Blog
Diaphragm Seal Pressure Gauges: When to Use One and How to Specify
Quick answer: a diaphragm seal (also called a chemical seal or pressure seal) is a flexible metal membrane placed between a corrosive, sticky, hot or hygienic process fluid and a pressure gauge or transmitter. The pressure passes through the membrane into a fill liquid, which carries it to the instrument, so the process fluid never enters the gauge. Use one when the media would corrode, clog or contaminate a standard gauge, or when cleaning and hygiene matter. Choose the seal material for the media, the fill fluid for the temperature, and remember that a capillary adds a height-related error. This guide explains how diaphragm seals work, when you need one, what to specify, with worked examples.
What is a diaphragm seal gauge?
A normal Bourdon gauge lets the process fluid fill the tube. That is fine for clean air, water and oil. It is a problem for paint, adhesive, slurry, food products, strong acids or crystallising solutions, which can clog the tube, attack the metal or contaminate the product. A diaphragm seal solves this with three parts:
- The diaphragm: a thin, flexible metal membrane bolted to the process connection.
- The fill fluid: a liquid sealed in the space behind the diaphragm and inside the gauge.
- The gauge or transmitter: the measuring instrument, connected directly to the seal or through a capillary tube.
Process pressure pushes the diaphragm inward, which pressurises the fill fluid, which acts on the gauge exactly as if the process fluid were there. The gauge reading is the same, but the process fluid never touches the instrument.
When do you need a diaphragm seal?
| Problem with the media | Example | Why a seal helps |
|---|---|---|
| Corrosive | Acids, caustic, chlorides | Seal material can be a resistant alloy or lining, while the gauge can stay standard |
| Viscous or sticky | Adhesives, paint, syrup | Flush membrane has no pocket to clog |
| Crystallising or solids-bearing | Sugar solutions, slurries, wastewater | Open, flush surface can be cleaned |
| Hygienic | Dairy, beverage, pharmaceutical | Sanitary connections and smooth surfaces for cleaning |
| Hot | Thermal oil, hot process fluid | A cooling element or capillary keeps heat away from the gauge |
| Toxic or hazardous | Chemicals that must not leak | Welded or sealed system reduces the leak path |
Manufacturers note that the three most important factors in choosing a seal are the temperature, pressure and consistency of the media.
Types of diaphragm seal arrangement
Direct mount
The gauge screws directly onto the seal, which bolts or screws to the process. Compact and low cost, with no capillary errors. The gauge sits close to the process, so heat and vibration matter.
Capillary (remote) mount
A flexible armoured tube joins the seal to the gauge, which can be mounted somewhere convenient and cooler. It suits hot processes, hard-to-reach points and vibration, but it adds the height error and temperature effects described below.
Cooling element
A finned section between the seal and the gauge lets heat dissipate. Useful for moderately hot media. See high temperature pressure transmitters and sensors.
Flush and sanitary connections
For food, dairy and pharmaceutical use, seals come with hygienic clamp-style connections and a flat, crevice-free face that can be cleaned in place.
Materials and fill fluids
- 316L stainless steel: suits many media and is the common starting point.
- Special alloys, such as Hastelloy C, Monel and tantalum, or PTFE linings: used for highly corrosive media. The choice depends on the medium, temperature, pH, chlorides and cleaning method.
- Fill fluids: chosen for the temperature range and compatibility with the process. Food and pharmaceutical work uses fluids suitable for incidental contact, in case a diaphragm fails.
Always take the exact limits from the maker’s datasheet. The fill fluid, diaphragm material and temperature all affect performance.
The capillary height error: a worked example
The fill fluid in a capillary has weight. If the gauge is mounted higher or lower than the seal, the fluid column adds or subtracts pressure at the gauge.
Worked example. A gauge is mounted 2 metres above the seal, and the fill fluid has a density of about 960 kg per cubic metre.
- Error = density × gravity × height = 960 × 9.81 × 2 = about 18,800 Pa, which is 18.8 kPa or about 0.19 bar.
- With the gauge above the seal, it would read lower than the true process pressure by that amount unless corrected at calibration.
If you know the height difference, the maker can compensate when setting the zero. If the gauge is later moved, the error returns. On a 0 to 1,000 kPa gauge, 18.8 kPa is almost 2% of full scale, so it matters. For low ranges it can swamp the measurement. Keep capillaries as short as the job allows and mount the gauge at the specified height.
