Process Instrumentation Tips for a Perfect Turndown of Process Instruments

Author / Editor: Bernd Reichert / Dr. Jörg Kempf

It’s a proven fact, that a larger turndown of a process instrument isn’t to be compared to a better performance in any case. This article specifies all that matters concerning the interpretation of the data sheet and the definitions.

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Pressure-current diagram with possible output characteristics
Pressure-current diagram with possible output characteristics
(Picture: Wika)

Turndown — this feature in a process instrument represents one difference, and maybe even the most important one, between a pressure transducer and a process transmitter. In pressure measurement, this is often the decisive argument why users will prefer a process transmitter for particular measuring tasks: It gives them the possibility to set the measuring span or scaling individually. The instrument is programmed in such a way that it only monitors the measuring range that is relevant for the process.

In most instances, the manufacturer supplies a sensor with the start of the measuring range at 0 bar relative or absolute. However, those values within the lower pressure range are sometimes not the focus of the plant monitoring. For example, an operator might only be interested in the upper 20 % of a measuring span, since a safety shutdown might need to be triggered at higher pressures, while lower pressures never occur in the course of the process. Being able to set the sensor so that this upper range is covered by the full span of the 4...20 mA signal is only available with process transmitters and not with pressure transmitters.

Turndown To Simplify Logistics

The turndown capability also brings a logistical advantage: Service departments require only one product of the same model in a standard design, which can then be matched to the respective process pressure using the turndown. In addition, faults can be resolved immediately with instruments from stock: It only needs minimal setting in the laboratory or workshop in order to make a replacement available in the event of a sensor failure.

In view of these features, turndown can be considered as a promising sales argument. Under increasing competitive pressure, manufacturers of process instruments are quite obviously positioning themselves behind the sports motto of “higher, faster, further” in order to get the maximum possible attention from potential customers. Especially high figures for turndown are specified in data sheets, so that the particular instrument will stand out with the direct comparison of specifications.

This strategy often works out. In general, it is not the buyers who specify the measurement technology for a plant. They order against the specifications which the planning engineers have established for the process instruments to be used. When determining who should put together which package for the process instrumentation, in recent times the specification of the turndown has been evolving as the factor to tip the scales. For example, one manufacturer states a figure of 20:1 in a bid, a competing company might however push 300:1, so the buyer will think about it and eventually — based on the numbers — go for the supposedly higher-performance instrument. The actual needs or application requirements can thus be easily lost out of sight.

When Does a Turndown Makes Sense. The answer you will find on the next page.