Clocking the Meter. Episode eighty-two of the Gas Service Animated series.
There is a laboratory-grade flow instrument on the side of the house, and the customer thinks it is just the bill. Clocking the meter turns the utility's dial into your proof of appliance input â no tools inside the cabinet, just a stopwatch, arithmetic, and discipline. Today: the whole procedure, and the two traps that skew it.
First, the authority chain â because it matters on paper. The mandatory rule is section 11.1.1: burner input adjusted to the proper value per the manufacturer, and overfiring prohibited, full stop. The code recognizes exactly two ways to determine input: clock the meter, or read manifold pressure with the orifice size. The clocking procedure itself lives in annex A â nonmandatory guidance â dressed by decades of utility practice. Say it that way when it matters.
The setup is half the accuracy. Fire the test appliance to steady state at high fire. Shut off every other gas appliance in the building â every one. And remember the quiet thief: a standing pilot still registers, about a thousand Btu per hour of slow creep on a half-foot dial. Note the pilots you cannot kill, or clock long enough that they wash out in the average.
The formula: c f h equals thirty-six hundred, times the dial size, divided by the seconds per revolution. Test dials come in half, one, two, and five cubic feet per revolution. Watch it work: the two-foot dial turns once in thirty-six seconds. Thirty-six hundred times two is seventy-two hundred; divided by thirty-six â two hundred cubic feet per hour.
Cubic feet become Btu through the heating value. Two hundred c f h at one thousand Btu per cubic foot is two hundred thousand Btu per hour. But one thousand is the customary default, not a fact â the district heating value comes from the gas supplier, and it drifts with the gas. And never clock propane on the natural gas assumption: L P vapor runs about twenty-five hundred Btu per cubic foot â the same dial math understates input two and a half times.
Now the error math that separates a reading from a guess. One second of stopwatch slop on a single twenty-second revolution is five percent of the answer. Time three revolutions and average, and the same slop shrinks under two percent. Use the smallest dial the meter offers, count full revolutions, and let the average do the honest work.
The verdict step. Compare the clocked input to the nameplate. Two hundred thousand measured against a one ninety-nine nameplate â half a percent over, healthy. Meaningfully over nameplate is overfiring â prohibited by 11.1.1 â corrected per the manufacturer at the orifice or the manifold pressure, never by wishing. And at altitude the target itself moves: you clock against the derated input, not the sea-level plate. Episode eighty-seven finishes that math.
Trap two: meter pressure. The dial measures volume, and at elevated pressure each cubic foot holds more gas. Clock a two p s i meter with the low-pressure assumption and the annex quantifies your mistake: about thirteen percent overfired if the flow is not adjusted. The annex table's values are corrected for meter pressure â seven inches, eleven inches, fifty-five point four. Know which meter you are standing in front of before you trust the math.
Clocking in one breath. Steady state, high fire, everything else off, pilots noted. Thirty-six hundred times the dial, over the seconds â three revolutions, averaged. Times the real district heating value. Compared to the right target â nameplate at sea level, derated at altitude, pressure-corrected on elevated meters. Overfired means fix it, per the manufacturer. Next: pressure itself â reading it, converting it, and testing with it.