Sensors - February 2001 - DCDD Pressure Switch

A unique pressure switch based on a double-contact, double-diaphragm configuration virtually eliminates false actuations due to shock or vibration.

Photo 1. The PSF100A pressure switch incorporates a double-diaphragm/double-contact configuration to protect it against false actuation due to shock and vibration.

Keeping costs low without compromising the design of a pressure switch is often a difficult task. Conventional inexpensive pressure switches are typically based on mechanical designs that are inherently susceptible to false actuations from shock and vibration. The more expensive alternatives are pressure transducers that use complex circuitry to convert a change in capacitance or inductance to a voltage output. A new solution, the PSF100A double-contact, double-diaphragm (DCDD) pressure switch (see Photo 1), can survive shocks up to 50 g or vibrations to 10 g at frequencies from 50 to 2000 Hz without giving spurious readings.

Sensor Design
The patented design of the switch is quite simple (see Figure 1).

Figure 1. In a normal state, with no pressure applied, both contacts of the double-make, double-break circuit configuration are open (A). When pressure is applied, both contacts are closed, completing the circuit (B). With no pressure applied, when the sensor is subjected to shock or vibration the contact arms move together in parallel. Since at least one contact is always open, the circuit cannot close in error.

Two parallel contacts are riveted to opposite sides of a center housing. In the assembly process, one of two diaphragms is placed over each contact. Covers are put in place next, and the unit is eyeleted together. To create a single air chamber out of the two sides of the device, the housing is ultrasonically welded together in a critical location. The resulting airway routes air from the pressure port to separate chambers located adjacent to each diaphragm on the opposite side of the contacts.

As far as the user is concerned, the device operates no differently from any other pressure sensor. In the switch's normal open state with no pressure applied, both of the contacts remain open. When differential pressure is introduced, the contacts close to complete the circuit. When the device is struck or shaken without pressure applied, however, its contact arms move together in parallel. Because at least one contact is always open, the circuit does not close in error.

The PSF100A pressure switch incorporates spring-tempered phosphorous bronze contacts with a thin layer of WE#1 electronic grade gold inlay. The same gold inlay is used at the contact point of the brass terminals that extend outside of the switch housing. The standard housing is injection-molded polycarbonate and the two diaphragms are made of polyurethane. If these materials are not chemically compatible for a particular application, the switches can be produced in several combinations using alternative materials such as polyetherimide or nylon for the housing and Teflon for the diaphragms.

This switch design is suited for interfacing with PCs, PLCs, or any logic circuit, and the gold inlay contacts function well up to 1.2 W. The switches carry a UL rating of 30 VDC at 40 mA max. If kept within these electrical ratings and under the proof pressure specifications, they will last more than 20 million cycles. The proof pressure is defined as the maximum amount of momentary differential pressure that the switch can withstand without suffering damage to the moving contact arms or a shift in the set point.

The standard rating is 8 psi for switches with a set point of

3 in.H₂O, and 15 psi for set points >3 in.H₂O. These overpressure tolerance levels exceed those specified for low-pressure transducers, which are typically more resistant than conventional switches to shock and vibration.

The PSF100A is available with set points ranging from 0.5-50 in.H₂O, and have a standard operating temperature of 40°F-150°F. The standard set-point tolerance of most of the switches is set at the factory as ±20% of the set point, with the exception of the 0.5 in.H₂O model. The PSF100A-0.5 is commonly used to detect the presence or absence of differential pressure. By definition, it has a factory set point between 0.2 in.H₂O and 0.5 in.H₂O, which loosely converts from 0.01 to 0.02 psi. Presence of a positive pressure is necessary in many applications from clean rooms to chicken hatcheries, and from pig farms to the ignition control units of combustion furnaces.

Applications
DCDD pressure switches are used on airplanes as indicators that advise of a malfunction in one of the cooling fans that protect the instrumentation. On military aircraft, helicopters, and missiles they provide confirmation of either remaining under or exceeding some air speed threshold. They are incorporated in the arc fault detection system on the Trident submarines, and in the sip and puff controllers for wheelchairs.

DCDD pressure switches are well suited to harsh environments because they require less cleaning and adjusting than conventional devices. One example is a progression die that monitors the buildup of slugs. If the slugs block the path of a steady stream of air, the switch contacts will open and the die will shut down before it is damaged. Other factory applications use DCDD pressure switches to sense an empty bag in a vacuum-operated liquid dispenser, initiate a timer in an injection-molding machine after a given pressure has been reached, and, in conjunction with a bubbler system, detect an empty ink bottle on an industrial inkjet parts marker.