Tuesday, April 15, 2014

Understanding Pressure Gauge Specifications.


Pressure gauges have many specifications which technicians should understand when selecting a gauge, including range, resolution, accuracy, proof pressure, and burst pressure.

The range is the lowest to highest reading on the gauge. It is important that the range of a gauge fit the pressures for which it will be used. You want a gauge able to read the highest pressure you might encounter, but you don’t want the top reading much higher than that. This is because accuracy is stated as a percentage of full scale. A gauge with an accuracy of 1% and a top reading of 300 psig has a possible error of 3 psi. If you make the top reading 600 psig, now the possible error becomes 6 psi. If the pressures you will be reading only go up to 250 psig, expanding the top pressure reading just makes the error larger.

Resolution is the smallest indication the gauge can display. A gauge that can display in tenths of a pound has a resolution of 0.1 psi. This is not the same as the accuracy. Just because a gauge CAN display in tenths of a pound does not mean the reading is accurate to tenths of a pound. However, having a small resolution does make the gauge more useful for precise readings than another gauge with the same accuracy and top scale. For example, there are both analog and digital gauges with the same accuracy, but an analog gauge with a scale of 0 – 500 psig cannot have a resolution in tenths of a pound – there is just not enough space. A digital gauge with the same accuracy and top reading has no problem displaying in tenths of a pound.

Accuracy is stated as plus or minus a percentage of full scale. Full scale is the highest reading on the gauge. To determine how far off a gauge can be and still be within specifications, multiply the percentage accuracy times the top reading. For example, a gauge with 1% accuracy and a top reading of 500 psig would have an accuracy of plus or minus 5 psi. Note that the accuracy is plus or minus a percentage of full scale, NOT, a percentage of the reading. For example, a reading of 400 psig could indicate a pressure anywhere from 395 psig to 405 psig. This is not too bad at 400 psig, where 5 psi only represents 1.25% of the actual reading. However, plus or minus 5 psi becomes a problem at lower readings. A reading of 25 psig could indicate a pressure anywhere from 20 psig to 30 psig.  Now 5 psi represents 20% of the actual reading. For accuracy reasons, you don’t want the top reading on a gauge to be any higher than it has to be to get the job done.

Temperature compensation is an important aspect of accuracy in digital gauges. The accuracy of the sensors used in digital gauges changes with temperature. A gauge that specifies an accuracy at a single temperature is only that accurate at that temperature. It will be less accurate at other temperatures. A temperature compensated gauge maintains its accuracy over a wider range of temperatures. The most accurate digital gauges will be temperature compensated.

Proof pressure is the highest pressure the gauge can withstand without damaging the sensor. This is typically considerably higher than the highest reading.

Burst pressure is the pressure at which something on the gauge physically breaks. It is higher than the proof pressure. Gauges should be selected with proof pressures and burst pressures considerably higher than the highest pressure you would expect to encounter. A gauge which would be perfectly acceptable for use with systems using R-134a might not be acceptable for use on systems using R-410a.

1 comment:

  1. Repeatability ..... The gauge reads the same pressure after repeated measurements by the same person.

    Reproducibility.... several measurements taken by different individuals are identical in value.

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