Product loss during retail motor fuel dispenser inspection

Table of contents


By: Christian Lachance, P. Eng.
Senior Engineer - Liquid Measurement
Engineering and Laboratory Services
Measurement Canada

Date:

Introduction

Retail motor fuel dispensers have traditionally been calibrated and inspected with open-neck 20 L test measures. The introduction of new testing methods and equipment has resulted in reports of discrepancies between these new methods and the traditional testing method. The new methods include closed-loop proving equipment and testing on low-volatility substitute liquids.

Previous investigations of the accuracy of the traditional test measure indicated that fuel evaporation during testing may be a significant contributing factor in the uncertainty surrounding this test method when used with gasoline.

This investigation was launched to determine the effect of fuel evaporation under a wide range of proving conditions that are typically encountered during field testing. The investigation also provides an analysis of different proving equipment items and method bias.

Test method

A small bidirectional 20 L pipe prover was used to deliver a known amount of gasoline through a dispenser hose and nozzle into a traditional 20 L test measure. The test was repeated with a vapour retention prover and a calibration cart prover as a comparative study and investigation into alternative methods of proving. The difference between the liquid volume measured with the test equipment and the liquid volume delivered by the pipe prover was then used to estimate liquid product loss/evaporation during testing.

The diagram below illustrates the test setup.

Figure 1. Evaporative loss study test setup.

Evaporative loss study test setup, as described below.

Five test runs were conducted at both low flow rate and high flow rate for each item of equipment under test. Flow rates of approximately 15 Lpm and 30 Lpm were typical of the low and high rates obtained. The average flowing temperature at the prover, prover pressure, product temperature in the test equipment, and the test equipment readings were recorded.

Testing was conducted on six different occasions over a period of seven months in order to assess the effects of varying ambient conditions and fuel properties. Samples of the fuel were taken before and after each series of tests and sent to a laboratory for Reid vapour pressure (RVP) measurement.

In order to understand and analyse the results, it is necessary to become familiar with the various sources of error when using the test measure and the pipe prover system. The following is a listing of the significant sources of error for the two systems.

Test measure

  • Calibration uncertainty
  • Liquid temperature change between meter and test measure
  • Liquid pressure change between meter and test measure
  • Test measure thermal expansion
  • Loss product during transfer
  • Reading error
  • Wetting variance due to different product characteristics
  • Drift caused by damage to the body of the test measure

Pipe prover

  • Calibration uncertainty
  • Liquid temperature change between meter and pipe prover
  • Liquid pressure change between meter and pipe prover
  • Pressure and temperature effects on the prover
  • Seal failure
  • Variations in connecting volume
  • Repeatability of the piston travel distance

Since the aim of this study was to assess the effects of evaporation, some of the above factors were eliminated or minimized through corrections.

All test equipment was calibrated directly against the pipe prover hose and nozzle assembly using water. This step was conducted to minimize calibration bias between the pipe prover and test equipment to less than ±5 mL.

Corrections were applied for liquid pressure and thermal expansion effects. Corrections were also applied for the effects of pressure and temperature on the proving equipment. The accuracy of these corrections is estimated to be approximately ±5 mL.

A few factors are random in nature, so they do not contribute significantly since the average of multiple runs was taken. These factors are reading errors by the test equipment operator (including resolution errors) and repeatability of the pipe prover.

The pipe prover seal integrity was verified by a leak test. Variations in connecting volume are addressed through proper procedure and minimizing the connecting volume.

The test equipment used in the study is very stable, and stability can be ensured by visual inspection.

The other factors that can contribute to the difference are product loss during transfer and, to a lesser extent, the wetting effect due to the use of a liquid other than the calibration liquid.

Equipment

Pipe prover

The pipe prover is a bidirectional design of 20 L. There is no pre- or post-run travel length, since the volume is defined between the end-of-travel positions. Forward and reverse flow direction is accomplished through the operation of the 4-way valve. The integrity of the piston seal and 4-way valve seal was verified by a leak test before each series of tests.

