Vibrations can sometimes violently load Bourdon tube pressure gauges. They cause pointer flutter and in addition, in case of sustained loading, harm to the measuring system, even to the point of total failure. The most efficient protection against these effects is supplied by measuring instruments with case filling.
The principle is simple: The pressure gauge case is filled with a liquid, usually glycerine or silicone oil. In the case of 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. As well, Placid measuring system is protected against premature wear, since 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. As a result, the pointer also operates largely free of vibration. However, this effect, which is necessary for immobilising the pointer, decreases other moving components of the movement. The effect is a significantly higher wear of these parts than with a pressure gauge with fill fluid.
WIKA confirmed this information some time ago in an internal laboratory test with different pressure gauge versions. Unfilled pressure gauges, pressure gauges with dampened movement and pressure gauges with case filling were subjected to an endurance test under practical conditions that have been harsher than those of the EN 837-1 pressure gauge standard. The investigation produced the next results:
Pressure gauge version (kind 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, this type of pressure gauge isn’t recommended for applications with vibrations. This verdict also applies to the version with dampened movement, particularly in applications with stronger and sustained vibrations. Bomb is, in cases like this, countered by a rapid wear of another moving parts. This version was, already, no longer functional well before the end of the test.
Figure 2: Cracks in the Bourdon tube or a worn-out link are examples of typical vibration damage in the mechanisms of pressure gauges. Such wear is prevented by case filling.
The pressure gauge with case filling was the only variant which remained fully operational. As a result of fill fluid, the risk of leakage is frequently used being an argument against this type of instrument. WIKA cases with filling are therefore designed and handled to lessen the chance of leakage because of vibration to the very least.
Note
For additional information on our selection of pressure gauges, go to the WIKA website.
See also our article
Filling liquids in pressure gauges: Usage and advantages