Tips for a Perfect Turndown of Process Instruments

Related Vendors

DÜPERTHAL Sicherheitstechnik GmbH & Co. KG

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When Does a Turndown Makes Sense

However, with turndown, does “more” necessarily mean a “better” product? In order to answer this question, one must bear in mind the objective of any scaling — to utilise the measurement signal in the optimal way, from the start to the end of the measuring range.

As an example, we can look at the monitoring of a tank. The operator wants to ensure that half of the tank contents is always available as a reserve, but on safety grounds the level must always be at least 20 % below the maximum. Thus, the required measuring range is between 50 % and 80 % of the maximum filling height. For this application, the operator will select a measuring instrument that can monitor the complete filling level — for example, ten metres or approx. 1,000 mbar. So the appropriate process transmitter for this can cover a pressure range of 0 to 1,000 mbar.

However, only a filling level of 50 % to 80 % is relevant, so the scaling would be set to correspond to 500 mbar to 800 mbar, or one can say to a measuring span of 300 mbar. The turndown required for this is calculated as the ratio of the basic measuring range (1,000 mbar) to the scaled measuring range (300 mbar) — so here 1,000:300 or 3.3:1. So the output signal will be measurable as 4 mA at 500 mbar pressure and 20 mA at 800 mbar pressure. These are the physical limits with an analogue output or with a 4...20 mA signal overlaid with a Hart bus signal (see diagram next page).

The example chosen here is actually based on practical experience and should demonstrate why turndown is useful in many cases. Compared to the possible basic measuring range of the instrument, the scaled range, at about one third, is relatively small. It would almost be minimal if one had taken a turndown factor of 300:1. Translated into the tank example, with a possible pressure profile of 1000 mbar, at 300:1 the operator would thus be wanting to measure just about 3.3 mbar. In other words, a maximum measuring height of ten metres would shrink to a total of 33 mm.

One aspect which must not be forgotten with a turndown deliberation is the measuring accuracy of the process transmitter. More on the next page.

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