Hardly a week passes that we don’t receive a call from a plumbing or electrical contractor, facilities engineer or design engineer wherein they’ve confused transducers and pressure switches. True, both may be classified as sensors but they are vastly different in their functionality and the information and safety they provide. So, what is the difference?
Pressure switches are definitely the more simplistic and ‘old school’ technology of the two. As the name implies, it is simply a switch (making contact or breaking contact) which is moved into one of two positions (opened or closed) by the pressure of the gas to which it is connected. It is not highly accurate (specifications range from ± 5% of full scale to ± 10% of full scale, nor is the pressure setting highly repeatable. For example, a commonly used switch in our industry has a working pressure range of 0 - 3,200 psig with a 10% of full scale accuracy/repeatability tolerance factor. Once set to a specific pressure (for example 1,200 psig), that means that the actual alarm condition may be triggered anywhere between 1,520 psig and 880 psig (10% of 3,200 = 320 1,200 + 320 = 1,520 1,200 - 320 = 880) and the alarm may occur at a different pressure within this range every time. Another drawback of pressure switches is that the only way to know if the switch is working properly is to test it. This requires that the pressure switch must be physically disconnected from the pipeline to allow the pressure to fall below the low pressure alarm set point and, if it’s a high/low switch, connected to a pump which raises the pressure above the high pressure set point. This type of testing must be done manually and is typically only done during a periodic (in most cases annual) inspection.
Transducers are the more sophisticated, ‘new school’ technology and are the choice of an ever increasing number of manufactures, replacing pressure switches. How do they work? Like the pressure switch, the transducer is connected to the medical gas pipeline. The pressure of the gas is dead-ended against a metallic (usually stainless steel) diaphragm which deflects or bends in accordance with the rise and fall of the gas pressure. The amount of deflection is measured by the transducer and this information is transmitted in milli-volts to a circuit board. It should be noted that all transducers must work in conjunction with a circuit board. Most manufacturers build the transducers onto their associated circuit board, so the distance that the information is transmitted in milli-volt format is minimized to a fraction of an inch. The information is then digitized by the circuit board and communicated to a remote logic board (such as an area alarm or manifold) these are the logic boards that contain the digital pressure and LED displays that are commonly viewed by hospital personnel. Transducers are highly accurate (within 3% of full scale) and highly repeatable (within 1 psig). Obviously these factors alone provide added safety vs pressure switches. In addition, since transducers operate within a normal milli-volt output range, it is easy for manufacturers to build in additional safety features to their associated circuit and logic boards which can immediately trigger or display alarm conditions or alert warnings when a transducer is not operating within its normal milli-volt output range. In essence, the transducer is tested multiple times every second – not just periodically or once a year.
In the rapidly and continuously changing world of electronics the costs of transducers have become much more affordable. When you compare transducers and pressure switches, transducers inherently provide features and benefits not available in pressure switches: