Design News for Mechanical and Design Engineers

This solid-state pressure switch combines a pressure transducer, digital display, four fully adjustable switches, adjustable deadband, and analog output with optional serial interface in a NEMA 4X housing.

A number of considerations, therefore, are important when selecting an economical pressure switch that gives optimal performance and reliablity:

• It may pay to go with a solid-state pressure switch when monitoring more than one point. For example, if the application depends on high, low, high-high and low-low conditions, you could specify a single unit with four integral switches, rather than four electromechanical units with only single-switch capability or two duplex switches. Use of the single solid-state switch greatly improves reliability and reduces installed costs.

• Even when monitoring a single point, solid-state devices are often less expensive. They can last the life of the product in high-cycle systems, and their higher accuracy can improve process quality and reduce scrap and waste.

• Temperature impacts sensor performance. Solid-state sensors generally have built-in temperature compensation via computer-controlled laser-trimmed thick-film temperature-compensation resistors to ensure accuracy over a wide temperature range.

• High temperatures can also damage sensors. If the fluid temperature exceeds switch rating (usually about 70C), install cooling coils between the sensor and the main system line. Each loop in the coil will reduce the temperature, on average, approximately -12.5C. Mounting the control unit remotely from the sensor will also provide additional thermal protection.

• Transient pressure spikes can irreparably damage a solid-state design. Dampening screws or snubbers reduce the orifice size between the pressure port and the sensor, preventing the spike from reaching and damaging the sensor.

One concern with solid-state switches used in industrial and process settings, however, is electromagnetic interference that can corrupt signal data. For this reason, one should select transducers, transmitters, and solid-state pressure switches that carry a CE Mark or other such designation, indicating that the switch is unaffected by interference. EMI/RFI does not affect electromechanical switches because the circuit is an electrical switch that is either open or closed.

Finally, signals from pressure switches to PLCs and DCSs are usually analog, and this type of data will continue to dominate most commercial and industrial controls for the near future. However, the growing need to frequently change setpoints and generate on-board diagnostics is driving the demand for digital signals. Interest in fieldbus is also on the rise and is a significant application driver. Process industries will likely treat fieldbus similar to the change to DCSs, first applied to new plant construction and then to plant modernization. To speak with a company applications engineer, call +1 323 583 6218, fax +1 323 589 3463 .