How do I handle high temperature media?

Pressure transducers are being used by many engineering firms and contractors for steam applications. Setra may specify a compensated range of -4º to 176ºF, but the temperature of the steam can be much higher. With the use temperature standoffs, the steam temperature will be greatly reduced by the time it reaches the transducer. 

Be aware that the media temperature is not the ambient temperature used in our specs. See Glossary of Pressure Terms for the definition of ambient conditions.

When selecting a pressure transducer for monitoring high temperature liquids or gases, be aware of the operating temperature range of the transducer. This specification can be found under environmental data on most manufacturers’ data sheets. The temperature compensated range specification of a transducer is the error introduced as temperature increases or decreases from 70°F. This thermal error affects both the span and zero setting of the transducer’s analog output and is generally listed as a percent of full scale per degree (% FS/°F). The static accuracy, the accuracy at 70°F, of the pressure transducer plus the thermal error introduced by a high temperature pressure source is the total possible error for a specific high temperature pressure measurement.

The expense of a special high temperature pressure transducer can be avoided by using a temperature stand-off (short length of tubing) to isolate the transducer from the high temperature pressure media. Table 1 shows various lengths and diameters of tubing for 303 stainless steel (SS) and brass that will limit the media temperature at the transducer to 0 to 200°F.  Table 1 assumes the ambient temperature around the transducer is 100°F, and the major source of thermal input is through the connecting tubing.

Ideally, the steam line tap should be on the bottom of the steam line since heat rises and steam will not replenish the line. Some examples of pigtails (also referred to as siphons) are below.

Steam will exit the pipe and fill the line to the transducer. Due to the length of tubing in an atmosphere of 100ºF, the steam will condense and create a water trap. The water trap creates isolation of the steam temp and the air pressurizing the transducer.