Trafag EXP 900/904/912 Ex Pressostat Pressure Switch

Flameproof electromechanical pressure switch for Zone 1 and Zone 21

The Trafag EXP 900/904/912 is an electromechanical pressure switch, or pressostat, built into an explosion-protected housing. It is certified under IECEx, ATEX and UKEX as II 2G Ex db eb IIC T6 Gb for gas atmospheres and II 2D Ex tb IIIC T80 °C Db for dust.

A bellows sensor operates a microswitch with one floating change-over contact (SPDT). The housing is rugged aluminium with an optional stainless steel version, rated IP66, and the switch can be mounted in any position.

Technical specifications of the EXP 900/904/912

The product code 91 microswitch is standard and not adjustable in its differential, which keeps the switch simple and repeatable. Because the housing is an increased-safety terminal compartment on a flameproof switch chamber, it combines two protection concepts, and the certificate conditions must be respected when the switch is installed. Keep the conditions of use from the certificate with the plant documentation, and record the set point after commissioning so that later checks can compare against it.

Variants and standard types

The type number identifies the adjustment style: 900 has a display and adjusting screw, 904 has no display but an adjusting screw, and 912 has a display and an adjusting knob. Sensor materials are 1.4435 with nickel-plated brass sensor housing, in G1/4 female or G1/2 male threads depending on range.

Trafag lists four standard products with extra short lead time. The EXP1.5 covers -0.9 to 1.5 bar with a fixed 0.2 bar differential, the EXP2.5 covers 0.2 to 2.5 bar at 0.2 bar, the EXP6 covers 0 to 6 bar at 0.4 bar and the EXP16 covers 1 to 16 bar at 0.9 bar. Options are a mounting bracket, a stainless steel housing and damping elements.

Selecting and installing the switch

Select the range so the set point sits in the middle of the scale, between about 10 and 90 percent, and check the overpressure rating against pressure peaks. Where peaks or pulsation occur, fit a damping element or snubber from Trafag accessory datasheet H72258.

Terminate cables with a suitably certified Ex e gland and follow the Trafag instructions for the cable entry and the switch rating. Note that the 150 degrees C media limit applies to the process side, while the ambient limit of 65 degrees C applies to the housing.

Documents and quotation

The attached document is the Trafag EXP 900/904/912 Ex pressostat datasheet (H72263t, 09/2024). Certificates and instructions are published separately by Trafag.

For a quotation, send the type (900, 904 or 912), range, set point, thread, housing material and any bracket requirement. See Pressure Instrumentation for related instruments.