| Plain Bourdon gauge | Diaphragm seal assembly | |
|---|---|---|
| Wetted part | Bourdon tube bore | Flat or flush diaphragm |
| Blocks with slurry | Yes | No |
| Exotic material possible | Limited | Yes — Hastelloy, tantalum, PTFE lined |
| Process temperature | Up to about 200 °C | Up to 400 °C with capillary |
| Added error | — | Small temperature and fill error |
| Cleanable in place | No | Yes on flush designs |
| Relative cost | 1x | 2x to 4x |
| Repairable on site | Gauge only | Seal must be refilled at works |
If it can crystallise, polymerise, settle, freeze or set — the answer is a seal. A blocked Bourdon tube does not read low, it reads whatever it was when it blocked, which is far more dangerous.
SS316L covers most services. Hydrochloric acid needs Hastelloy or tantalum, hydrofluoric needs Monel, and some services need a PTFE lining. Getting this wrong is the usual reason a seal fails in weeks.
A direct seal bolts the gauge onto the seal. A remote seal connects them with a filled capillary — use it when the process is hot, when the gauge must be readable somewhere else, or when vibration would shake the gauge apart.
Silicone is the default. Food and pharmaceutical service needs a food grade fill. High temperature needs a high temperature fill. Oxygen service needs an inert fill — the wrong one there is a safety issue, not a performance one.
A seal adds a temperature-dependent error, larger with a long capillary and a small diaphragm. On a 0-10 bar range it is negligible; on a 0-250 mbar range it is not. For very low ranges use the largest diaphragm you can fit.
A remote seal assembly is filled with oil, and that oil expands and contracts with the temperature around the capillary — not just the process. A capillary run across a sunlit terrace in a Faridabad May behaves differently from one in an air-conditioned rack. Where the ambient swings widely, keep the capillary short, shade it, or use a fill fluid chosen for the temperature range.
Clean water, air, oil, steam through a siphon, or ordinary process gas — with a snubber if the pressure pulses.
Slurry, crystallising media, corrosives, hot process, sanitary lines, or anywhere a blocked tapping would be a safety problem.
A pressure transmitter with remote seal, if the reading has to reach a control system as well as an operator.
Before committing to a seal, ask whether a simple accessory solves it. A siphon handles steam. A snubber handles pulsation. A chemical seal pot handles a mildly aggressive liquid. Each costs a fraction of a diaphragm seal assembly, and on the right duty they do the same job.
It adds a small error that varies with ambient temperature, capillary length and diaphragm size. On normal ranges it is well within the gauge class; on very low ranges choose a large diaphragm and a short capillary.
No. The assembly is filled and sealed under vacuum at the works. A damaged diaphragm means a replacement or a factory refill.
A snubber is a restrictor that damps pressure pulses from pumps and compressors. It costs very little and roughly doubles gauge life on reciprocating service. Use one.
On vibrating service, yes — it damps the pointer and extends life. On service where the ambient goes below about -20 °C, use a silicone fill instead.
SS316L for most duties. Hastelloy C for chlorides and strong acids, Monel for hydrofluoric, tantalum for the worst acid services, and PTFE lining where nothing metallic will do.
Tell us the medium and whether it can block a narrow tube. We will confirm whether a seal is genuinely needed or whether a siphon or snubber solves it for a fraction of the cost.
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