Pressure Test Guages 1

[Sharik]

This is a silly question but so many people here are worked up about it:

When doing a pressure test and using an analog pressure guage, the range of the guage is supposed to be 1.5 to 4 times the test pressure (basically so the test pressure is in the middle third of the guage range).

But if you are using a digital pressure guage would this standard still apply? Could you use a calibrated digital guage with a range of 0 - 600 psig (accurate to 2 decimal places) for a 15 psig pressure test?

 

[zeusfaber]

You need to be a bit careful of the difference between "accuracy" and "resolution" here.

A pressure gauge which reads 0-600 psi, but was (calibrated) accurate to 2 decimal places would need to be accurate to roughly a thousandth of a percent - which seems a bit too good to be true. More likely is that it can detect pressure changes of that sort of order, but may very well be quite wrong about what the true value of the underlying pressure is.

The specification at the back of your gauge's handbook and the calibration certificate should both give you a good idea of how accurate the gauge will be when measuring pressures of around 15 psi. You're probably looking for figures stated in terms similar to those on Page 3 of

http://docs-europe.electrocomponents.com/webdocs/064b/0900766b8064bae8.pdf.

Once you know what sort of errors to expect, you can check whether they lie within your required accuracy (which should be stated as part of the test process).

A.

 

[zdas04]

Another way of saying that is that digital gauges are certified to an accuracy of +/- some percent of full range. So a 0-600 psig gauge that is +/-1% is +/-6 psig. So if your test reads 15 psig, that means that your reading is certainly between 9 and 21 psig. Is that enough uncertainty for your test? Might be, probably isn't.

If you have a 0-50 psig 1% gauge then a 15 psig reading is 14.5-15.5 psig which is probably good enough.

I have a vacuum to 300 psig electronic test gauge (really expensive) that is certified to +/-0.15% so any reading has an uncertainty of +/- 0.47 psig (full range is -14.73 to +300 psig) and it always gives me decent, repeatable data.

David

 

[GenB]

Codewise.
Read the Code careful. I think there is a clause where you can use digital gauges and the rules are quite different than std analog gauges
I think you are okay using the larger range. I will read it too.

 

[AIOR]

Hi,

Shariku ot Bobika.

This is not a “Newton Binomial problem”. Per UG-102 (b) a rule about a gage range “not less than 1.5 not more than 4 times the test pressure” is only applicable to a dial pressure gages. For digital pressure gages a wider range is allowed.

Thanks

 

[Sharik]

Thanks for the replies. I do know about ASME Section VIII, Div. 1, UG-102(b) but it only states a larger range is permissible for digital guages but it doesn't state any upper limits. The accuracy range (+/-) can make a large range guage useless for a low pressure test. I was hoping someone would have a reasonable range for digital guages they were using.

 

[JR97]

Sharik,

At our company we impose the ASME Section III requirement. Specifically NB-6412(b)which requires digital pressure gages have combined caliberation and readability error less than 1% of test pressure. The important portion of the requirement is 1% of test pressure. Since all gages are rated by the manufacturer as an accuracy at full scale, e.g. 1% FS; the lower end of the useability is defined.

For example, if a 1000 psi gage with acuracy of .25% FS is purchased, all readings on the gage will be within 2.5 psi of actual (theoretically). The 2.5 psi accuracy would be 1% of the test pressure or better for tests at 250 psi or higher. Therefore, a 1000 psi digital gage could be used for tests exceeding 250 psi.

Hope this helps,

JR97