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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 then see below for instructions on how to vent the gauge.

If your gauge is installed in a position where the fill plug is not facing up then see below for instructions on how to cool the gauge to a known reference temperature.

Venting the Gauge

Only vent the gauge if the fill plug is facing up (air bubble below the the fill plug location). If your gauge is installed in a position where the fill plug is not facing up then refer to the instructions on how to cool the gauge to a known reference temperature.

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?
To ensure an accurate reading you should vent the gauge whenever taking a pressure reading.

Cooling the Gauge
If you can not install the gauge in a position where the fill plug is facing up:

  • Before installing warm the gauge up to room temperature (this is your reference temperature).
  • Vent the gauge (see "Venting the Gauge" above).
  • Install gauge.
  • Ensure that the gauge is at the reference temperature when taking a pressure reading. If the gauge is hotter than the reference temperature then wrap a cool wet towel around gauge case to cool the gauge to your reference temperature. If the gauge is cold then wrap a warm wet towel around gauge case to heat gauge up to your reference temperature.

Ensuring that your gauge is at your reference temperature when taking a reading will minimize any pressure offset due to temperature change.

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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