Flowing product temperature and pressure is taken at the inlet and outlet. A proving run is conducted by first circulating product until temperature is stabilized. Stability is assumed when the inlet and outlet are within 0.3 °C and when the outlet temperature is stable.

The volume of the prover is corrected for the temperature effect of the steel. Product temperature and pressure at the prover are measured so as to allow for liquid compressibility and thermal expansion corrections. The temperature of the liquid inside the prover is taken as the average flowing product temperature at the outlet during a run. The product pressure inside the prover is taken from the product pressure at the inlet before the run is initiated.

The prover was fitted with a standard dispenser hose and nozzle assembly. The nozzle was not fitted with a splashguard.

Test measure

A traditional 4 inch diameter neck stainless steel test measure was used for the testing. In order to increase the resolution of this equipment, it was fitted with a removable plunger/displacer for the neck.

For all tests, the test measure product temperature was measured with an immersion probe after the run. This temperature measurement was used for the test measure steel expansion correction and the liquid thermal expansion correction.

No precautions were taken to minimize splashing or vapour loss during the test.

Vapour retention prover

The vapour retention prover (VRP) design consists of a 20 L, 2 inch neck prover with the drain piped to a reservoir. The neck is capped and has a vapour line running to the reservoir. A bellows is added to the reservoir. When the prover is drained, air is taken from the reservoir and bellows. When the prover is filled, displaced vapours fill the bellows. This equipment is essentially sealed and provides an environment where the air is saturated with fuel vapour. The saturated environment is considered to significantly reduce product evaporation.

Product temperature is measured from a permanently mounted thermometer with the probe directly immersed in the liquid.

Calibration cart

This is essentially the same design as above but with a vent valve instead of bellows, so fresh air in introduced via the vent valve when the reservoir is emptied.

Calculations

The volume of liquid delivered by the pipe prover is obtained from the following:

V prover = V base prover × cts prover

Where:

  • Vprover = volume of liquid delivered by pipe prover at prover temperature (Tprover) and prover pressure,
  • Vbase prover = volume of pipe prover at reference temperature (15 °C),
  • ctsprover = correction for prover steel expansion due to prover temperature.

The volume of liquid measured by the test measure, taking into account:

  • the temperature effect of the steel of the test equipment,
  • the liquid thermal expansion from the prover to the test equipment, and
  • the liquid expansion due to the pressure drop from the pipe prover to the test equipment.

was obtained from the following:

V tm = ((V base tm + Reading) × cts tm × (ctl tm ÷ ctl prover)) ÷ cpl prover

Where:

  • VTM = test equipment volume measurement of the liquid delivered by the pipe prover corrected to prover temperature and pressure,
  • VbaseTM = to deliver volume (water) of test equipment at reference temperature corrected for bias with pipe prover,
  • ctlTM ÷ ctlProver = correction for liquid expansion due to temperature difference from the pipe prover to the test equipment,
  • ctsTM = correction for test equipment steel expansion due to thermal effects,
  • cplProver = correction for liquid expansion due to pressure drop from the pipe prover to the test equipment.

The difference between the test equipment liquid volume measurement, with all corrections applied, and the calculated liquid volume delivered by the pipe prover is obtained with:

Difference compared with pipe prover after all corrections = VProver − VTM

This value represents the combined effect of vapour loss and test equipment wetting error.

The difference between the test equipment liquid volume measurement, before any corrections, and the calculated liquid volume delivered by the pipe prover is obtained with:

Difference compared with pipe prover before corrections = VProver − (VbaseTM+ Reading)

This value represents the test equipment error when no corrections are applied to the test equipment reading.

