A new gas piping system is to be pressure tested at 10 psig. Take as given the rule that the test gauge must be selected so that the test pressure falls between 20 percent and 80 percent of the gauge's full scale range. Which gauge satisfies that rule?
- A 30 psi gauge
- BA 10 psi gauge, matching the test pressure exactly
A gauge whose full scale equals the test pressure puts the needle at the very top of the dial, where accuracy is poorest and where a small overpressure pegs it. It fails the 80 percent limit.
- CA 100 psi gauge, giving plenty of margin above the test pressure
At 100 psi full scale the test sits at 10 percent of the range, below the 20 percent floor. The needle barely moves off the peg and a real pressure loss can go unnoticed.
- DA 200 psi gauge, so that the needle can never be driven off the scale
A 200 psi gauge puts the test at 5 percent of scale, which is worse still. Protecting the gauge from being pegged is not the objective; reading a small change accurately is.
Why A is correct
Dividing the test pressure by the two limits gives the acceptable full scale range: 10 divided by 0.80 is 12.5 psi at the low end and 10 divided by 0.20 is 50 psi at the high end. A 30 psi gauge falls inside that band and places the needle near mid scale, which is where a gauge is most readable and most accurate.
What this question is testing
Whether the candidate can invert a percentage rule to find an acceptable range rather than testing options one at a time, and whether they understand why a test pressure must land in the middle of a gauge's range. It also checks whether they treat the gauge as part of the evidence a test produces rather than as a piece of hardware chosen for convenience.
On the job
Gauge selection is what turns a test from a formality into evidence. On a 10 psi test held for the required time, a 100 psi gauge simply will not show the small drop that reveals a weeping joint, and the inspector will say so. Keep a small gauge in the box for low pressure work and a larger one for high pressure tests, and label them, because the wrong one grabbed in a hurry can cost you a return trip.
Memory technique
Aim for the middle of the dial; a needle at the top or the bottom proves nothing.
Exam tip
Divide the test pressure by each percentage limit to get the acceptable full scale range, then pick a gauge inside it.
Where to look it up
Chapter 4 of the fuel gas code, the inspection and testing section covering test pressure, duration and test gauge requirements. Jurisdictions adopting NFPA 54 have parallel provisions, so confirm what your state has adopted.