Vibrations can sometimes violently load Bourdon tube pressure gauges. They cause pointer flutter and in addition, in case of sustained loading, damage to the measuring system, even to the point of total failure. The most effective protection against these effects is provided by measuring instruments with case filling.
The principle is simple: The pressure gauge case is filled with a liquid, usually glycerine or silicone oil. Regarding vibrations, the case filling optimally dampens the vibrations of the Bourdon tube, transmission mechanism and movement. It therefore prevents pointer flutter, and thus the displayed measured value remains clearly readable. Concurrently, the measuring system is protected against premature wear, because the fill fluid acts as a lubricant for the moving components. This considerably extends the service life of the pressure gauge.
Figure 1: View of the inside of a pressure gauge with a dampened movement. The circle marks the seat of the pot with the silicone oil that accommodates the pointer pinion.
Highly viscous silicone oil
Instead of a musical instrument with case filling, a pressure gauge with silicone-dampened movement is often chosen. In this design, the pointer pinion moves in a pot of highly viscous silicone oil. Subsequently, the pointer also operates largely free of vibration. However, this effect, which is necessary for immobilising the pointer, slows down other moving components of the movement. The effect is really a significantly higher wear of these parts than with a pressure gauge with fill fluid.
WIKA confirmed these details some time ago within an internal laboratory test with different pressure gauge versions. Unfilled pressure gauges, pressure gauges with dampened movement and pressure gauges with case filling were put through an endurance test under practical conditions which were harsher than those of the EN 837-1 pressure gauge standard. The investigation produced the following results:
Pressure gauge version (type of dampening)
Zero point offset after 50 hrs / 200 hrs
Condition of the instruments after 200 hours
Unfilled / without dampening
2,3% / 3.0%
Conditionally functional
Dampened movement
2,3% / 66%
No longer functional
Liquid-filled
0,6% / 0,8%
Fully functional
Fast wear
In the test, the unfilled variant ended up being relatively resistant. However, given the inevitable pointer flutter, such a pressure gauge is not recommended for applications with vibrations. This verdict also applies to the version with dampened movement, particularly in applications with stronger and sustained vibrations. The pointer stability is, in this case, countered by way of a rapid wear of another moving parts. This version was, already, no more functional well before the end of the test.
Figure 2: Cracks in the Bourdon tube or perhaps a worn-out link are examples of typical vibration damage in the mechanisms of pressure gauges. Such wear is prevented by case filling.
Frightening with case filling was the only variant which remained fully operational. Due to the fill fluid, the chance of leakage is often used being an argument against this type of instrument. WIKA cases with filling are therefore designed and handled to lessen the risk of leakage due to vibration to a minimum.
Note
To learn more on our selection of pressure gauges, go to the WIKA website.
See also our article
Filling liquids in pressure gauges: Usage and advantages