You may have noticed that some pressure gauges are liquid-filled. These measuring devices are often installed in industrial equipment, hydraulic and other systems. What is that liquid inside and what function does it perform?

Not all needle pressure gauges, commonly called manometers, are filled with liquid and you know it. If you measure the pressure in your car’s tires with a gauge, you noticed that there is no fluid behind the glass. Pressure gauges of home boiler systems are also usually not filled with liquid. Therefore, it is possible that you have not even seen such a device:

A liquid-filled pressure gauge. (Édouard Hue, Wikimedia(CC BY-SA 4.0)

As you can see, almost the entire face of this meter is immersed in a clear fluid. This particular membrane manometer is used in the wine industry, but similar devices can be seen in all kinds of applications. That liquid inside is either a light oil, or glycerin. The entire mechanism of the pressure gauge is submerged in this liquid, not just the needle face. Of course, oil or glycerin protects the mechanism from corrosion and dust, and lubricates the device. However, the main function of the liquid in a manometer is somewhat different.

Liquid-filled pressure gauges are installed in systems where the pressure is constantly and rapidly changing, especially if the pressure is somewhat pulsating. In other words, these devices are better suited for measuring dynamic pressure. For example, a reciprocating compressor causes sudden jumps in pressure with each stroke. The pressure also changes continuously in industrial robots with pneumatic or hydraulic systems. The problem is that when the pressure changes suddenly, it is difficult to monitor it accurately, because the needle easily jumps past the actual pressure value due to inertia.

A glycerin-filled pressure gauge. (Deyan Levski, Wikimedia(CC BY-SA 3.0)

And that’s why pressure gauges are filled with oil or glycerin – liquid dampens the movements of the needle, preventing quick jerks. This is important not only when the pressure in the system changes dynamically, but also when the equipment vibrates. Manometer mechanism is quite simple, but it can be a bit springy, which is less than ideal when there are vibrations. If the needle is jumping around due to vibrations it will be difficult to tell what pressure is actually being maintained. Furthermore, those vibrations can cause premature wear of a pressure gauge and can make it produce unreliable readings. For example, manometers of various agricultural sprayers are often filled with oil to prevent the needles from moving too much due to tractor vibrations.

Although there are many different pressure gauges, most membrane manometers are still using a tube mechanism invented by Eugene Bourdon in the 19th century. In simple terms, it is a slightly coiled metal tube that sort of wants to straighten up due to internal pressure. Although it sounds like a primitive device, Bourdon tube manometers can be extremely accurate. Especially when they are filled with a liquid that dampens unwanted vibrations and slows down the pointer needle.

Bourdon-tube mechanism of a pressure gauge. (Nilsausta, Wikimedia(CC BY-SA 4.0)

So why aren’t all manometers filled with liquid? Not all systems require such precision or resistance to vibrations and pressure spikes. If the pressure does not change dynamically, if there is no need for very accurate readings, if the needle of the gauge is not affected by strong vibrations, a liquid-filled pressure gauge is simply not necessary.

A dry pressure gauge. (Sasha Taylor , Wikimedia(CC BY-SA 4.0)

Liquid-filled pressure gauge housings must be sealed very well. And the mechanism itself must be adapted to work in oil or glycerin. This means that liquid-filled pressure gauges are more expensive. They are also a little bit heavier. Most of the time, a simpler dry manometer does the job just fine.


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