Good morning all, I have recently join control automation.
I have quick question regarding Wika pressure gauge and diaphragm seal if any of you throw some light on from your experience. that will highly appreciated.

A project where we did procured wika 232.34 PG along with L990.26 Diaphragm Seal, Silicone fluid. Ambient condition is -40F to +40F. L990.26 is 3/4" 300# Process connection, 1/2" Instrument connection. but once we got Diaphragm Seal Calculation Program confirmation which says this assembly will fail under -40F ambient conditions. I do not understand, why it would fail under an ambient condition, I could not get detail information from Wika catalog about it. It could be due to small volume displacement with smaller size diaphragm diameter could cause a issue during cold ambient condition but I could not confirm it from any manual or so. Can you please share your knowledge and experience about this condition? thank you.

 

I suspect that the term 'fail' might well mean that the gauge will not meet its accuracy spec, for one reason or another, as opposed to failing to hold pressure. It could be that some 'unseen' component, like the grease used on the gauge internals is only rated to -40F. Grease gets to viscous at low temperature and inhibits movement of the movement.

You might consider contacting Wika directly to see if they can discover what triggers the limit at -40F.

 

Thank you David for your time for reply, I think its important to know how much volume displacement required for the to bourbon tube to sense the pressure so it starts deflating, I think for smaller ranges pressure gauges larger diameter diaphragm seal is the important point for the model selection, which allow more displacement. but unfortunately you can not get this information from product manual and it is also depends on how knowledgeable and technically informed vendor is about this kind of things.

 

You are correct on several counts - the importance of fluid displacement and the need for understanding the importance when matching a diaphragm seal to a pressure element.

The spec sheet for the seal lists its displacement. A distributor can probably find the pressure gauge displacement value if you press them. But I'm sure the reason Wika went to the trouble of creating a PC program for qualifying remote seals is to avoid human errors when matching up the components.

Here's Wika's explanation about their PC program from their WIKA Handbook Pressure & Temperature Measurement, U.S. Edition:

"1.6.5 Software-aided selection of diaphragm seals

The quality of a chemical seal measuring system (diaphragm seal, capillary tube and measuring instrument) greatly depends on a combination of various parameters. These include displacement
volume, control volume, temperature and pressure related changes of volume of the system fill fluid, dimensional data of the measuring system, and the characteristics of the liquid under varying
operating conditions.

In addition to these complex parameters and their dependence on one another, three fundamental questions must always be answered:
- is the displacement volume of the diaphragm seal adequate?

- what is the measurement error created by temperature and pressure changes?

- what is the response time?

These questions could be answered by "trial and error" using a simple set of the established parameters. Intuition based on experience may even occasionally produce useful answers, but this approach cannot be considered reliable. The optimum configuration for a particular application can only be determined by systematic variation of all the influencing parameters. The iterative solution of the equations for these parameters is an ideal task for a software. WIKA’s solution was to develop its own software program for the optimum configuration of diaphragm seal systems. The data from approximately 40 system fill fluids, more than 50 diaphragm seal models and all the most popular pressure gauges, pressure transducers and transmitters are saved in this program.

The values calculated by the software are the most reliable when accurate operating condition data is input into the system. So, a questionnaire was created for collecting the application-specific data, such as measuring range, ambient temperatures and maximum acceptable response time. This input data, which differs from application to application, is supplemented by further parameters. The WIKA software also considers parameters as the density, viscosity, thermal coefficient of expansion, compressibility factor and vapor pressure values of system fill fluids, plus the characteristic curves of diaphragm seals and diaphragm shape, diameter, height differentials, etc.

The large number of parameters and their various effects creates a large variety of diaphragm seal system configurations. Therefore, no attempt has been made here to illustrate these effects graphically."

So there's a lot that goes into the program. When the program establishes a temperature limit, there's a lot of factors taken into account.

Given that Wika puts their German engineering behind their products, I'd tend to trust
- that they've taken the displacement into account
- their assessment of the temperature limit.

 

Thank you for your detailed reply, Based on my understanding, I completely agree with you on this. as spec designer( technologist, engineers etc) we have limited resources to confirm if selected diaphragm seal has adequate volume displacement for selected pressure range? You are right on some extent as some of diaphragm seal spec sheet has volume displacement number (not all spec sheet has volume displacement) but how could one be so sure that given volume displacement will be adequate for selected pressure gauge ? This technical information must be available to end user for model selection purpose otherwise end user will mostly dependent on inside technical sales people to provide these type of very much important information. Thank you for time.