Tips for a Perfect Turndown of Process Instruments

Related Vendors

DÜPERTHAL Sicherheitstechnik GmbH & Co. KG

Donadon SDD Srl

Anton Paar GmbH

WIKA Alexander Wiegand SE & Co. KG

Why to Take Measuring Accuracy into Account

One aspect which must not be forgotten with a turndown deliberation is the measuring accuracy of the process transmitter — this reduces proportionally with turndown. Let us assume that the tank operator decides for a high-quality transmitter with an accuracy of 0.1 % of the measuring span. Then, with his turndown of 3:1, he would already have a maximum permissible error of 0.3 % with respect to the set scaling. Converted into the level height, this gives an inaccuracy of 3,000 mm x 0.33 % = ±10 mm.

In the context of the level measurement, this would give a measuring result that would be completely acceptable and normal. With some process transmitters a turndown of up to 5:1, with respect to the measuring accuracy, is stated as neutral. For this, however, in the case of the tank example, one would need to go for a measuring cell that was better by a factor of 5, in order not to exceed the permissible error of 0.1 % of the entire measuring range, i.e. for a sensor with an accuracy of 0.02 %.

That a scaling in no way improves the measuring characteristics of a transmitter is particularly true when one translates the extreme turndown of 300:1 back to the tank example. Here, one would have to tolerate an error of 33 % of the set measuring range, which is to say an error of 11 mm in a measurable filling height of 33 mm. In practice, one can no longer speak of measurement in a case like this.

The quite widely-held belief that the larger the turndown in the data sheet, the better the performance of the instrument, is thus not automatically confirmed. What a number or ratio promises, may be possible to implement technically without problems. But whether it is actually suitable for the task in the process needs closer inspection. The selection decision for process instrumentation is complex, given the other basic factors that must be considered — environmental conditions such as pressure, temperature and humidity, the process conditions of temperature and vibration and the duration of application with possible wear.

In comparison with these, the importance of turndown in the purchasing decision is put into perspective. In the end, only one thing must be ensured: to get exact measuring results — after an exact setting of all process parameters — which do not exceed an acceptable level of measurement uncertainties.

* The author is Manager Process Transmitters, Wika Alexander Wiegand SE & Co. KG, Klingenberg/Germany.

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