Size is an important consideration when selecting a diaphragm seal to protect your expensive pressure measuring instruments. Size in this case does not refer to the external dimensions of the diaphragm seal device, but to the size of the internal flexible diaphragm seal that transfers the pressure from the process media to the fill fluid. (The fill fluid then transfers the pressure to the instrument for reading.)
A Diaphragm Seal Must be Big Enough to Get the Job Done
A diaphragm seal must be big enough to displace a sufficient volume of fill fluid to drive the pressure gauge or transmitter through its full range. This means that pressure instruments measuring relatively high pressure ranges can use small diaphragm seals, but those measuring lower pressure processes need larger diaphragm seals to create enough flow to take the instrument to the top of its range.
Something to keep in mind is the lower the pressure range, the larger the diaphragm seal needs to be. That is a good rule of thumb because lower pressure systems require the greater volume displacement of a larger diaphragm to create sufficient displacement to drive the system.
By the same token, smaller diaphragms (lower volume displacement) are usually sufficient for higher pressure applications.
A corollary to the rule of thumb above is the larger the gauge, the more displacement is needed by the diaphragm to drive the system. By the same token, smaller gauges are generally preferred for low pressure applications.
The general rule of the lower the pressure, the larger the diaphragm required and vice versa applies to diaphragm seals used with pressure gauges or pressure transmitters.
Using Larger Diaphragm Seals can Increase Measurement Accuracy
Another important consideration is that the use of a smaller diaphragm seal on a relatively low-pressure process will lead to less accurate measurement.
For example, in a relatively low-pressure application using a 1.5-inch diaphragm seal on a pressure transmitter used to measure the liquid level of a tank, your error would be as high as 12.1 inH2o. If you move up to a 2-inch diaphragm seal, your error drops all the way to 1.7 inH2O; and if you use an extra-large 3-inch seal, you will have a mere 0.5 inH2O error.
The size of the seal is just one of a number of important factors to consider when choosing a diaphragm seal to protect your instruments. Give the pressure gauge and transmitter experts at WIKA a call and let us walk you through the diaphragm seal selection process. We’ll make sure you’ve got the right instrument and the right seal for the job.
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