S&T Committee 2014 Final Report

Appendix D – Item 331-1: N.4.2.1. Determination of Error on Vehicle-Tank Meters

S&T - D1 

Appendix D 

NIST Handbook 44 – Vehicle-Tank Meters

Item 331-1: 

N.4.2.1. Determination of Error on Vehicle-Tank Meters with Multiple Flow 

Rates and Calibration Factors

How Slow Flow Errors Affect VTM’s

Imagine a typical VTM which is calibrated using a 100 gallon prover for a bulk delivery company whose 

customers’ tanks are typically between 100 and 1,000 gallons. The meter has an electronic register 

programmed with a slow flow rate for start-up and shut-down, and a high-flow rate for typical 

deliveries. Startup and shutdown deliveries are 10 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 long 0.4 gallons on a 100 gallon test. Instead, he calibrated the normal flow 

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

transaction quantity deviates from the prover size. On a 100 gallon delivery the meter would appear to 

be accurate, but on a 500 gallon delivery a -0.4 gallon error has been introduced. That is within 

tolerance, but if all of his meters have similar errors in the same direction, typical deliveries will be in the 

operator’s favor at the expense of his customers.

When calibrating at the normal speed, the 

meter registers 20 gallons of product for the 

startup and shutdown, but actually delivers 

20.08 gallons. (10.04 gallons delivered for every 

10.00 gallons registered at slow speed.) If the 

service technician calibrates the meter to zero 

at normal speed, the meter will actually deliver 

79.92 gallons for every 80.00 gallons it registers 

at that flow rate.

Every subsequent delivery of 100 gallons should 

receive exactly the right amount. Every delivery 

exceeding 100 gallons will be ‘short’ and every 

delivery less than 100 gallons will be ‘long.’
S&T Committee 2014 Final Report

Appendix D – Item 331-1: N.4.2.1. Determination of Error on Vehicle-Tank Meters

S&T – D2

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 500 gallon delivery in this example, the 

meter would register 10 gallons on startup but 

actually deliver 10.04 gallons. It would then 

jump to normal rate and deliver 79.92 gallons 

for every 80 gallons it registers until it goes into 

shutdown mode when it slows down and again 

delivers 10.04 gallons as it registers only an 

additional 10 gallons.

The error would be well within maintenance tolerance so the Weights and Measures official need only 

be concerned if the slow flow errors on all the meters for a particular product are in the same direction. 

At that point, the official should determine the direction of the error on a typical deliver to determine if 

the equipment is being properly maintained. Device users can ensure they have no problems with this 

requirement by making sure that slow flow errors are not predominantly in one direction.