For the purposes of the analysis, the values for the individual corrections were calculated as:

  • Test equipment steel correction = (VbaseTM + Reading) × ctsTM
  • Liquid thermal expansion correction = (VbaseTM + Reading) × ctlTM ÷ ctlProver
  • Liquid expansion due to pressure drop = (VbaseTM + Reading) ÷ cplProver

Since the correction values are small relative to the measurement value, an alternate and approximate method of calculating the test equipment volume is as follows:

VTM ≈ (VbaseTM+ Reading) + test equipment steel correction + liquid thermal expansion correction + liquid expansion due to pressure drop

Vapour pressure

The fuel vapour pressure at proving conditions was calculated from the model provided in API Manual Petroleum Measurement Standards, chapter 19.4, Appendix B using a value of s = 3 for the slope of the ASTM distillation curve at 10% evaporated, in degrees F per percentage point.

The following graph provides the results of this model for the range of gasoline encountered during the study. The product vapour pressure was lowest in the summer at 50 RVP and highest in the winter at 110 RVP.

Figure 2. True vapour pressure of refined petroleum stocks.

True Vapour Pressure of Refined Petroleum Stocks. Data are available in the table below.
Description of Figure 1
Data for Figure 2—True vapour pressure of Refined Petroleum Stocks.
Vapour Pressure(kPa) Temperature (°C)
RVP = 50
26.0 16.7
12.6 −2
13.7 0
14.9 2
16.1 4
17.4 6
18.8 8
20.3 10
21.9 12
23.6 14
25.4 16
27.3 18
29.3 20
31.4 22
33.7 24
36.1 26
38.6 28
41.3 30
44.1 32
47.0 34
RVP = 80
16.2 −10
17.6 −8
19.0 −6
20.6 −4
22.3 −2
24.1 0
26.0 2
28.0 4
30.1 6
32.4 8
34.8 10
37.3 12
40.0 14
42.9 16
45.9 18
49.0 20
52.4 22
55.9 24
59.6 26
RVP = 110
16.0 −20
17.5 −18
19.0 −16
20.6 −14
22.3 −12
24.1 −10
26.1 −8
28.2 −6
30.4 −4
32.7 −2
35.2 0
37.9 2
40.7 4
43.6 6
46.7 8
50.1 10
53.5 12
57.2 14
61.1 16
65.2 18

Results

The "Volume difference before corrections" values represent the test equipment error when no corrections are applied to the test equipment reading.

The "Volume difference after all corrections" values represent the combined effect of vapour loss and test equipment wetting error.

The "Equipment steel temp. correction" is the correction for test equipment steel expansion due to thermal effects.

The "Liquid Temperature Correction" is the correction for liquid expansion due to the temperature difference between the pipe prover and the test equipment.

The "Liquid pressure correction" is the correction for liquid expansion due to the pressure drop from the pipe prover to the test equipment.

All values are the average of 5 runs of 20L test quantity.

