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.
Electromagnetic Interference (EMI) can affect transducer performance. High field strengths tend to affect transducer outputs. In some cases, these fields can completely saturate internal amplifiers to the point where erroneous outputs are produced regardless of the pressure input.
There are shielding and grounding techniques that are remedies for the effects of EMI. Also, wires should be carefully routed from the transducer to its receiving device so as to avoid EMI areas. Solutions are specific to each problem presented.
Only a few transducer manufacturers specify EMI protection. This is stated as: percentage of full scale error divided by a frequency range up to a maximum field strength. Example: Full scale output error typically is less than 1% over the frequency range of 20 kHz to 2 GHz at field strengths up to 100V/m. While these specifications are difficult to relate to the real world, the fact that manufacturers supply EMI data in their specifications indicates they have had some experience dealing with this interference.
There are many design features that may not appear in published specifications that can be important over the life of the application.
- is the transducer sensitive to mounting orientation (attitude)? Will pressure readings be consistent if the attitude changes when the equipment moves?
- are special wrenches or tools needed to install the transducer?
- will external design and materials of construction stand up to physical abuse? In mobile equipment, a protruding transducer can make a convenient step for maintenance personnel climbing on equipment. Moreover, falling debris is another potential source of physical damage. When possible, transducers should be accessible, but in areas not subjected to potentially damaging conditions, and
- can electrical connections be made in a reliable and fool-proof fashion? Can leads come to the transducer from any direction? Is reverse polarity protection provided?
It should now be evident that no one transducer is better than all others. An ideal transducer for one application could be unsatisfactory for another. With the wide variety of transducer products to choose from, knowing what features to look for and how to interpret specifications for a particular application will help you choose transducers with confidence.
The checklist above is designed to help users organize application information that has an effect on transducers. However, even when the checklist is completed, it still may be difficult to select a specific transducer for a specific application. For example, one transducer may cost five or ten times more than another, but offer comparable performance characteristics. This cost difference can usually be attributed to additional capabilities built into the more expensive transducer. Immunity to electrical noise or the ability to sustain pressure spikes are two characteristics that add cost to a transducer without improving basic performance parameters. However, specifying a transducer without these characteristics, in an application clearly needing them, ultimately will result in an unsuccessful application.
Furthermore, the more expensive transducer may actually cost less when considering economics of an entire system. This is because the less expensive transducer may require additional components that make it function in an otherwise unacceptable environment.
In general, capabilities that add cost to a transducer, so it can perform under less than ideal conditions, can be divided into four categories:
- special performance capabilities that make a standard transducer compatible with a special application
- the environment, both the fluid and of that surrounding the exterior of the transducer
- electrical requirements, both of the input and output signals, and
- physical and mechanical requirements, regarding size strength, etc.
In general, as the number of these capabilities increases, so does cost of the transducer. Some characteristics substantially add to a transducer's cost while others do not. This means it is important to evaluate each application to decide what characteristics are absolutely essential to an application and which are desirable, yet cost effective.
Types of pressure measurement
Selecting a pressure transducer goes beyond choosing one with acceptable performance. It must be configured to measure any of four common forms of pressure.
Gauge pressure (psig) (barg) quantifies fluid pressure relative to ambient air pressure. In the case of a diaphragm-type transducer, Figure (a), the fluid side of the diaphragm sees the measured pressure; the other side sees ambient air pressure. Because a transducer measuring gage pressure is vented to the atmosphere, it could be exposed to atmospheric contamination and condensation unless precautions are taken.
Absolute pressure (psia) (bara) measures pressure relative to a vacuum. In the case of a diaphragm-type transducer, Figure (b), one side of the diaphragm sees fluid pressure, the other sees a full vacuum.
Sealed reference pressure (psis) (bars) is measured relative to a reference pressure whose magnitude is at, or close to, standard atmospheric pressure. In the case of a diaphragm-type transducer, Figure (c), one side of the diaphragm is exposed to the fluid pressure while the other side is exposed to to a chamber sealed from the atmosphere and containing pressurized gas at standard atmospheric pressure. The pressure transducers recommended for hydraulic service are sealed gage to ensure that the units sensitive internal components remain moisture and dirt free.
Differential pressure (psid) (bard) quantifies the pressure difference between two points within a system. The measurement also must consider the magnitude of the system's line pressure. Measurements usually are taken from two different fluid inputs within the system using a transducer designed specifically for differential-pressure calculations, Figure (d), or by installing a separate transducer at each of the two fluid inputs. (Output from each transducer is routed to a common signal processor that produces a signal proportional to line pressure as well as the difference between the two pressures.)
Line pressure is important not only for monitoring system operation, but also for making differential pressure measurements more meaningful. For example, an 8 psi (0.544 bar) pressure drop across a filter may be acceptable for a system operating at 120 psi (8.16 bar), but unacceptable for one operating at 80 psi (5.44 bar).
Standard atmospheric air pressure (zero gauge pressure) is 14.7 psia (1 bar). Actual atmospheric pressure normally ranges from 14.2 to 15.2 psia (0.966-1.03 bar). Recognizing the small variations from nominal, the user realizes that in the range of hydraulic pressures, the possible difference between a vented gage and sealed gage transducers output would be extremely small. In the order of less than 0.5 psig (0.034 bar) in 2000 psig (136 bar), there is an error of only 0.025%. Because most pneumatic systems operate at pressures much lower than hydraulic, the difference between vented gage and sealed gage measurements may be more significant.