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My liquid filled gauge isn't reading correctly, do I have a faulty gauge?

The gauge is most likely fine and you are seeing the effect of temperature change on the filled gauge.

How do I fix it?

Cut the Nipple Plug
Most of our 0-15 PSI gauges come with a nipple plug. Install the gauge so that the nipple plug is facing up, then cut the nipple off the plug. This will allow the case pressure to be equalized at all times ensuring accurate readings regardless of gauge temperature. IMPORTANT: DO NOT cut the nipple on the plug if the nipple is not facing up.

If your gauge does not have a nipple plug or your gauge is installed in a position that does not allow you to cut the nipple plug you will need to vent or cool the gauge to ensure an accurate reading (see below).

Cooling the Gauge
If you vented the gauge before installing and the gauge has gotten warmer you can reduce the pressure buildup inside the gauge case by wrapping the gauge case with a cold, wet towel. Once the temperature of the gauge is at ambient temperature it will read correctly.

If you didn't vent the gauge before installing, cool the gauge to room temperature (80-90F) and vent the gauge. Your gauge will now be accurate at this temperature. If the gauge heats up you can cool the gauge back down to this temperature with a cold wet towel for accurate readings.

Venting the Gauge

Venting is easy to do and takes seconds. To vent the gauge carefully push the side of the vent plug (the part under the plug lip) with your thumbnail to equalize the gauge case pressure with the atmosphere (reference pressure). Keep the fill plug vented for 2-3 seconds to ensure the case pressure is allowed to equalize with the outside pressure. Do not remove the plug. See photo below for an illustration of how to vent the gauge:

Venting a liquid filled gauge to equalize case pressure.

It is OK if some liquid also escapes, just wipe any liquid that escapes off with a cloth. Loss of the liquid fill will not affect operation of the gauge.

How often should I vent the gauge?
You should vent the gauge whenever the temperature of the gauge has changed significantly (either heated up or cooled down).

Why is venting/cooling necessary?
All mechanical liquid filled gauge cases are sealed (to keep the liquid in) so as they heat up pressure will build up in the case (approx 1psi for every 30-40F temperature change). This case pressure exerts a force on the mechanical movement and offsets the actual process pressure. This effect is minimal and typically only noticeable when measuring lower pressures, i.e. on a 0-15 psi gauge.

American National Standards Institute (ANSI) approves American National Standards which include the American Society of Mechanical Engineers (ASME) standard ASME B40.100. This standard addresses venting of sealed gauges within the following section:

Sealed Cases: Liquid filled or not, will exhibit error as a result of exposure to ambient or media temperature different from that at which the case was sealed unless compensation is provided. This error is caused by internal case pressure changes and depends on fill media, extent of fill and other factors. The error is constant over the entire scale, and if the temperature is stable, within limits, it can be corrected by resetting the pointer. An increase in temperature generally causes an increase in internal case pressure with a resulting decrease in indicated pressure. The opposite occurs for a decrease in temperature. For a given temperature change, the percentage of error noted on the gauge is a function or the range of the gauge. If, for example, the temperature increase causes the internal pressure to increase by 3 psi, then on a 30 psi gauge, this will cause a 10% error, whereas on a 0-100 psi gauge, the error will be 3%. For higher ranges, the percentage of error becomes proportionately less.

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