S&T Committee 2014 Final Report

Appendix C – Item 330-4: N.4.2.5. Determination of Error on Wholesale Devices

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Appendix C 

NIST Handbook 44 – Liquid-Measuring Devices

Item 330-4: 

N.4.2.5. Determination of Error on Wholesale Devices with Multiple Flow Rates and 

Calibration Factors

How Slow Flow Accuracy Affects LMD’s

Because the legal tolerance on slow flow tests is so great (+/-0.5%) compared to industry standards (typically +/- 

0.05%), and because slow flow tests themselves are so time consuming, registered service agents may be tempted 

to skip slow flow tests entirely during seasonal re-calibrations. Even if one ignores the fact that the Liquid 

Measuring Device Code in NIST Handbook 44 requires that a special test be done at the slow flow rate, there 

remains a very good reason that slow flow rates should always be tested. If the error at the slow flow rate is 

unknown, then it is impossible to calibrate the high flow rates to deliver with the extreme accuracy sought by 

industry on quantities which are greater or less than the test prover used at the time of calibration.

Imagine a typical whole sale meter which is calibrated using a 1,000 gallon prover at a terminal where the 

customers’ trucks have pocket sizes between 1,000 and 4,000 gallons. The meter has an electronic register 

programmed with a slow flow rate for start-up and shut-down, a high-flow rate for typical deliveries, and a midspeed fallback rate for when the pumps can’t keep up with demand. Startup and shutdown deliveries are 100 

gallons each regardless of total quantity delivered. 

Now imagine that the service agent calibrating the meter didn’t check the slow flow rate and didn’t know that the 

meter was short five gallons on a one thousand gallon test. Instead, he calibrated the fallback and normal flow 

rates without testing the slow flow and introduced a linear error which increases the farther the transaction 

quantity deviates from the prover size. On a 1,000 gallon delivery the meter would appear to be accurate, but on a 

3,400 gallon delivery a three gallon error has been introduced. That is a 0.09% error which is almost twice the 

typical industry goal. 

When calibrating at the normal and fallback speeds, 

the meter registers 200 gallons of product for the 

startup and shutdown, but actually delivers only 

199 gallons. (99.5 gallons delivered for every 

100 gallons registered at slow speed.) If the service 

technician calibrates the meter to zero at normal and 

fallback rates, the meter will actually deliver 

801 gallons for every 800 gallons it registers at those 

rates.

Every subsequent delivery of 1000 gallons should 

receive exactly the right amount. Every delivery 

exceeding 1000 gallons will be ‘long’ and every 

delivery less than 1000 gallons will be short. 
S&T Committee 2014 Final Report

Appendix C – Item 330-4: N.4.2.5. Determination of Error on Wholesale Devices

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To determine the error on a typical delivery, the 

service agent needs to calculate the error 

introduced by the startup and shutdown gallons, 

and then the error introduced at the higher flow 

rates.

For a 3,400 gallon delivery in this example, the 

meter would register 100 gallons on startup but 

only deliver 99.5 gallons. It would then jump to 

normal rate and deliver 801 gallons for every 800 

gallons it registers until it goes 

into shutdown mode when it slows down and 

again delivers only 99.5 gallons of the 100 gallons 

it registers. Delivery error is +3 gallons (0.09%).

The math would be reversed if the meter had been 

five gallons long on a 1,000 gallon slow flow test at 

the startup and shutdown speed. The meter would 

deliver 100.5 gallons for every 100 gallons it 

registered at startup and shutdown, but only 799 

gallons for every 800 gallons registered at the normal 

delivery rate. The total delivery is 3 gallons (0.09%) 

short. Under-registration, which is favorable to 

consumers in most situations, can be detrimental to 

them when it occurs at the slow flow speed.

Does it matter considering that the error introduced is so much smaller than the tolerance allowed in the liquid 

measuring code? It does to industry, or they wouldn’t set such tight accuracy standards for themselves. And it 

does to Weights & Measures officials who must consider the predominant direction of error in addition to 

tolerance. Everyone’s time is wasted chasing extreme accuracy at the normal delivery rate if the accuracy of the 

startup and shutdown rate has been ignored.