Redundant temperature sensing: ‘Goodbye, calibration’

01 October 2006

Sensor drift will cause inaccurate measurements, but ifm has come up with a solution: a self-monitoring sensor that is guaranteed to go for five years without calibration.

In many processes of the food, chemical, and pharmaceutical industries the exact temperature measurement is a prerequisite for a smooth operation and a specified product quality.

Yet there’s only one way to guarantee a temperature probe is working properly—calibrate it regularly. In
Germany, the cost for a three-point calibration as defined by Deutscher Kalibrierdienst (DKD) is about Û280. For a critical process, the calibration may have to be performed four times a year, which amounts to Û5,600 over a 5-year period. Even if it’s only twice a year you’re still paying a lot of money just to keep what you already have.

And, no matter how much money a company spends on calibration, if a sensor starts to drift between calibrations, it is difficult to predict the response, and there is a risk of inaccurate measurements.

Michael Schimanowski, product manager for temperature sensors at ifm electronic thinks his company has a better 5-year plan. ifm has launched its redundant TAD sensor that monitors itself and detects sensor drift almost as soon as it happens. If one sensor goes bad, the other can take over. ‘No calibration necessary—just fit and forget,’ says the ifm advertising brochure. To back up their claim, ifm offers a five year warranty on the sensor.

Why do temperature sensors go bad so quickly? The deterioration of the sensor accuracy is caused by thermal or mechanical stress. Mechanical stress can be frequent pressure fluctuations, whereas thermal stress is mainly caused by large temperature changes in a short time. A harsh process will shorten the life
of the sensor.

Diverse redundancy TAD’s primary sensor is the industry standard platinum resistance measuring element (Pt1000) with a positive temperature coefficient (PTC), which means its resistance becomes greater as the temperature gets higher, almost in a linear fashion.

By contrast the second sensor has a Negative Temperature Coefficient (NTC) and is highly non-linear. ifm chose the two completely different types of sensors, with different response curves, because it would be unlikely they would both go bad at the same time. Mr. Schimanowski calls this ‘diverse redundancy.

‘Diverse because the measuring elements used have different characteristics, and redundant because two measuring elements are integrated into the probe. So in normal operation the temperature sensor works with two different measuring elements. As a result of this, the process can be finished safely with the second measuring element if one element fails.’

The electronics of the temperature sensor takes the average value of the measured temperatures and provides an analogue output (4...20 mA) proportional to the determined temperature. Drift thresholds and
parameters of the self-monitoring routines are set up with FDT configuration software.

With self-monitoring, a potential drift is instantaneously detected, allowing an early reaction. Even during this period when it is known the sensor is drifting, it provides a reliable signal. An increasing drift continues to be monitored and is reliably signalled when exceeding the drift alarm limit.

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