Hi,
What should be the maximum allowable capillary length for diaphragm seal to be used with DP transmitter?
How much accuracy and response time will vary if we go beyond allowable capillary length?

 

what distance are you dealing with.

 

I am having 10 mtr distance. We follow 6 mtr max distance for DPT with remote seal.

 

Your corporate limit of 6m of capillary length is somewhat less what is available for commercial remote seal assemblies from major vendors (depending on diaphagm size).

Commercial capillary length limits are likely due to
- a xmtr-capillary-diaphragm combination accuracy limits at reference conditions
- the vacuum effect on the high side (vessel bottom connection port) when the transmitter is elevated above datum, and gravity pulls the fill fluid away from the transmitter's high side diaphragm. The higher the elevation above datum, the longer the capillary, the higher the vacuum effect. A transmitter has some vacuum limit spec that it can handle. This spec varies from transmitter brand to brand.


Here's a couple of maximum capillary length specs from major vendors:





Here's one note that provides for an increased temperature error uncertainty factor for longer capillary lengths



All that said, my experience with major vendors is that their lawyers frequently have a say what can be sold, so configurations are limited to those that do not exceed published accuracy specs. You can always ask, but some major vendors will probably limit the capillary length to their published maximums.

But there are lots of remote seal shops - a shop with a vacuum pump and some valving to evacuate and backfill seals and capillaries to fabricate any custom configuration of transmitter, seal and capillaries, even if the assembly is probably not all-welded connections.

10m capillary length sounds like a reasonable limit with large diaphragms. One European vendor, Badotherm, offers to calculate the TPE, Total Probable Error for any custom configuration. Siemens remote seal catalog pages have comprehensive temperature error tables and a calculation for temperature error at operating conditions (ambient temperature). You could do the calc for 10m capillary at your operating conditions and see whether the error is acceptable (have to add in transmitter error, but the transmitter is typically minimal compared to ambient temp error).

 

Thanks for your response