Table 1. Results for the August 23, 2005 test.
Test equipment Flow Pipe prover temp. (°C) Pipe prover pressure (kPa) Volume difference before corrections (mL) Equipment steel temp. correction (mL) Liquid temp. correction (mL) Liquid pressure correction (mL) Volume difference after all corrections (mL) Liquid vapour pressure (kPa)
Test measure high 29.2 257.8 −62.1 13.1 13.5 −6.6 −41.2 42.6
Calibration cart high 29.3 261.3 −26.1 15.2 −8.7 −6.7 −26.1 40.9
VRP high 30.7 256.5 −30.0 16.6 −6.7 −6.7 −26.5 42.6
Test measure low 27.3 95.8 −63.0 11.4 10.1 −2.4 −43.1 40.2
Calibration cart low 25.8 94.4 −44.4 11.0 3.9 −2.4 −31.4 35.6
VRP low 25.3 114.4 −29.6 10.7 −0.3 −2.8 −21.8 35.3
Table 2. Results for the December 7, 2005 test.
Test equipment Flow Pipe prover temp. (°C) Pipe prover pressure (kPa) Volume difference before corrections (mL) Equipment steel temp. correction (mL) Liquid temp. correction (mL) Liquid pressure correction (mL) Volume difference after all corrections (mL) Liquid vapour pressure (kPa)
Test measure high −5.0 259.2 −7.6 −19.6 12.3 −5.0 −20.7 26.8
Calibration cart high −4.8 250.9 5.8 −20.7 4.1 −4.9 −16.2 27.9
VRP high −2.4 262.0 −9.8 −18.2 2.7 −5.2 −28.2 30.0
Test measure low −6.0 99.3 −8.1 −20.7 16.9 −1.9 −14.6 25.6
Calibration cart low −3.8 121.3 14.4 −19.4 −4.1 −2.4 −11.6 29.3
VRP low −3.0 126.8 4.8 −18.7 1.4 −2.5 −15.4 29.4
Table 3. Results for the February 15, 2006 test.
Test equipment Flow Pipe prover temp. (°C) Pipe prover pressure (kPa) Volume difference before corrections (mL) Equipment steel temp. correction (mL) Liquid temp. correction (mL) Liquid pressure correction (mL) Volume difference after all corrections (mL) Liquid vapour pressure (kPa)
Test measure high 3.6 300.6 −35.0 −11.3 10.7 −6.3 −42.5 36.5
Calibration cart high 4.0 303.3 −5.3 −11.6 5.0 −6.4 −18.6 37.4
VRP high 6.2 296.4 −7.2 −9.1 0.8 −6.3 −22.1 40.7
Test measure low 2.6 137.9 −21.5 −12.1 6.3 −2.9 −30.7 35.4
Calibration cart low 4.0 131.0 −12.9 −11.4 1.1 −2.7 −26.3 37.6
VRP low 5.2 137.9 −4.7 −10.1 −1.9 −2.9 −19.9 39.3
Table 4. Results for the January 11, 2006 test.
Test equipment Flow Pipe prover temp. (°C) Pipe prover pressure (kPa) Volume difference before corrections (mL) Equipment steel temp. correction (mL) Liquid temp. correction (mL) Liquid pressure correction (mL) Volume difference after all corrections (mL) Liquid vapour pressure (kPa)
Test measure high 8.2 289.5 −40.1 −6.8 10.5 −6.3 −43.1 42.6
Calibration cart high 8.4 292.3 −7.2 −7.2 7.5 −6.3 −13.4 43.0
VRP high 8.7 289.5 −10.4 −6.6 2.8 −6.3 −20.7 43.8
Test measure low 5.7 124.1 −30.0 −9.0 3.6 −2.6 −38.6 39.3
Calibration cart low 3.5 128.2 4.5 −11.8 −2.8 −2.7 −12.9 36.7
VRP low 3.9 136.5 4.7 −11.3 −4.7 −2.9 −14.3 37.3
Table 5. Results for the January 25, 2006 test.
Test equipment Flow Pipe prover temp. (°C) Pipe prover pressure (kPa) Volume difference before corrections (mL) Equipment steel temp. correction (mL) Liquid temp. correction (mL) Liquid pressure correction (mL) Volume difference after all corrections (mL) Liquid vapour pressure (kPa)
Test measure high 1.2 295.1 −38.4 −13.6 12.4 −6.0 −46.7 32.7
Calibration cart high −0.1 308.9 −6.5 −16.0 8.2 −6.3 −21.1 31.3
VRP high 0.1 297.8 −5.1 −15.6 4.4 −6.0 −22.9 31.7
Test measure low 0.9 142.0 −44.8 −14.0 13.5 −2.9 −49.3 32.2
Calibration cart low −0.3 140.6 −7.0 −16.3 11.0 −2.8 −15.7 30.9
VRP low −0.8 159.9 −6.8 −16.5 4.4 −3.2 −22.6 30.7
Table 6. Results for the May 25, 2005 test.
Test equipment Flow Pipe prover temp. (°C) Pipe prover pressure (kPa) Volume difference before corrections (mL) Equipment steel temp. correction (mL) Liquid temp. correction (mL) Liquid pressure correction (mL) Volume difference after all corrections (mL) Liquid vapour pressure (kPa)
Test measure high 30.8 383.8 −62.9 16.2 3.3 −10.0 −52.3 42.2
Calibration cart high 31.6 358.5 −46.6 15.7 5.3 −9.4 −34.3 43.3
VRP high 31.5 255.1 −25.8 17.1 −0.5 −6.7 −15.5 43.4
Test measure low 23.1 146.5 −39.1 8.5 −1.7 −3.6 −35.6 32.8
Test measure low 20.9 146.5 −22.0 6.7 −12.6 −3.5 −31.3 30.8
Calibration cart low 24.7 159.9 −14.8 9.6 −7.3 −4.0 −16.4 34.9
Calibration cart low 26.6 125.8 −31.5 11.1 −0.7 −3.2 −24.0 36.9
VRP low 26.3 164.6 −21.6 11.8 −2.8 −4.1 −16.6 36.6
VRP low 23.9 126.8 0.8 9.8 −14.6 −3.1 −7.2 34.3

