Primary vs Secondary Standards

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Pressure testing equipment calibration is important in ensuring that you collect accurate and reliable data through your work. While different devices and systems will require different calibration techniques, all calibration depends on either a primary or secondary (otherwise known as “transfer”) standard. But what, exactly, is the difference between primary vs secondary standards?


To put it simply, a primary standard is a factual universal measurement while a secondary standard is a device directly calibrated previously by the primary standard.

What is a Primary Standard?

A primary standard is a standard based only on one’s knowledge of the only three fundamental units:

  • Mass
  • Length
  • Time

In pressure work, this reduces to either a manometer (where the height of the fluid column gives you the pressure) or a dead weight tester, where it is only necessary to know the area of the piston and the value of the mass applied.


Generally, the deadweight tester is too cumbersome to use directly as a pressure gage; this has lead to a plethora of devices to use as secondary standards and as process instruments.


At Harwood we maintain our controlled-clearance deadweight gages by sending our weights and pistons out to be recertified on a regular basis. We also recalibrate our secondary standards against out primary on a regular basis.

What is a Secondary Standard?

A secondary standard is any measuring device that has been calibrated against a primary standard.


In pressure work, the error band on the primary standard is generally so many percent of reading – the error is the summation of the uncertainty in the weight measurement plus the uncertainty of the piston measurement; secondary devices are usually reported as +/- a percent of full scale. For most production work, that is sufficient. However, if the device being calibrated is, itself, going to be used as a standard to calibrate others, it should have a primary calibration.

Harwood Engineering has been providing high pressure testing services and pressure gauge calibration for more than 65 years. Contact us today with any questions you may have about primary vs secondary standards or your pressure testing needs.