This webinar will focus on the theory and application of calibrating analog pressure gauges. Topics include gauge grades, exercising, and considerations for test.

1. Tips For
Calibrating
Analog
Pressure
Gauges
John Lopez, Fluke Calibration

2. Some introductory thoughts
 There’s generally more than one way to get the job
done. What way is best? “It depends”
 What do I mean by this?

3. Standards, how to determine what you
need to calibrate with?
 Pressure media
 Liquid vs gas
 Range of coverage
 Accuracy required

4. Standards, how to determine what you
need to calibrate with?
 Pressure media
 Generally, lower pressures are easier to
achieve with gas media (air or N2)
 Generally, higher pressures are easier to
achieve with liquid media
 But as always, “it depends”
 Can your devices tolerate liquid? If so, which
liquids?
 Can your standards tolerate liquid?
 Mixed media is possible, may require two
standards or cleaning

5. Standards, how to determine what you
need to calibrate with?
 Range of coverage
 Of course you need to span every point on
the workload
 But depending on the standards chosen, you
may need more or less
 Fewer but more accurate standards may end
up being less expensive and more efficient

6. Standards, how to determine what you
need to calibrate with?
 Accuracy versus Resolution
 Very common request: “I need X digits of
resolution”
 This may be true (QA standards, etc.) but,
accuracy is not resolution.
 On a digital instrument, resolution generally is
larger than accuracy by design so that it does
not impact accuracy

7. Standards, how to determine what you
need to calibrate with?
 Low accuracy, high resolution: a high-res
display with human entered values
 High accuracy, low resolution: a calibrated
balance and mass set with 500g masses

8. Standards, how to determine what you
need to calibrate with?
 Gauge grades/specific details
 Generally expressed as either a percent of scale of the
gauge (“1 percent gauge”) or as percents of the lower
quarter, middle, and upper quarter of the gauge (“3-2-3
gauge”).
 Specific ASME guidance is a purchased standard but there
are summaries on the web
 Actual process usage
 Very common to have or specify more accurate
instruments but to use a less restrictive standard (“I need
to be +/- 10 PSI”)
 This is OK – if your QA processes allow for it. It depends.

9. Standards, how to determine what you
need to calibrate with?
 Accuracy: it’s generally easiest to work in the same
language
 Trying to convert “a 100 PSI 3-2-3 gauge” to a “a 0.05%
scale 300 PSI standard” on the fly is confusing
 Create a table and compare apples to apples
 100 PSI 3-2-3 is 2% of 100 PSI worst case, so +/-2 PSI
 300 PSI standard that is +/- %0.05 scale is +/- 0.15 PSI

10. Theory of bourdon tubes
 What’s a bourdon tube anyway?
 “bore-don”
 Same principle as one of those party
blow things
 Pressure attempts to unroll a coil

11. Theory of bourdon tubes
 In the gauge, the coil is metal and
connected to an indicator. Simplified
view:

12. Theory of bourdon tubes
 More realistic internal view showing
gear train and linkages

13. Considerations for test
 Speaking of the mechanical
linkages…
 Several considerations that will
impact results and repeatability
of the results

14. Considerations for test - mechanics
 Exercise routines
 Exercise is taking the gauge from
its low scale (vented) condition
to full scale and back
 May be done several times
 Why do this?
 Is this required?

15. Considerations for test - mechanics
 “Dithering”
 Tapping the gauge to help the indicator settle
in to a position
 Not required but will affect the results, so
 Consistence of use or disuse is important

16. Considerations for test - mechanics
 Pneumatic standards
 Can be used on devices that are contaminated with liquids
or particles
 Care must be taken
 Cleaning procedures can be challenging
 Filters can create pressure differentials
 Traps may not be 100% effective
 Bottom line is that you do not want to damage your
standard

17. Considerations for test - mechanics
 Cardinal points
 Much easier to set the device under test to an
exact position and verify reading on standard
 Avoids human interpolation of the indicator
being between marks
 Parallax
 “Off to one side” problem
 Generally only a problem on higher end gauges
 Narrowed indicators and mirrored backs assist
with avoiding this

18. Considerations for test - vacuum
 Vacuum – a special case
 Most portable pneumatic standards can only generate around -
12, -13 PSI (24 to 28 inHg)
 A vacuum pump can be used to reach full vacuum (if required)
 Generally the vacuum pump is run and a metering valve is used to
take points

19. Considerations for test
 Adiabatic effects/settling time
 A closed system will fluctuate some
 Allowing a consistent settling time or setting consistent
stability requirements will lead to more consistent results

20. Digital gauges
 Higher resolution than analog gauges
 Equivalent and often better accuracy
 May require a better standard unless they are used interchangeably
 Lower internal volume, lack a bourdon tube and gear train
 Easier to clean out, dithering and exercise have lesser effects

21. Questions or Comments?
Email Christina Spearman cpearman@transcat.com
Transcat: 800-828-1470
www.transcat.com
For related product information, go to:
www.transcat.com/brand/fluke-calibration