February 2004

Temperature is a Big Factor in Vehicle-Tank Meter Test 

By Dick Suiter 

 

Nearly all weights and measures officials are aware that a gallon of heating oil or 

gasoline is defined as having a volume of 231 in3

 at a reference temperature of 60 oF. If 

you measure a gallon of product at 60 oF and then vary the temperature, the volume of the 

product will expand as you increase the temperature above 60 oF and will contract as you 

reduce the temperature below 60 oF. However, if you change the temperature of the 

product before it is measured, you will still deliver a volume of 231 in3 for each indicated 

gallon, assuming there is no meter error. 

Now let’s consider the effect of temperature on a test measure or prover. Graduated 

neck-type volumetric field standards intended to be used for testing petroleum products 

are calibrated to deliver their nominal capacity at the same reference temperature of 

60 oF. If you vary the temperature of the product being introduced into the test vessel, the 

metal of the vessel will expand or contract as the temperature goes up or down. Thus, the 

actual volume of the vessel will increase as the temperature of the product increases 

above 60 oF or decrease if the product temperature is less than 60 oF. The amount of the 

change in volume is directly related to the cubical coefficient of thermal expansion for 

the type of metal used to construct the test vessel and is proportional to the nominal 

volume of the test vessel. 

The common metals used for construction of volumetric field standards are mild steel and 

stainless steel. The accepted cubical coefficient of thermal expansion for the metals used 

to construct volumetric field standards is 0.0000186/oF for mild steel and 0.0000265/oF 

for stainless steel. 

For example, if you have a stainless steel volumetric field standard, the volume of the 

standard in gallons will change by an amount equal to 0.0000265 times the nominal 

volume of the standard times the difference in the temperature of the product, in the 

standard, from 60 oF. If you want the result to be in cubic inches you merely add the 
additional step of multiplying the change in volume in gallons by 231. If you are testing 

a vehicle-tank meter using a 100-gallon stainless steel prover and the temperature of the 

product passing through the meter and coming from the nozzle of the delivery hose is 

80 oF, the change of volume of the prover is significant. The change is calculated as 

follows: 

0.0000265 H (80o ! 60o) H 100 gal H 231 or 12 in3

If the nominal capacity of the prover doubles to 200 gallons the change in volume at 

80 oF also doubles to 24 in3

. 

In the example above with a product temperature of 80 oF, if the reading in the sight glass 

of a 100-gallon prover is +65 in3 without consideration for temperature of the prover, that 

reading is within the maintenance tolerance of 0.3 gallons or 69.3 in3. At a prover 

temperature of 80 oF the metal of the prover has expanded by approximately 0.6 in3 for 

every degree of temperature change. Thus, the volume of the prover has in fact increased 

by 12 in3

, meaning that the level of the product in the sight glass has effectively been 

lowered by that amount. If the prover reading is corrected for the temperature difference, 

the true reading is now +77 in3

, meaning the meter error is greater than the applicable 

maintenance tolerance of 0.3 gallon or 69.3 in3

. 

As another example, consider a test of a vehicle-tank meter using a 200-gallon stainless 

steel prover where the product temperature is 35 oF. If the reading in the sight glass is 

plus 150 in3 without consideration for temperature, that reading is well beyond the 

allowable maintenance tolerance 0.6 gallon or 138.6 in3

. At a prover temperature of 

35 oF the change in volume of the prover is as follows: 

0.0000265 H (35E ! 60E) H 200 gal H 231 or ! 30.6 in3

The volume of the prover has in fact decreased by 30.6 in3, meaning that the level of the 

product in the sight glass has effectively been raised by that amount. If the prover 

reading is corrected for the temperature difference, the true reading is now +119.4 in3

, 

meaning the meter error is well within the applicable tolerance of 0.6 gallon or 138.6 in3. 

The impact of temperature correction becomes more significant when acceptance 

tolerances are applicable to the meter being tested. Regardless of whether maintenance 

or acceptance tolerances are applicable, it becomes readily apparent that neglecting to 

take the effect of temperature on the prover volume into account when using larger 

volumetric field standards to test meters, such as vehicle-tank meters or retail motor-fuel 

dispensers with a flow rate greater than 30 gpm, greatly increases the probability of 

inappropriately rejecting or approving meters. While it is not difficult to calculate the 

appropriate correction for a volumetric field standard at any product temperature, there 

are tables that have been developed for some of the more common size provers. It also is 

not difficult to develop a correction table for any size prover. 
If you have questions about availability of an existing correction table or developing a 

table for your prover size or anything else related to this article please contact Dick Suiter 

by e-mail at rsuiter@nist.gov or by phone at (301) 975-4406.