In order for pressure gauges to continuously provide accurate readouts, they must be regularly calibrated using the correct technique. Although calibrating these precision gauges takes time and involves thorough attention, it is entirely necessary to avoid costly errors. Whether you are an expert in this field or have never worked with pressure gauges before, it is beneficial to review the essential details associated with the calibration process. In this blog, we will discuss everything you need to know when it comes to pressure gauge calibration.
Before discussing gauge calibration, it is first necessary to review some fundamental operating principles. The pressure measured by a pressure gauge is a physical quantity defined by "force per unit area." Although this definition is globally accepted, the units in which it is expressed are not. Depending on the industry, pressure may be expressed in bar, at, atm, or torr, although the pascal is the standard unit used in most engineering settings. Another consideration is the reference to which the pressure is being measured. For example, absolute pressure is measured from absolute zero, while gauge pressure uses atmospheric pressure as the zero point. It is critical to determine the type of reference system used by a particular gauge when performing calibration.
The vast majority of pressure gauges in use are of the analog variant. These devices are equipped with a dial and needle which react to the slightest changes in an attached pressure scale. Since these gauges are commonly employed in applications requiring narrow tolerances, most are manufactured to meet the specifications set forth by the American Society of Mechanical Engineers.
The most common calibration procedure involves setting the pressure gauge on a pressure comparator, which is a device that can generate a set amount of force over a given area. In addition, a master gauge should be attached to the other side of the device in order to ensure accurate pressure readings. These devices are heavily scrutinized and are regularly tested to display accuracies of +/- 0.1%. After getting both elements in place, it is then necessary to determine the gauge's least count. The least count may be determined by observing the first significant mark on the dial. Next, the operator will devise an equation with the largest mark as the numerator and the least count at the denominator. The result of this equation will be the increment in which the gauge should be tested. For example, if the least count is 1 and the greatest is 10, the tester should calibrate at 1 pascal, 2 pascals, 3 pascals, etc.
It is important to determine accuracy class when calibrating a gauge. For example, some applications allow for inaccuracies up to +/- 10psi, while others have more stringent requirements at +/- 1psi. In order to accurately label a gauge as calibrated, the tester should take into consideration the accuracy as a reference. Additionally, if an alternative method or media is used as part of testing, it is necessary to ensure the gauge is not damaged or dirty. In general, it is a good rule of thumb to never employ testing media that may degrade or permanently damage the device. When using liquid media in testing, it is critical that the test and master gauge be at the same height since pressure is also dependent on height in the context of a liquid media.
Before performing the actual test, the operator may consider priming the gauge by applying its maximum pressure for a short amount of time before slowly releasing it. This strategy will ensure the smoother movement of the needle by reducing friction. Another useful recommendation is to continue increasing the pressure until the needle reaches the next largest mark since it much easier to read than subsequent increments. Since the number on the master gauge will also increase, the tester will still be able to calculate the difference between the two with accuracy.
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