March 2006 

Meter Testing and Corrections – Corrections During Liquid 

Meter Testing 

By Tina Butcher 

When conducting tests of liquid-measuring devices such as loading-rack meters, LPG 

meters, and vehicle-tank meters, several corrections are used to account for the effects of 

influences such as pressure (in a closed vessel such as an LPG prover) and temperature. It 

is necessary for the weights and measures official to understand why these corrections 

must be made. 

Why Make Corrections? 

To ensure a fair test of any weighing or measuring device, it is important that no aspect of 

the test procedure used by the inspector or service agent unfairly (or unreasonably) 

affects the results of the test. Test standards must be accurate and appropriate. The 

Fundamental Considerations of NIST Handbook 44 specify that, when a standard is used 

without correction, its combined error and uncertainty must be less than one-third of the 

applicable device tolerance. Likewise, the procedures used by the inspector or service 

agent should not introduce significant errors into the test process. For example, it is 

important that an inspector set up equipment correctly, read a prover accurately, adhere to 

established drain times, and follow recognized test procedures. 

What Corrections Need to Be Made? 

Influences in effect at the time of the test must be also be accounted for. Temperature and 

pressure can affect measurement results, and corrections need to be made during some 

types of meter testing to account for the effects of these influences. Three corrections that 

should be considered, particularly during the testing of larger meters and LPG, are 

illustrated in the following diagram and explained in the text that follows. These 

corrections are particularly important for products such as LPG that have a high 

coefficient of expansion. 
(1) Correction for Product Temperature 

This correction accounts for a change in product volume resulting from a difference 

between the temperature of the product at the meter and the temperature of the product 

at the prover. 

Product expands as temperature increases and contracts as temperature decreases. For 

example, a given volume of product measured at 61 oF occupies more space than that 

same product measured at 59 oF or 60 oF as illustrated in Figure 2. 

The rate at which a product expands and contracts in response to temperature is referred 

to as its “coefficient of expansion.” For reference, the widely accepted coefficient of 

expansion for gasoline is 0.00069/°F and for diesel is 0.00050/°F. 

Consider an example where the meter measures the volume at a product temperature of 

59 °F. What happens if the temperature of the product rises to 61 °F as it travels between 

the meter and the prover? As the temperature rises, the product will expand, raising the 

product level to a higher volume in the prover than it would have been had the 

temperature remained the same as it was at the meter. To compare the volume indicated 

on the prover with the volume indicated on the meter without making any adjustments or 

corrections would not be a fair test since you would essentially be comparing a 59 °F 

volume with a 61 °F volume. Consequently, for most meter tests, a correction must be 

made to ensure that the prover readings and the meter readings are both based upon 

volumes at the same temperature. 

For a non-temperature compensated meter, there are two methods for accomplishing this 

correction. One is to adjust the product volume at the meter to its volume at 60 °F, adjust 

the volume at the prover to 60°F, and then compare the two adjusted volumes. The 

second method is to adjust the volume at the prover to the volume it would be if it were at 

the same temperature as the meter temperature and then compare the corrected value to 

the meter reading. Most NIST training courses on meters use the first method, which is 

illustrated in the example below. The American Petroleum Institute (API) publishes 
tables (including Table 6B, “Generalized products: Correction of Volume to 60 °F”) that 

list factors used to correct volumes to 60 °F, making this method reasonably quick and 

easy. To obtain a temperature and pressure volume correction factors CD for generalized 

crude oils, refined products, and lubricating oils, contact either API or ASTM 

International (originally known as American Society for Testing and Materials) at 

http://api-ec.api.org or http://astm.org, respectively. 

To adjust a volume to 60 °F using the API tables, first find the factor corresponding to 

the temperature of the product. Multiply the factor by the volume; the result is the 

“volume corrected to 60 °F.” That is, what the volume would be if its temperature were 

actually adjusted to 60 °F. 

Product Type: 

Gasoline with an API gravity of 60.0: 

Meter Indicated Volume: 200.0 gallons 

Product Temperature at Meter: 59 °F 

API Correction Factor for 59 EF: 1.0007 

Meter Volume Corrected to 60EF: 200.0 x 1.0007 = 200.14 gallons 

Prover Indicated Volume: 201.00 gallons 

Product Temperature at Prover: 61 °F 

API Correction Factor for 61 EF: 0.9993 

Prover Volume Corrected to 60EF: 201.0 x 0.9993 = 200.9 gallons 

The results of 200.14 gallons at the meter and 200.9 gallons at the prover can now be 

compared since both have been adjusted to the corresponding volumes at 60 °F. 