Discussion

Product temperature range

The results show a very wide range of liquid temperatures, −5 °C to 30 °C, which was due to the use of an above-ground fuel storage tank. The temperature range is therefore more representative of extreme field conditions as opposed to typical field conditions.

Product vapour pressure

The range of product RVP varied from 50 to 110 RVP and the product vapour pressure during testing was between 25 and 45 kPa. Since fuel is normally formulated with low RVP in summer and high RVP in winter, it is expected that low product vapour pressure will be encountered when the fuel storage temperature will be low relative to ambient temperature. We also expect to see the reverse high vapour pressure when the product storage temperature is high relative to ambient temperature.

Steel thermal expansion correction

The most significant correction in this study was the correction for equipment steel expansion. This effect is approximately 1 mL per °C when 304 stainless steel is used. It ranged from −20 mL to 17 mL, and was due to the extreme product temperature range experienced in this study. We would expect smaller variations in typical field conditions.

Liquid temperature correction

The correction for liquid temperature change between the pipe prover and the test equipment is approximately 2.5 mL per 0.1 °C difference for gasoline. The two factors believed to influence the temperature differential are ambient/product temperature differences and evaporative cooling.

It is estimated that the evaporation of 40 mL of fuel is equivalent to a temperature drop of 0.25 °C on 20 L of fuel. In practice, however, the temperature drop will be less because not all cooling heat is transferred to the liquid. For the majority of tests, the fuel temperature was very close to the ambient temperature. As a result, the effects of ambient/fuel temperature differences could not be analyzed.

The test measure liquid temperature correction averaged about 10 mL. The calibration cart and VRP showed correction values averaging just above 0. This supports the assumption that higher evaporation rates in the test measure will result in greater evaporative cooling of the product. However, there was no significant correlation between the amount of temperature correction and product VP. It should be noted that the accuracy of the temperature measurement is approximately ±0.2 °C, which is equivalent to a ±5 mL correction.

Liquid pressure effect correction

The pressure inside the pipe prover was approximately 250 kPa (35 psi) at high flow and 125 kPa (18 psi) at low flow, resulting in a small expansion of the liquid as it was transferred to the ambient pressure in the test equipment. The expansion for 20 L was approximately 6 mL at high flow and 2 mL at low flow. Similar expansion would occur during dispenser testing, depending on the metering pressure.

Difference in volume after all corrections

This graph shows the results of the study in terms of volume measurement difference between the pipe prover and the equipment used. Each point is the average of five runs.

Figure 3. Difference in measured volume after all corrections.

Difference in Measured Volume After All Corrections. Data are available in the table below.
Description of Figure 3
Data for Figure 3—Difference in Measured Volume After All Corrections.
Difference (mL) Product vapour pressure (kPa)
Test measure high flow
−41.2 42.6
−20.7 26.8
−42.5 36.5
−43.1 42.6
−46.7 32.7
−52.3 42.2
Calibration cart high flow
−26.1 40.9
−34.3 43.3
−16.2 27.9
−18.6 37.4
−13.4 43.0
−21.1 31.3
VRP high flow
−26.5 42.6
−28.2 30.0
−22.1 40.7
−22.9 31.7
−20.7 43.8
−15.5 43.4
Test measure low flow
−43.1 40.2
−14.6 25.6
−30.7 35.4
−38.6 39.3
−49.3 32.2
−35.6 32.8
−31.3 30.8
Calibration cart low flow
−31.4 35.6
−16.4 34.9
−24.0 36.9
−11.6 29.3
−26.3 37.6
−12.9 36.7
−15.7 30.9
VRP low flow
−21.8 35.3
−15.4 29.4
−19.9 39.3
−22.6 30.7
−14.3 37.3
−16.6 36.6
−7.2 34.3

Vapour pressure effect, TM

The graph shows a close correlation between the measured volume difference for the test measure and the product vapour pressure during testing. As expected, the difference increases with product vapour pressure. This is consistent with the assumption that vapour loss is the main contributor with a regular test measure, as higher product vapour pressure will induce greater evaporation rates.

Vapour pressure effect, VRP and calibration cart

Both the calibration cart and the VRP showed a relatively consistent difference of about 20 mL in volume measurement. As seen in the next graph, the average bias for the VRP based on the last two runs is approximately 15 mL. This would seem to indicate that this prover is sensitive to any air entrained when the reservoir is drained and perhaps some conditioning of the air in the prover.

Other than the liquid pressure expansion from the pipe prover to the test equipment (6 mL at high flow), the cause of this bias is not known but is expected to be due to a combination of:

  • a small amount of evaporation and perhaps some atomization during transfer,
  • wetting effects,
  • bias errors in the study.

Wetting effect is caused by the variance in the amount of residue left in the "to deliver" test equipment when a product other than water is used.

Figure 4. Difference in measured volume vs product VP (VPR values based on last two runs only).

Difference in Measured Volume vs Product VP (VPR values based on last two runs only). Data are available in the table below.
Description of Figure 4
Data for Figure 4—Difference in Measured Volume vs Product VP (VRP values based on last two runs only).
Difference (mL) Product vapour pressure (kPa)
TM high flow
−41.2 42.6
−20.7 26.8
−42.5 36.5
−43.1 42.6
−46.7 32.7
−52.3 42.2
Calibration cart high flow
−26.1 40.9
−34.3 43.3
−16.2 27.9
−18.6 37.4
−13.4 43.0
−21.1 31.3
TM low flow
−43.1 40.2
−14.6 25.6
−30.7 35.4
−38.6 39.3
−49.3 32.2
−35.6 32.8
−31.3 30.8
Calibration cart low flow
−31.4 35.6
−16.4 34.9
−24.0 36.9
−11.6 29.3
−26.3 37.6
−12.9 36.7
−15.7 30.9
VRP high flow
−13.5 42.8
−27.6 30.2
−24.5 40.9
−20.9 31.8
−21.4 43.6
−14.7 43.4
VRP low flow
−12.0 35.4
−11.5 29.5
−16.9 39.4
−23.6 30.7
−10.8 37.5
−8.2 35.5

Volume difference before corrections

The following graph shows the results of the measured volume difference before corrections. This is somewhat representative of the expected bias between closed-loop proving and proving using the test equipment under evaluation.

Figure 5. Difference in measured volume before corrections.

Difference in Measured Volume Before Corrections. Data are available in the table below.
Description of Figure 5
Data for Figure 5—Difference in Measured Volume Before Corrections.
Difference (mL) Product vapour pressure (kPa)
TM high flow
−62.1 42.6
−7.6 26.8
−35.0 36.5
−40.1 42.6
−38.4 32.7
−62.9 42.2
Calibration cart high flow
−26.1 40.9
−46.6 43.3
5.8 27.9
−5.3 37.4
−7.2 43.0
−6.5 31.3
VRP high flow
−30.0 42.6
−9.8 30.0
−7.2 40.7
−5.1 31.7
−10.4 43.8
−25.8 43.4
TM low flow
−63.0 40.2
−8.1 25.6
−21.5 35.4
−30.0 39.3
−44.8 32.2
−39.1 32.8
−22.0 30.8
Calibration cart low flow
−44.4 35.6
−14.8 34.9
−31.5 36.9
14.4 29.3
−12.9 37.6
4.5 36.7
−7.0 30.9
VRP low flow
−29.6 35.3
4.8 29.4
−4.7 39.3
−6.8 30.7
4.7 37.3
−21.6 36.6
0.8 34.3

As expected, the spread of results is wider, about 15 mL to −65 mL, compared with the range of −10 mL to −50 mL for the corrected results. A smaller range of results would be expected in typical field conditions, since the product temperature range and resultant steel expansion effects would be lesser. This is demonstrated in the following graph, with only the steel thermal correction added.

Figure 6. Difference in measured volume vs product VP (Test equipment only corrected for steel thermal expansion).

Difference in Measured Volume vs Product VP (Test Equipment  Only Corrected for Steel Thermal Expansion). Data are available in the table below.
Description of Figure 6
Data for Figure 6—Difference in Measured Volume vs Product VP (Test equipment Only Corrected for Steel Thermal Expansion).
Difference (mL) Product vapour pressure (kPa)
TM high flow
42.6 −49.0
26.8 −27.2
36.5 −46.3
42.6 −46.9
32.7 −52.0
42.2 −46.6
Calibration cart high flow
40.9 −10.9
43.3 −30.9
27.9 −14.9
37.4 −16.9
43.0 −14.3
31.3 −22.5
VRP high flow
42.6 −13.5
30.0 −28.0
40.7 −16.3
31.7 −20.7
43.8 −17.0
43.4 −8.6
TM low flow
40.2 −51.6
25.6 −28.8
35.4 −33.7
39.3 −39.0
32.2 −58.8
32.8 −30.6
30.8 −15.4
Calibration cart low flow
35.6 −33.4
34.9 −5.3
36.9 −20.4
29.3 −5.0
37.6 −24.3
36.7 −7.4
30.9 −23.4
VRP low flow
35.3 −19.0
29.4 −13.9
39.3 −14.8
30.7 −23.3
37.3 −6.6
36.6 −9.8
34.3 10.6

Conclusion

This study provides an estimate of product loss combined with wetting effects during testing with test measures. When care is not taken to minimize splashing during testing, the results indicate that the combined vapour loss and wetting effect error correlates closely with product vapour pressure. In this study, the error was found to increase from 10 mL to about 50 mL for a corresponding increase in product vapour pressure from 25 kPa to 45 kPa. When other sources of error are included, the differences between the volume measured by the test measure and the volume delivered by the pipe prover ranged from 15 mL to −65 mL. But with the product temperatures of −5 °C to 30 °C observed in these tests, correcting for expansion of the test measure steel reduced these differences to the 0 to −60 mL range.

The desired accuracy ratio of test equipment to device under test is 1:3. Unless vapour loss and wetting effect are addressed during the use of a test measure, this accuracy target will not be met.

The performance of the VRP and calibration cart indicates that vapour loss can be significantly reduced with these designs. When test equipment steel corrections were applied, the measured volumes were in agreement with the delivered volumes, assuming a tolerance equal to ⅓ the retail dispenser tolerance.

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