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For all their advantages, thermocouples are increasingly being recognized as ill suited for pharmaceutical validation work, such as chamber and storage area temperature mapping. There are a number of reasons why - too many things can go wrong when you use thermocouples. In a FDA-regulated environment, this is a major concern. Processes and equipment must be validated and, in order for that to be done, thermocouple based systems require a great deal of detailed attention.
To understand why thermocouples are so troublesome, it helps to know how they work.
Thermocouples are temperature sensors that measure temperature by generating a small voltage signal proportional to the temperature difference between the junctions of two dissimilar metals. One junction (the hot junction) is typically encased in a sensor probe at the point of measurement; the other junction (the cold junction) is connected to the measuring instrument. The measurement instrument measures the voltage signal and the cold junction temperature then computes the temperature using mathematical equations applicable to the type of thermocouple used.
In theory, thermocouple systems work very well. In practice, measurement errors are common and can be difficult to control. Most of these errors are attributable to the very small electrical signal that is generated by the thermocouple device and the dependence that small signal has on the junctions and imperfections in the system circuitry. These factors can result in high levels of system noise and can result in cold junction errors that can invalidate the readings. Other factors that can contribute to thermocouple system errors include:
- the length, thickness, polarity, and quality of the thermocouple wiring and installation
- the number of connections in the system
- the stability of the instrument environment
- the accuracy and responsiveness of the cold junction temperature sensor
- the magnitude of thermal gradients along the length of the wiring runs
- the accuracy of the temperature calibration, and
- the capability of the measurement instrumentation to adapt to all of these factors
In order for a thermocouple-based system to be reliable, all of these sources of error need to be tightly managed. In most applications this will involve a pre-calibration before use, and a post-calibration following use. It also requires careful preparation of thermocouple probes, wiring and connections. Temperature gradients and kinking of wires must be avoided. Cold-junction temperatures need to be kept as stable as possible. Probes need to be attached to the exact instrument channels for which they were calibrated. And so on.
All in all, it can take a lot of work and careful attention to get thermocouple readings you can trust.
It's instructive to note that the possible errors in a thermocouple system are significant enough that a leading supplier of thermocouple based systems recommends field-calibrating their systems before and after each validation application - and further recommends the use of two different calibration standards for each calibration. The complexity doesn't end there, though. The temperature calibration process is, in and of itself, complicated, and, if not managed properly and performed by trained personnel, can be a major additional source of error.
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