Marsh-Bellofram

Pneumatic Relay

Type 75

The Marsh Bellofram Type 75 relay uses signal pressure to accurately control output pressure over a wide range of flow and supply pressure variation. Under varying flow conditions output pressure is maintained by use of an aspirator tube, which adjusts the air supply valve opening in accordance with the flow velocity. A balanced supply valve, utilizing a rolling diaphragm, makes the relay virtually immune to changes in supply pressure. Maintenance is simple due to the unit construction, and the relay can be serviced without removing it from the line. Signal to output pressure ratios of 1:1, 1:2, 1:4 and 1:6 are available. Maximum output is 150 psig (10.3 BAR).

Type 75 - The basic relay offers excellent precision along with high forward flow rates. Type 75 High Relief Relays - These relays provide extra fast "blowdown" for very rapid release of output pressure. The extra relief feature makes this relay suitable for cylinder return stroke actuation, air hoists, and similar applications requiring fast exhaust.

Type 75 Negative Bias - The Type 75 Relay is also available with a 4 + 1 psig (0.3 + 0.07 BAR) negative bias spring mounted internally. This bias spring automatically subtracts 4 + 1 psig (0.3 + 0.07 BAR) from any signal pressure introduced. The relay then multiplies the net signal pressure by its ratio value to obtain final output pressure.
This option is particularly useful in obtaining zero pressure from pneumatic devices such as I/P transducers that normally cannot be adjusted this low, as well as obtaining higher outputs from such devices. Typical applications of the Type 75 Relay with fixed negative bias include the electronic control of the applications listed for the standard Type 75 Relay.