All ZOC16, ZOC17, and DSA modules have four way valves for each sensor. Application of a control pressure causes a piston in each valve to compress an o-ring, forming an air tight seal. All of these valves are capable of, and have been tested to, a minimum of one million operations.

All ZOC 22, 23 and 33 modules also have four way valves for each sensor. These valves use inflatable bladders instead of pistons. Although the method is somewhat different, the bladders are able to form air tight seals.

Control pressures are used to switch the valves in a module to one of the standard operating modes: Zero, Operate, Calibrate, and Isolate. Electronic signals are not required to switch these valves.

Each module requires two or three control pressures to operate the valves, depending on the valve logic. ZOC16, ZOC17 and all DSA modules require three pressures. Two of the pressures control the normal valve modes. The third pressure controls a separate purge valve.

ZOC22, ZOC23, and ZOC33 modules also require two or three control pressures. The standard versions of these modules require two control pressures. The Duplex versions of these modules require a third control pressure to operate the Duplex valve.

The different modes of operation are obtained by applying the control pressures in a logical sequence. Truth Tables may be found in the appropriate module hardware manual that show the logic required for proper operation.

All DSA series, ZOC16, and ZOC17 modules are" normally open" valves. That is, no control pressure is required to put the module in the Operate mode. Control pressures are required to switch the valves to the other modes of operation. The control pressure used for these modules must be 85 to 105 psi depending on the pressure range of the module.

All ZOC22, ZOC23, and ZOC33 modules require a control pressure for each mode of operation. The control pressure for these modules must be 60 to 65 psi.

Is it possible to minimize the number of control pressures needed?In some cases, yes. For DSA modules, a single control pressure may be used for "Zero Calibrations" Also, CTL 1 and CTL 2 may be applied from the same source to switch the module into the Calibration mode. ZOC22, ZOC23, and ZOC33 modules must have all control pressures available.