What is in this article?:
Pressure is a key factor in almost every fluid-power circuit. Pressure transducers convert fluid pressure to an electrical signal for monitoring and control of pressure in electronic control systems.
Pressure spikes are microsecond to millisecond bursts of pressure that can reach 15 times the normal system operating pressure. For example, if a valve shifts abruptly to block flow, a shock wave can be generated within the system. Likewise, if a hydraulic system is moving a load and the load suddenly stops, the system may react with a brief surge of pressure.
System control electronics — such as PLCs with millisecond scan times — are not fast enough to detect spikes of such short duration. Often, the first indication that a system is generating pressure spikes is a positive shift in a pressure transducer's zero output. System control electronics commonly indicate the shift in transducer output as a pressure out-of-range condition, which could cause the system controller to shut down.
Pressure transducers are the components most vulnerable to damage from pressure spikes. Transducers, with much quicker response than mechanical gauges, react to spikes and can show signs of having been over-pressurized. This is not because the transducer is less durable than the mechanical gauge it replaced. Actually, a transducer designed for severe service should have been specified. Spikes also damage the machines that generate them. The erratic flow of liquid, common in systems that generate spikes, reduces efficiency and accelerates wear on valve ports and seals.
(Note that pressure spikes do not pose serious problems in pneumatic systems because the air is compressible, which tends to dampen shock. Cyclic pressure surges, caused by pulsation from compressors, pose a greater potential problem because the pressure surges — while not as sharp — occur repeatedly and frequently.)
Pressure spikes usually can be detected with an oscilloscope through a transducer of, say, five times the normal operating pressure range. Once it is determined that spikes exist in a system, any of several practices can be used to prevent them from damaging the transducer. A transducer with a higher pressure rating can be used. However, doing so sacrifices accuracy in the normal operating range because a transducer with a wider operating range has poorer resolution.
As an alternative, a snubber can be used to dampen the spike. A snubber is an orifice installed in piping between the transducer and the source of the spike. A potential disadvantage of this practice is that it slows the response of the measurement. If neither measurement resolution nor response can be compromised, a transducer that can tolerate spikes should be specified. Obviously, these transducers cost more.