can be cited as representative. The food industry's demand for cleaning of process pipework by mechanical means, requires a change of mechanical design for the temperature sensor. Processes in biotechnology are only possible with very close tolerances and extremely clean surfaces. An absolute error of approximately 1 K at 100°C or up to 5 K at 600°C is currently the state of the art. In enzyme technology deviations of just 0,1 K from the correct process temperature can already decide between good product or scrap in the 10% range of magnitude.
Not only problems of electromagnetic compatibility (EMC) but also cost reasons are transferring the processing of measurement signals by measuring transducers increasingly from the control room into the field. There is a distinct trend in temperature metrology to reduce the size of the control cabinet, which was fitted with busbar measuring transducers, or to do away with it altogether and replace it with on-site head transmitters.
For the declaration of CE conformity and for purposes of ISO 9000 traceability, far more measuring sensors have to be installed than has so far been the case. The observance of close and standardised tolerances and the high quality demanded in production requires high-end but low-cost technology for mass use. Extremely economical production of the temperature sensors and their processing electronics is necessary. This demand can only be met by exhaustive exploitation of modern technologies. It seems likely, therefore, that the integration of sensor and electronics on a high-temperature silicon or ruby base will become established in the measurement ranges up to approximately 180°C and possibly even to 250°C.
There are signs of a further trend for the temperature metrology sector:
Sensor temperature is the most important correction variable for many sensors of other measurement variables. This applies in particular to sensors based on organic chemistry. In these cases the sensor element for the temperature measurement is also integrated. The temperature measurement is thus delivered as a 'side product' for follow-up intelligent processing by the microprocessor. In other words, temperature becomes 'one of many' measured values. Similarly, the networking of sensors with a fieldbus means that the temperature measuring sensor is no longer a single special isolated measuring point - in spite of all its special conditions of use - but part of an entire network. These new conditions of application are well illustrated by comparison with the human organism. The touch sensors, temperature sensors and pain sensors in the skin, although individually specialised, add up to a uniform network with compatible interfaces that can only provide the necessary information when working together. No single manufacturer of sensors is able to produce all types economically. There is mounting pressure, therefore, for sensor manufacturers to cooperate and standardise. This is the only way to be able to serve each industrial branch with its characteristic combination of probes and sensor types economically and with the know-how needed in this sector.
For more information contact Alpret Control Specialists, 011 249 6700, [email protected], www.alpret.co.za
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