The FCX-OHS/OHSLIGHT transmitter: Measuring humidity at high temperatures

Measurement of the humidity at temperatures > 100°C is important in a large number of industrial applications ranging from drying process chambers to large baking ovens. The high process temperature is critical for the selection of the humidity sensor. Not too many sensors can measure sufficiently correct at these high temperatures. Pewatron has developed the high-accuracy (+/-2 %Vol H2O ) and reliable, long-life FCX-OHS/OHSLIGHT transmitter for use in various industrial applications and for operating temperatures in a wide range of temperatures from 100°C to 300°C. The FCX-OHS stand-alone transmitter is widely used to control humidity in all zones of large industrial baking ovens, and has thus become an important process optimization tool for maximizing the output and quality of bakery products. The FCX-OHSLIGHT transmitter is being used for high-throughput smaller ovens, for instance rotating rack ovens, where often an integrated PLC is used to control the signal output from the transmitter.

The measurement principle of the sensing element in both the FCX-OHS and the FCX-OHSLIGHT transmitter is based on the oxygen displacement compared to dry air with a content of 20.95 % oxygen. With the increase of humidity in the atmosphere, the oxygen concentration will reduce to 0% in case of complete humidity saturation and for high temperatures > 100°C. The transmitter generates an output signal of 4 to 20 mA (FCX-OHS) or 20 to 4 mA (FCX-OHSLIGHT) corresponding to 0 to 100 %Vol H2O and temperatures above 100°C.

The stainless steel probe (M22 x 1.5) with the humidity sensor integrated into the tube, is ideal for wall mounting by using a removable flange, but can also be mounted in other positions on smaller ovens (for instance top-down). The length of the probe can be chosen between 30 cm and 2500 cm. The sensor and the construction materials used for the sensor probe (metal parts, wires and connectors) are temperature resistant above 300°C and also resistant to high pressures of up to 20 bar. The probes are connected to the transmitter electronics using a 4-pole high-temperature cable. The sensor element can be equipped with a dust and particle protection filter.

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