Article Ref: H-016 

Weights and Measures Connection, Vol. 6, Issue 3 Page 1

March 26, 2015

Draining Test Measures and Provers

Byline: G. Diane Lee

I often think about the amount of money spent on gasoline per year just to get to and from work. Like 

many, I travel a considerable distance to work, which accounts for a lot of gas and money per year. It is 

reassuring to know that I can rely on the work of weights and measures officials who inspect and test 

gasoline dispensers to ensure accurate measurements. Throughout the United States, thousands of 

volume standards are calibrated in weights and measures laboratories every year. Weights and 

measures inspectors then use these standards to test gasoline dispensers to provide consumers 

assurance that they are getting what they pay for at the pump. These tests also protect device owners 

and ensure device owners are receiving fair payment. The accuracy of these tests depends on the 

inspector following the correct test procedures. One very important procedure in the testing of gasoline 

dispensers is that of draining test measures and provers. Incorrectly draining a test measure or prover 

can contribute a considerable amount of error in testing. This article reviews the correct procedures for 

draining five-gallon test measures and bottom-drain provers; answers frequently asked questions 

concerning these procedures; and provides information on additional training tools available on this 

topic. 

PROCEDURES FOR DRAINING TEST MEASURES AND PROVERS

NIST Handbook 44 “Specifications, Tolerances, and Other Technical Requirements for Weighing and 

Measuring Devices” Section 3.30. Liquid Measuring Devices, paragraphs N.4.4.1. and N.4.4.2. addresses 

the procedures for draining test measures and provers.

N.4.4.1. Pour and Drain Times for Hand-held Test Measures. – Hand-held test measures 

require a 30 s (± 5 s) pour followed by a 10 s drain with the measure held at a 10° degree to 15°

angle from vertical.

N.4.4.2. Drain Times for Bottom Drain Test Measures or Provers. – Bottom drain field 

standard provers require a 30 s drain time after main flow cessation.

When draining a hand-held test measure you must pour the product back to storage by first touching 

the neck of the test measure to the neck of a metal funnel. A plastic funnel may cause a static electric 

charge and must not be used to return product back to storage. The contents must be poured slow 

enough so that the main flow stops after 30 s ± 5 s. Then, the remaining liquid is drained for 10 s at a 

10° to 15° angle from vertical.

When draining a five-gallon truck mounted prover, the truck is typically grounded to the dispenser 

before testing begins. Some jurisdictions also ground the prover to the storage tank on the truck. You 

should check with your organization’s safety officer or other expert to ensure that equipment is properly 

grounded. The release valve on the prover is opened and the product is drained from the prover to the 
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storage tank below. Observe the sight glass as the liquid drains from the prover. After the main flow 

stops, the remaining liquid is allowed to drain for 30 seconds and the valve is closed.

Although these procedures may not seem difficult to a more seasoned inspector, no matter what your 

experience level, the following frequently asked questions may provide new information or confirm or 

enhance your existing knowledge of draining test measures and provers. 

Frequently Asked Questions:

What is a test measure? What is a prover?

Both a test measure and prover are volumetric standards used to test gasoline dispensers and other 

liquid measuring devices. Requirements for these field standards are included in NIST 105-3, 

“Specifications and Tolerances for Graduated Neck-Type Volumetric Field Standards.“ 

“Test measures” are small, hand-held volumetric measures that are typically 20 L (5 gal) or smaller in 

size. See Figure 1 - Test Measure.

“Provers” are bottom-drain volumetric measures and may be free standing or permanently mounted on 

a truck, platform, or trailer that are typically 20 L (5 gal) or larger. See Figure 2 - Prover. 

Why is it important to follow proper drain procedures?

When liquid is drained from a test measure or prover, a residual amount of liquid will remain clinging to 

the walls. It is impractical to dry the inside of the test standard between uses; thus, when a standard is 

calibrated, it is calibrated “to deliver” its content. A “to deliver” calibration means that the standard is 

calibrated to compensate for this clingage so that tests can be run one after another without having to 

take time out to thoroughly dry the standard before each test. Using proper drain procedures ensures 

that each time you perform an accuracy test on a gasoline dispenser, you start with the same amount of 
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Weights and Measures Connection, Vol. 6, Issue 3 Page 3

liquid clinging to the inside of the standard between each test and helps to ensure accurate 

measurements from test to test. 

If the inside of a standard is dry, the standard must be wet down (e.g., fill the standard to capacity and 

follow the draining procedures in this article) before using it to test gasoline dispensers. During periods 

of non-use, such as, driving from one location to another location for testing or breaking for lunch while 

testing at one location, care must be taken to ensure that testing resumes with a standard that has been 

properly wet down. 

When draining a test measure, how do I hold the test measure at a 10 to 15 degree angle? 

One way to determine if you are holding the test measure at a 10° to 15° angle from vertical when 

draining it is to think of the arm that you use to hold the handle of the test measure as your vertical line 

(a line that extends up and down) (See Figure 3). This arm makes a 90° angle with the ground (See 

figure 3 and 4). Holding the bottom of the test measure with your other hand, adjust the test measure 

to the 10 degree to 15° angle from vertical as shown in Figures 3 and 5. 

Figure 5 – Test 

Measure at a 10° 

Angle

Figure 3 – Inspector Draining a Test 

Measure at a 10° to 1`5° Angle.
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Weights and Measures Connection, Vol. 6, Issue 3 Page 4

Figure 6 – Inspector Draining a Prover

How do I correctly measure the drain times for test measures and provers?

When draining test measures or provers, inspectors must use a stop watch or other timing device with a 

second indicator to determine the correct drain time. 

Test measures must be emptied by pouring the 

contents slowly enough that the main flow stops 

after 30 s and then drained for 10 s at a 10° to 15° 

angle. While it may be difficult to look at a stop 

watch or watch with a second hand or digital 

indicator while pouring the contents of the test 

measure, an option is to wear the stop watch 

around the neck and periodically look down at the 

stopwatch to determine if you need to adjust the 

timing of your pour. A watch with a second hand or 

indicator worn on the arm used to hold the handle of the test measure could also be used. Turn the 

watch so that you can see the second hand or indicator as you pour and then drain liquid from the test 

measure. 

On a bottom-drain prover, open the drain valve with one hand and hold the timing device in the other 

hand. After the main flow stops, begin timing the drain for 30 s. See Figure 6 - Inspector draining a 

prover. 

Training Tools 

The NIST, Office of Weights and Measures (OWM) provides other training tools on draining test 

measures and provers. You may visit the weights and measures contacts system at 

https://tsapps.nist.gov/WMD/default.aspx to locate webinars being provided on this topic. Additionally, 

a retail motor fuel dispenser inspection video and shorter videos including this topic are available on our 

website at http://www.nist.gov/pml/wmd/. 

Incorrectly draining a test measure or prover can contribute a considerable amount of error in testing. 

Following the test procedures in this article will help to ensure that accurate test results are obtained 

when testing gasoline dispensers and help to ensure accurate commercial measurement transactions.

For additional information on draining test measures and provers, contact Diane Lee at (301) 975-4405.