Applications require different levels of accuracy. Measuring the pressure of a home water system does not necessarily need to be as accurate as a medical sterilizer application.
A measurement of any parameter is only valuable if the user has confidence that it is accurate, but even the highest quality pressure gauge can become less accurate over time. To maintain optimal reliability and repeatability of your gauge, you must verify its calibration periodically.
There are a number of factors to consider when checking calibration. This article will describe these considerations and explain how checking the calibration of a pressure gauge can ensure continued accuracy for your application.
What is Calibration and Why Check it?
Calibration is the process of making an adjustment to ensure an instrument indicates within specified accuracy limits.
Calibration verification is the act of checking the accuracy of an instrument by comparing it to a standard of known accuracy, and then quantifying the accuracy as a percentage of span.
When it comes to pressure measuring equipment, calibration ensures that it continues to provide accurate measurements through the following factors:
- Accuracy: how close or far off a given set of measurements (observations or readings) are to their true value. Accuracy of pressure gauges is typically expressed as a percentage of full scale.
- Tolerance: the maximum permissible deviation from a specified value; may be expressed in measurement units. A gauge with a span of 100 psi and an accuracy of ±1% of span will have a tolerance of ±1.0 psi at any point on the measurement scale.
- Precision: how close or dispersed the measurements are to each other and their repeatability.
So, why should you check your pressure gauge’s calibration?
Calibration may be required for:
- A new instrument
- After an instrument has been repaired or modified
- When a specified time period or usage (operating hours) has elapsed
- Before and/or after a critical measurement
- After an event such as a shock, vibration, or exposure to an adverse condition
- Sudden changes in weather or environment
- Whenever observations appear questionable
- As specified by a requirement, customer specification, instrument manufacturer recommendation, etc.
Accuracy vs. Uncertainty
Uncertainty of measurement is the doubt that exists about the result of any measurement. In everyday speech, this might be expressed as “give or take.” A stick might be two meters long give or take a centimeter or 20 cm ±1 cm, at a level of confidence of 95%.
Accuracy or error is the difference between the measured value and the true value of what’s being measured. Accuracy is a qualitative concept, defined as the closeness of the agreement between the result of a measurement and a true value.
Uncertainty is a quantification of the doubt about the measurement result. Uncertainty of measurement acknowledges that no measurements can be perfect and is defined as a parameter, associated with the result of a measurement, that characterizes the dispersion of values that could reasonably be attributed to the measured.
It is typically expressed as a range of values in which the true value is estimated to lie, within a given statistical confidence, but it does not attempt to define or rely on a unique true value.
Ashcroft Tools for Calibration
Not only does Ashcroft offer instruments to meet your application needs, but we can also provide you with the tools and knowledge to ensure these instruments remain accurate and reliable for their entire service life.
ASME recommends that when verifying the calibration of a pressure gauge that the standard to which the gauge under test is being compared is at least four times more accurate. For example, a 0-100 psi gauge with ±1% of full span accuracy (tolerance of ±1.0 psi) should be verified with a pressure standard capable of measuring within ±0.25 psi or less.
A primary standard in metrology is a standard that is sufficiently accurate and not calibrated by, or subordinate to, other standards. Primary standards are defined via other quantities like length, mass and time, and they are used to calibrate other standards referred to as working or secondary standards.
A secondary standard must be calibrated against a primary standard.
There are a number of instruments that Ashcroft offers for pressure calibration:
Ashcroft® Deadweight Tester 1305D
- Traceable primary pressure standard
- ±0.1% of reading accuracy
- Includes complete set of weights for pressure generation
- Dual Range system includes low- and high-pressure piston assemblies
- Portable, includes metal toolbox for protection and portability
- Includes tools and adapters to accommodate most calibration requirements
Ashcroft® ATE-2 Handheld Calibrator
- Quick Select Pressure Modules
- 25 In H2O to 10,000 psi
- Accuracy from ±0.025% to ±0.1%
- Traceable calibration
- Accurately measures three parameters: pressure, temperature and voltage/current
Ashcroft® Digital Test Gauge 208X
- Accuracy of ±0.25%, ±0.10% or ±0.05% of full scale
- Temperature Compensated
- Ranges Vacuum to 7000 psi
- Gauge, Absolute, Compound
There are also analog test gauges available:
- Accuracy: ±0.1% full scale (Grade 4A)
- Pressure Ranges: Vacuum to 100,000 psi
- Dial size: 6 in., 8 ½ in., 12 in. and 16 in.; aluminum solid-front case
- Accuracy: ±0.25% full scale (Grade 1A)
Pressure Ranges: Vacuum to 10,000 psi - Dial size: 4 ½ in., 6 in. and 8 ½ in.
- Case: aluminum; solid-front safety design
- Accuracy: ±0.5% full scale (grade 1A)
- Pressure Ranges: Vacuum to 1,000 psi
- Dial size: 3 in.
- Case: stainless steel; open front
We don’t like to pressure you, but we have more information.
Now that you know more about the importance of periodically checking the calibration of your pressure gauge, you can help ensure it stays in working order to operate accurately and reliably in your process.
For more information on pressure gauges, read some other articles we’ve written:
- How Often Should I Check the Calibration of My Pressure Gauge?
- How To Identify Your Pressure Gauge and Get the Proper Replacement
- How Does Temperature Affect Pressure Gauge Performance?
- How Do I Select the Right Pressure Gauge Range?
Feel free to contact us today to talk to one of our industry experts and get all your measuring instrumentation questions answered.
And learn more about how to avoid equipment failure by reading our eBook: