By Markus Affolter President Wilbrecht LEDCO, Inc.
Process control applications in the pharmaceutical manufacturing industry rely in large part on the well-known physical variables of temperature and pressure. For these critical applications-where the manufactured products directly affect the health and wellness of patients as well as the reputation and stability of pharmaceutical and medical companies-accuracy and reliability are key. As an important component of the process control circuit, pressure switches can be mechanically set to switch an electrical circuit and activate a process when a specified temperature (converted into a pressure measurement) or pressure level is achieved.
While a broad range of sensors are available for temperature and pressure control in process automation applications, a simple solution, like a pressure switch activating power directly without the use of an interconnected controller or electronics circuit, is often the best-and in many cases, the most cost-effective. In addition to design simplicity and ease of use, these switches offer superior resistance to electromagnetic disturbances and extreme temperatures common to pharmaceutical processing applications. However, the mechanical design does make them subject to certain limiting factors, including dead zone passing and switch differential movement, which may negatively affect the pressure switch's speed and precision capacities.
The Mechanics of Electromechanical Pressure Switches Electromechanical switches are activated through deflection of a flexible membrane that separates regions of different pressure, with the deformation of the membrane (D) dependant on the difference in pressure between the two faces (Fig. 1). To measure the differential pressure, the two sides are open to the two pressure sources. The reference face can be open to the atmosphere to measure gauge pressure, and when the other side is open to a sealed volume with fixed reference pressure, absolute pressure is measured.
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When the pressure in the chamber increases, the deforming membrane-which is located in close proximity to the switch-expands outward and pushes the pin plunger of the switch. Once the actuating point is reached, the switch commutes from its ON1 position to its ON2 position and causes the specified process to initiate.
The most common type of electro-mechanical switches used is a snap-action switch, in which an internal spring-loaded pole initiates the changeover movement. An actuating force is applied on the plunger and is transmitted to act on the pole. If the actuating force is higher than the internal force supplied by the spring, activation occurs. Ideally, the activation caused by the switch's changeover from the ON1 position to the ON2 position would be instantaneous; however, the limiting factors of mechanical switches affect the final pressure switch accuracy, causing a delay in the changeover movement.
Dead Zone Passing In most pharmaceutical process applications, the pressure (which may be caused by increasing temperature) build-up occurs gradually and because of this, the switch moves slowly from its initial position (Pr), to its actuating position (Pa), to the secured position (Pa2') where the circuit is actually closed, and on to its final position (Pfc) (Fig. 2). The same is true of its return trip-the switch moves slowly back towards the initial position. During the time that elapses while the switch commutes the distance between Pa circuit 1 opening to the point where the actuating force builds up enough to close circuit 2 (Pa2'), the switch is in a transition zone, or dead zone. Depending on the mechanical construction of the snap-action pressure switch, this dead zone may represent a critical pressure range in which no secure switching can be guaranteed and both electrical circuits remain open.
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While the distance the pin must travel from the actuating position (Pa) to a secured position (Pa2') often exceeds 10um for many switches-including V4 switches, which are common due to their compact footprint and competitive price range-some new switches are specifically designed to reduce dead zone passing by up to 10 times. For these switches, manufacturing operations have been modified to allow dead zone passing of just 1um, significantly increasing the accuracy and reliability of the switch and providing an ideal solution for many high-precision pharmaceutical applications.
Switch Differential Movement (Hysteresis) Along with the dead zone passing issue described above, mechanical switches are subject to another limiting factor at this point. As the pressure decreases, the switch should open/close the circuits at the exact same position as they were originally opened/closed as the pressure/temperature increased. However, mechanical switches do not move the same way under increasing pressure as they do under decreasing pressure, due to mechanical resistance in the individual parts of the switch. The switch does not commute back at the actuating position (Pa) but at a later position. This distance between the actuating point and the release point is referred to as differential movement, also known as hysteresis, and is another limiting factor of mechanical switches.
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For applications that rely on accurate detection of small pressure changes for optimal switching accuracy, a switch designed to minimize differential movement is a necessity. Although typical, inexpensive and small V4 switches feature a large hysteresis of 0.15mm or even higher, switches that reach consistent differential movements of less than 0.05mm are now available-offering a three times improvement from the industry norm.
Pressure and temperature switches based on mechanical switches provide a simple and cost-effective solution for processes that rely on temperature and pressure, but the used electrical switches vary greatly in their design and limiting factors and should be carefully selected for precision applications, as their limitations directly influence the limitations and specifications of the final pressure switch and therefore the quality of the whole pharmaceutical production process.
Wilbrecht LEDCO, Inc., manufactures custom microswitches, sealed position switches, precision LED assemblies and metal foil resistors. For more information, contact Wilbrecht LEDCO, Inc., St. Paul, MN at 651-659-0919 or toll-free at 888-323-8751. Email: firstname.lastname@example.org. Web: http://www.wilbrechtledco.com and http://www.www.microprecision.us. Wilbrecht LEDCO, Inc. is a division of Microprecision Electronics.