November 2005 

Reading, Correction or Error? 

Issues in Volume Calibration 

By Georgia Harris 

Numerous round robins in every regional group have demonstrated that there is confusion among 

metrologists over whether a reported value is a gauge reading, a correction to be applied (in the 

lab or in the field), or an error on the prover. 

Gauge Reading: the observed value of the meniscus on the prover - without any corrections for 

temperature or delivery from the standard (e.g., no Z60 formula). Apositive gauge reading does 

NOT mean anything without additional information included such as the calibrated volume of 

the standard, whether the standard was set to nominal capacity before delivery, and what the 

temperatures are in the standard and unknown. 

Error: the value by which the prover deviates from the nominal volume. A positive error means 

the prover is large. A negative error means the prover is small. 

Correction: the value to be applied in use. A positive correction is the amount that is added to 

the gauge reading in field use. 

If there is confusion among the metrologists who are doing the calibrations, what kind of 

confusion must exist in meter testing applications when these values are provided to the users? 

Setting test measures and provers to nominal values at reference temperatures (and pressures in 

the case of LPG) can help avoid problems with use of errors and corrections, but only when used 

at the reference temperatures. Providing the full calibrated volume of the prover can help avoid 

problems and enable subsequent temperature corrections. 

Temperature corrections must be made to the prover volume in field applications when the 

provers are used! See the February 2004 article by Dick Suiter, on "Temperature is a Big Factor 

in Vehicle-Tank Meter Test" in the WMD Archives at: http://ts.nist.gov/ts/htdocs/230/235/newsletterarchive.htm. Dick provided the equations to be used to calculate the actual volume change 

of the prover and provided several examples of the volumetric impact in meter testing. The table 

and graph on the following page provides a simplified view of the impact for stainless steel and 

mild steel 100 gallon provers. 

For example, if a 100 gallon prover is larger than nominal and a meter delivers exactly 100 

gallons at 60 oF, the gauge reading on the prover will be lower than nominal and the correction 

for the prover needs to be considered. Further, if a stainless steel prover is larger than nominal by 

6 cubic inches at 60 oF, and the prover is then used at 50 oF, the prover is smaller at that 

temperature and is actually the nominal size of 100 gallons! 
Example: 100 gallon prover

CCE = cubical coefficient of expansion 

 Mild Steel Stainless Steel 

CCE 1.86 x 10-5/F 2.65 x 10-5/F 

Temp F cu in gal cu in Gal 

10 -21 -0.093 -31 -0.133 

20 -17 -0.074 -24 -0.106 

30 -13 -0.056 -18 -0.079 

40 -9 -0.037 -12 -0.053 

50 -4 -0.019 -6 -0.026 

60 0 0.000 0 0.000 

70 4 0.019 6 0.026 

80 9 0.037 12 0.053 

90 13 0.056 18 0.080 

100 17 0.074 24 0.106 

110 21 0.093 31 0.133 

120 26 0.112 37 0.159 

Recommendations: 

1. Test measures and provers should be adjusted to nominal values at 60 oF at calibration 

whenever possible. 
2. The actual calibrated volume (e.g., 100.001 gallons or 99.999 gallons) should be reported on 

all calibration reports to avoid confusion among terms of error and correction, and their use in 

meter testing. 

3. The temperature of the water at the time of calibration is also needed on the calibration report 

because temperature is such a critical factor for volume calibrations.