On a pump discharge or compressor line, a dry gauge pointer often shakes so hard that nobody can read it and the movement wears out within months. Filling the case with a damping liquid calms the pointer, protects the gears and keeps the dial readable for years.
Vibration and pulsation are the main enemies of a mechanical pressure gauge. A liquid fill damps both, but only when the right fluid, range and venting practice are chosen for the service.
What Is a Liquid Filled Pressure Gauge?
A liquid filled pressure gauge is a mechanical dial gauge whose sealed case is filled with a damping liquid such as glycerine or silicone oil. The measuring element is usually a Bourdon tube, and the liquid surrounds the tube, the movement and the back of the pointer.
The liquid does not take part in the pressure measurement itself. Its job is to slow down rapid movements of the pointer and the gear train, lubricate the moving parts and keep moisture out of the case.
You will see these gauges on pump skids, hydraulic power packs, compressors and mobile machinery. They follow the same accuracy classes and dial sizes as dry gauges, which are explained in pressure gauge accuracy classes.
Why Vibration and Pulsation Destroy Dry Gauges
Vibration comes from the machine the gauge is mounted on, while pulsation comes from the process pressure itself. A reciprocating pump or compressor produces a pressure wave on every stroke, and the Bourdon tube follows each wave.
In a dry gauge, the pointer then flutters so fast that the operator sees only a blur. WIKA explains that if the vibration hits the resonance of the movement, the pointer can swing through a deflection angle of more than 180 degrees.
The segment and pinion gears wear quickly under this constant motion, and the pointer may even break off. A pressure snubber damps the process side, while the case fill damps the mechanism itself.
WIKA fills its liquid gauges to only about 80 to 90 percent of the case volume. The small air space allows the liquid to expand with temperature without bursting the window or leaking.
How the Fill Liquid Damps the Pointer
The viscous liquid acts like a shock absorber on every moving part. Slow pressure changes still pass through almost unchanged, but fast oscillations are filtered out, much like a low pass filter in electronics.
The liquid also coats the gear teeth and pivots, which reduces wear. Because the case is sealed and full, humid air cannot enter, so the dial does not fog and the movement does not corrode.
Glycerine vs Silicone Oil Fill
Most common fill, high viscosity, excellent damping at normal ambient temperature.
Lower viscosity, stays fluid at low temperature, wide temperature range.
Chemically inert fluorinated fluid, safe with strong oxidisers.
Ashcroft describes glycerine as generally the most viscous fill, which makes it the better damping agent. Silicone is significantly less viscous, yet it still gives useful protection against pulsation and keeps the pointer responsive when the weather turns cold.
Ashcroft quotes a range of 20 to 150 °F, roughly minus 7 to 66 °C, for glycerine case fill and minus 40 to 200 °F, about minus 40 to 93 °C, for silicone. WIKA recommends silicone oil when temperatures are particularly high, above 60 °C, or very low, below minus 20 °C.
On outdoor gauges in Ladakh, Himachal or other cold regions of India, specify silicone oil fill. Glycerine thickens so much in winter that the pointer lags badly behind the real pressure.
Temperature and Pressure Limits From EN 837
The WIKA model 213.53 data sheet shows typical limits for a liquid filled pressure gauge built to EN 837 1 and ASME B40.100. Accuracy is class 1.6 for 50 and 63 mm dials and class 1.0 for the 100 mm dial.
| Parameter | Glycerine Fill | Silicone Oil Fill |
|---|---|---|
| Ambient temperature | minus 20 to 60 °C | minus 40 to 60 °C |
| Medium temperature | minus 20 to 100 °C | minus 40 to 100 °C |
| Steady pressure, 63 mm | 3/4 of full scale | 3/4 of full scale |
| Fluctuating pressure, 63 mm | 2/3 of full scale | 2/3 of full scale |
| Fluctuating pressure, 100 mm | 0.9 of full scale | 0.9 of full scale |
The medium limit of 100 °C applies at the gauge socket. For steam or hot water, fit a siphon tube so that condensate cools before it reaches the Bourdon tube, and follow common installation mistakes to avoid heat soak.
EN 837 1 also allows an extra temperature error of up to 0.4 percent of span for every 10 K that the gauge departs from the 20 °C reference. Hot pump rooms can therefore add a noticeable error on top of the class accuracy.
Range Selection Formula
Working ratio = Maximum working pressure ÷ Full scale × 100
For pulsating service on 63 mm gauges, keep the working ratio at or below 66.7 percent
Example:
Full scale 10 bar, class 1.6, maximum working pressure 6 bar
Permissible error = 1.6 ÷ 100 × 10 = 0.16 bar
Working ratio = 6 ÷ 10 × 100 = 60.0 percent
Result: within the 2/3 limit for fluctuating pressure
A good rule is to choose a full scale about twice the normal working pressure, so the pointer sits near mid scale. This improves readability and leaves margin for pressure spikes, as covered in pressure gauge selection factors.
Gauge Range and Accuracy Calculator
Second Worked Example: Temperature Error
Take a 0 to 16 bar class 1.6 gauge on a pump skid where the case sits at 50 °C. The class error is 1.6 ÷ 100 × 16 = 0.256 bar.
The gauge is 30 K above the 20 °C reference, so the extra temperature error can reach 3 × 0.4 = 1.2 percent of span, or 0.192 bar. The worst case total is therefore about 0.45 bar, which matters when you compare field gauges with a transmitter.
A filled gauge fitted to a hot pipe can build internal pressure as the liquid expands. That trapped pressure pushes on the Bourdon tube from outside and can shift the zero until the case is vented.