Temperature effects on capillary systems
The fill fluid expands and contracts with temperature. If the capillary or the seal is hotter or colder than at calibration, the volume of fluid changes and the reading shifts. Longer capillaries and larger temperature swings give bigger errors. Makers publish the expected error for a given length and fluid.
Standards and sizes
EN 837-3 covers diaphragm and capsule pressure gauges, including nominal sizes from 50 to 250 mm and ranges up to 25 bar. A gauge with a seal fitted is a system, so you should check both the gauge standard and the seal’s own data. Seal systems for higher pressures follow the maker’s specification.
Worked selection example 1: a dairy
A dairy needs to monitor pressure on a milk line that is cleaned daily with hot caustic and acid.
- Requirement: hygienic, cleanable, no pockets for milk to collect.
- Seal: flush stainless steel diaphragm with a hygienic clamp connection.
- Material: 316L stainless steel for caustic and acid cleaning.
- Gauge: stainless steel gauge or transmitter, with a range chosen so working pressure is around 50% to 66% of the scale.
Worked selection example 2: a paint line
A coating line has pressure gauges that clog with paint every month. A flush diaphragm seal on each tapping stops paint entering the gauge. The seal face can be wiped or flushed. The fill fluid is chosen for the room temperature, and the gauges are mounted at the same height as the seals to avoid zero errors.
Worked selection example 3: a hot oil line
A line carries oil at 250 degrees Celsius. A capillary seal with a high-temperature fill fluid lets the gauge sit away from the heat. The designer checks the allowable fluid temperature, the capillary length limit and the temperature error, and asks the maker to set the zero at the final mounting height.
Installation checklist
- Confirm the seal and fill fluid are suited to the media and temperature.
- Mount the seal on a clean port with the correct gasket or seal.
- Do not damage or press on the diaphragm. It is thin and easily dented.
- Keep the capillary supported and free from sharp bends and mechanical stress.
- Mount the gauge at the height specified for calibration.
- Never remove the gauge from the seal, since that can lose the fill fluid.
- Include the system in your calibration plan. See pressure gauge calibration.
Common mistakes
- Choosing the gauge standard but ignoring the seal material.
- Forgetting the height error of a capillary.
- Using a seal with a fill fluid that cannot handle the temperature.
- Unscrewing the gauge from the seal and losing fluid.
- Dropping or denting the thin diaphragm.
- Skipping cleaning access on sticky or crystallising media.
What we can supply
Our current range of pressure gauges includes stainless steel Bourdon gauges, capsule gauges for low pressure and the WIKA A-10 sensor with 316L wetted parts, which suits many clean and mildly aggressive media. Diaphragm seal systems are a specialist item, so send us your media, temperature, pressure and connection through the contact page and we will tell you honestly what we can supply for your job.
Quick diagnosis for diaphragm seal systems
| Symptom | Possible cause |
|---|---|
| Reading offset at zero | Gauge height changed from the calibrated position |
| Reading wanders with temperature | Capillary or fill fluid effects |
| Slow response | Long capillary or viscous fill fluid |
| No reading | Ruptured diaphragm, lost fill fluid or blocked port |
A note on cost and when not to use a seal
A diaphragm seal adds cost and, with a capillary, adds errors. If the media is clean and compatible with the gauge, a plain gauge is simpler and cheaper. Use a seal where the media would damage or clog the gauge, or where hygiene or safety requires it.
Frequently asked questions
What is a diaphragm seal used for?
To protect a gauge from corrosive, sticky, hot or hygienic media while still passing the pressure to it.
Does a diaphragm seal change the reading?
It can, through capillary height and temperature effects. Specify them and calibrate the system as a whole.
Can I add a seal to an existing gauge?
Not casually. The gauge and seal are filled and calibrated as a system. Ask the supplier.
What material should I choose?
Start from the media, temperature, pH, chlorides and cleaning method, and confirm with the maker.
Is it the same as a snubber?
No. A snubber damps pulsation. A diaphragm seal isolates the process fluid from the gauge.
Next steps
Read the analogue pressure gauge guide, what is a pressure gauge and our supplier guide.