Note that it is important to use “matched” thermometers (thermometers that read the 

same temperature within a certain allowance) during this process to ensure there are no 

significant differences between the thermometer used at the meter and the thermometer 

used at the prover. 

(2) Correction for Prover Temperature 

This correction accounts for the change in prover volume due to the expansion or 

contraction of the prover with changes in temperature. 

Just like the product being metered expands or contracts with changes in temperature, the 

metal from which a prover is made will also expand or contract with temperature 

changes. Graduated neck-type volumetric field standards which are used to test meters 

measuring refined petroleum products are calibrated to deliver their nominal capacities at 

a reference temperature of 60 °F. If the temperature of the prover drops below 60 °F, the 

metal will contract, thus reducing the volume of the prover. If the temperature of the 

prover rises above 60 °F, the metal will expand, thereby increasing the volume of the 

prover. 

The rate at which the prover metal expands and contracts in response to temperature is 

referred to as its “cubical coefficient of expansion.” Provers are commonly constructed 
from stainless steel or mild steel. The coefficient of expansion for stainless steel is 2.65 x 

10-5 (or 0.0000265) and for mild steel is 1.86 x 10-5 (or 0.0000186). 

To correct for the effect of temperature on the prover metal, the following equation is 

used: 

Coefficient of Expansion x (Temperature of Prover – 60 °F) x Prover Nominal Capacity 

In the example above, for a 200-gallon prover at 61 °F, the correction would be 

calculated as follows if the prover were constructed of stainless steel: 

0.0000265 x (61-60) x 200 = 0.0053 gallons x 231 = 1.22 cubic inches 

This means that the volume of the prover has increased by 1.22 cubic inches at 61 oF. 

Thus, the reading in the prover would be 1.22 cubic inches less than it would be if the 

temperature were at the prover reference temperature of 60 °F. To apply the correction, 

1.22 cubic inches would be added to the prover reading of 201.00 gallons. 

Consider what would happen if the temperature were 81 °F. The effect is much more 

significant: 

0.0000265 x (81-60) x 200 = 0.1113 gallons x 231 in3/gal = 25.7 cubic inches 

In this case, the prover would have increased its capacity by 25.7 cubic inches and this 

amount would need to be added to the prover reading in order to account for the effect of 

temperature on the prover metal. 

For a more detailed description of this correction for vehicle-tank meter testing, see Dick 

Suiter’s February 2004 W&M Quarterly article, “Temperature is a Big Factor in VehicleTank Meter Test.” To access the article, go to the WMD website at www.nist.gov/owm; 

select “Weights and Measures Quarterly Newsletter Archive” under the “Publications” 

section and look for Series B-007 under the “Measuring” category. 

(3) Correction for Prover Pressure 

This correction accounts for the change in prover volume due to the expansion or 

contraction of the prover with changes in pressure. 

In a closed system such as that during LPG meter testing, pressure can affect 

measurement results. It is not necessary to make this correction to tests of “open” systems 

where pressure is not a factor, such as tests of vehicle-tank meters or loading rack meters 

delivering refined petroleum products which are liquids at atmospheric temperatures and 

pressures. 

Volumetric field standards which are used to test meters measuring liquefied petroleum 

gas and anhydrous ammonia are calibrated at a reference pressure of 100 pounds per 

square inch (psi). If the pressure of the prover rises above 100 psi, the prover will expand, 

thus increasing its volume from its calibrated volume. Likewise, if the pressure drops 

below 100 psi, the prover will contract, thus decreasing the volume from its calibrated 

volume. 
The rate at which this change occurs is not a linear function; the response of a particular 

prover to pressurization may be unique to that individual prover and is not predictable 

from its material or design. When a prover is calibrated, a pressure correction table is 

prepared, showing the appropriate corrections at various pressures. The correction for 

pressure is obtained from this chart and is added or subtracted from the prover reading 

according to the chart. 

Note that a large difference between the product pressure at the meter and the product 

pressure in the prover may indicate a restriction in the system. A difference greater than 

about 5 psi should prompt you to investigate and correct any system problems prior to 

continuing with the test. 

For questions about the information in this article, please contact Tina Butcher by 

telephone at 301-975-2196 or by e-mail at tbutcher@nist.gov.