7 Smart Tips for Using a Liquid Filled Pressure Gauge
Venting is often forgotten. Most filled cases have a rubber plug with a small nipple or lever, and gauge makers advise opening it after installation so the internal pressure matches the atmosphere, especially on low ranges where a small case pressure causes a visible zero error.
Pressure spikes from quick closing valves can still bend the tube even when the pointer looks calm. Protect it with the methods in pressure gauge over range protection and read about the hammer effect in pressure gauges.
Use a gauge with a back or top vent plug that can be closed for transport and opened in service. Store spare filled gauges upright so the fill does not seep past the plug.
When Not to Use a Liquid Filled Pressure Gauge
Never use a glycerine or silicone oil filled gauge on oxygen or other strong oxidisers. If the Bourdon tube cracks, the fill can contact the oxidiser and ignite, which is why oxygen gauges are cleaned for oxygen service and marked to use no oil.
Ashcroft notes that applications with oxidising media need an inert fill such as halocarbon, a fluorinated oil that costs more than glycerine. Many plants still prefer a dry, oil free oxygen gauge with a solid front and blow out back for maximum safety.
Also avoid filled gauges where very fast response is needed, such as some test benches, and where the liquid could contaminate a hygienic process after a leak. In food or pharma lines, use a diaphragm seal with a food grade fill, as described in fill fluid selection for diaphragm seals.
Liquid Filled vs Dry Gauges
- Steady, readable pointer under vibration and pulsation.
- Longer life of gears and pivots.
- No fogging or internal corrosion.
- Good shock resistance on mobile machinery.
- Fill viscosity changes with temperature.
- Case pressure build up can shift zero if not vented.
- Not allowed with oxygen and some oxidisers.
- Slightly higher cost and weight than dry gauges.
A dry gauge remains fine on steady lines such as instrument air headers or clean water mains. Where remote reading is needed, compare options in pressure gauge and pressure transmitter, since a transmitter with damping can replace a gauge in control loops.
Inspection and Calibration Checks
- Fill level between 80 and 90 percent with a small air bubble.
- Liquid is clear, not yellow or cloudy.
- No leaks at the window gasket or fill plug.
- Pointer returns to zero after venting at zero pressure.
- Pointer moves smoothly without sticking during a test.
- Accuracy verified against a reference at 5 points up and down.
Calibrate a liquid filled pressure gauge on a comparator or dead weight tester just like a dry gauge, but let the pointer settle a little longer at each point. The general method is shown in how to calibrate a pressure sensor.
WIKA Model 213.53 Data Sheet PDF
Why Gauges Are Filled With Glycerine Video
Liquid Filled Pressure Gauge FAQ
A liquid filled pressure gauge is a mechanical dial gauge whose sealed case is filled with glycerine, silicone oil or another damping liquid. The Bourdon tube, the gear movement and the back of the pointer all sit inside that liquid.
The fill damps pointer flutter caused by machine vibration and process pulsation. It also lubricates the gear train and keeps humid air and moisture out of the case.
Glycerine is the most viscous of the common case fills, so it damps fast pointer movement very well. It is also cheap, clear and compatible with most gauge case and window materials.
Its weakness is that it thickens in cold weather and may discolour when it runs hot. Ashcroft places the useful glycerine window at roughly 20 to 150 °F, so silicone oil replaces it outside that band.
WIKA recommends silicone oil when the gauge sees temperatures above about 60 °C or below about minus 20 °C in service. Its data sheet for model 213.53 allows silicone filled gauges down to minus 40 °C ambient.
Silicone oil is much less viscous than glycerine, yet it still damps pulsation well. It therefore keeps the pointer responsive on cold outdoor pipelines and in very hot pump rooms.
No, a glycerine or silicone oil filled gauge must never be used on oxygen service. If the Bourdon tube leaks, the oil can meet the oxidiser and ignite violently.
Use a gauge cleaned for oxygen service and marked to use no oil. Ashcroft notes that oxidising media need an inert fill such as halocarbon, although many plants simply prefer a dry oxygen gauge.
The liquid and the small trapped air bubble expand when the gauge warms up after installation. This builds pressure inside the case that can press on the tube and shift the zero reading.
Opening the vent plug equalises the case with the atmosphere around it. The effect is most visible on low ranges, where a small internal pressure forms a large fraction of the span.
Case fill damps the pointer and movement, but the Bourdon tube still sees every pressure pulse from the process. Severe pulsation can therefore fatigue and crack the tube over months of service.
On reciprocating pumps and compressors, combine the fill with a snubber or restrictor in the connection. Also keep the working pressure below about 2/3 of full scale for fluctuating service.
The accuracy of a liquid filled pressure gauge depends on dial size and accuracy class, not on the fill itself. WIKA quotes class 1.6 for 50 and 63 mm dials and class 1.0 for 100 mm dials to EN 837 1.
Temperature away from 20 °C adds up to 0.4 percent of span for every 10 K of change. Include this error when you compare a hot field gauge with a transmitter.
Related Articles
- Bourdon Tube Pressure Gauges
- Pressure Gauge Selection 7 Factors
- Pressure Snubber Working Principle and Selection
- Pressure Gauge Accuracy Classes
- Siphon Tube for Pressure Gauge
External References
- WIKA Data Sheet PM 02.12, Model 213.53 Liquid Filled Gauge
- Filling Liquids in Pressure Gauges, WIKA Blog
- Pressure Measurement, Wikipedia
What We Learn Today
- A liquid filled pressure gauge uses glycerine or silicone oil in the case to damp pointer flutter, lubricate the movement and keep moisture out.
- Glycerine suits normal ambient conditions, while silicone oil is chosen for cold or hot sites, with WIKA allowing silicone down to minus 40 °C.
- Keep pulsating working pressure below 2/3 of full scale, vent the case after installation and never use oil filled gauges on oxygen.