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SOP 18

Standard Operating Procedure for

Calibration of Graduated Neck-Type Metal Volumetric Field Standards

Using a Slicker-Plate Type1

1 Introduction 

Purpose of Test

This procedure may only be used to calibrate small non-pressurized, graduated 

neck-type, metal field standards such as the 5 gal (or 20 L) standards used by 

weights and measures officials to test liquid dispensing equipment, gasoline pumps

within a narrow range of laboratory and water temperatures (e.g., laboratory and 

water temperatures between 50 °F and 70 °F) and for test measure graduations that 

are at least 1 in3. Outside these limitations, SOP 19 is required. 

Conformity Assessment

The test measure or prover being calibrated should be evaluated for conformance 

to appropriate specifications and tolerances (using the checklist provided in NIST 

Handbook 105-3, Specifications and Tolerances for Graduated Neck Type 

Volumetric Field Standards, 2010) if being used for legal weights and measures 

applications. (Alternatively, if requested by the customer, evaluation against OIML 

R 120 (2010), Standard capacity measures for testing measuring systems for liquids 

other than water, may be referenced). Where compliance is required by law, 

conformity evaluations should be conducted prior to performing calibrations. See 

Section 6.2 for reporting results.

Limiting factors:

This procedure uses a slicker-plate type standard to fill an unknown test measure 

of equal nominal volume in one delivery. It requires water temperature stability 

during the transfer from the standard to the unknown test measure within the limits 

shown in Table 1. SOP 19 is a more appropriate procedure when temperature 

corrections are needed due to lack of water equilibration, temperature differences 

between the standard and unknown provers, performance of the calibration outside 

the stated temperature range, and for unstable environments. Even water that is 

temperature-equilibrated with the laboratory environment may not provide 

adequate measurement results depending on test measure resolutions. 

If the limits in Table 1 are exceeded, or materials other than stainless steel and mild 

steel are used, use SOP 19. These limits are to ensure that the impact of errors 

resulting from temperature differences on measured values is less than the 

 1 Non-SI units are predominately in common use in State legal metrology laboratories, and/or the petroleum industry 

for many volumetric measurements, therefore non-SI units have been used to reflect the practical needs of the 

laboratories performing these measurements as appropriate. Most laboratory standards for this calibration procedure 

are 5 gal “slicker-plate” type standards. Very few laboratories have 20 L “slicker-plate” type standards.
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resolution and repeatability on a 20 L or 5 gal test measure with 1 in3 graduations. 

For graduations less than or equal to 0.5 in3, errors due to temperature variations, 

including the ability to measure temperature and ensure stable temperatures in both 

the standard and test measure, require the use of SOP 19.

Table 1. Temperature limitations between 50 °F and 70 °F for 1 in3 graduations.

Graduation Size 

in3 Standard Material Unknown Material Temperature limit between standards (°C)

1 Stainless steel Stainless steel < 0.5 °C

1 Stainless steel Mild steel < 0.2 °C

Prerequisites 

Verify that the unknown prover has been properly cleaned and vented with 

all petroleum products removed prior to submission for calibration to ensure

laboratory safety. 

Verify that current calibration certificates with measurement values and 

uncertainties are available for all of the standards used in the test. All 

calibration values must have demonstrated metrological traceability to the 

international system of units (SI). Metrological traceability may be to the SI 

through a National Metrology Institute such as NIST.

Verify that the slicker-plate type standard has sufficiently small standard 

uncertainties for the intended level of calibration (i.e., less than 0.2 in3 with 

an approximate 95 % level of confidence).

Verify the availability of an adequate supply of clean, preferably soft water 

(filtered and thermally equilibrated as appropriate) (GLP 10). Water does 

not need to be distilled or deionized for use in this procedure. The equations

used in GLP 10 for the calculation of water density (air saturated) may be 

used without a significant impact on the measurement results. 

Verify that the operator has had specific training and is proficient in SOP 

18, GMP 3, SOP 17 and is familiar with the operating characteristics and 

conditioning of the standards used.

Verify that the laboratory facilities meet the following minimum conditions 

to enable meeting the expected uncertainty achievable with this procedure: 
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Table 2. Laboratory environmental and equilibrated water conditions.

Procedure Temperature Relative Humidity

Volume transfer

10 °C to 21 °C

(50 °F to 70 °F)

Stable to ± 1.0 °C / 1 h

35 % to 65 %

Stable to ± 20 % / 4 h

Field tests

A “field” calibration is considered one in which a calibration is conducted 

in an uncontrolled environment, such as out-of-doors. Calibrations 

conducted under field and laboratory conditions are not considered 

equivalent, and uncertainties must reflect the conditions of the calibration.

SOP 19 is a more suitable procedure for non-laboratory conditions. The care 

required for field calibrations includes proper safety, a clean and bubblefree water supply, measurement control programs, and a stable temperature 

environment shaded from direct sunshine to allow the prover, field standard, 

and clean test liquid (water) to reach an equilibrium temperature with 

minimal evaporation. Environmental conditions must be selected to be 

within stated laboratory conditions as shown in Table 2, during the 

measurements. All data and appropriate environmental conditions must be 

documented regardless of test location. 

An increased number of check standard verifications are required to ensure 

continued suitability of calibration values generated in field conditions as 

well as to verify the validity of any standards taken out of a secure 

laboratory environment once the standard(s) are returned to the laboratory. 

2 Methodology 

Scope, Precision, Accuracy 

This procedure is applicable for the calibration of a small test measure within the 

limitations of the standards available. The precision attainable depends on the care 

used in the various volumetric adjustments and readings, in the strict observance of 

drainage times, and the internal cleanliness of the various volumetric vessels which 

can influence their drainage characteristics. The accuracy depends on the 

uncertainties of the calibrations of the standards used.

Summary 

Water is delivered from the standard to the vessel under calibration. Because the 

"to deliver" volume of the latter is calibrated, the delivery must be into a "wetteddown" vessel. The wet-down also ensures consistent retention in the slicker-plate 

type standard. The gauge scale of the test vessel is adjusted to a correct reading, as 

necessary, and then sealed. Hand held test measures require a 30 s (± 5 s) pour 
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followed by a 10 s drain, with the measure held at a 10 degree to 15 degree angle 

from vertical, during calibration and use. Provers are emptied by gravity drain, 

followed by a 30 s final drain after the cessation of the main flow.

Standards and Equipment 

Calibrated slicker-plate type standard made of stainless steel.

Calibrated thermometer, with a resolution and uncertainty less than 0.1 °C.

Meniscus reading device. See GMP 3.

Timing device (calibration is not required; uncertainty of the measurement 

only needs to be less than 5 s for a 30 s pour or drain time).

Procedure 

Cleanliness Verification - Fill and drain both standard and vessel to be 

calibrated and check for any soiling that would affect drainage, as evidenced 

by clinging droplets, greasy films, and the like. Clean either or both with 

non-foaming detergent and water, as necessary, and rinse thoroughly. Note:

Many laboratories have a policy regarding cleanliness of submitted 

volumetric standards to minimize water contamination with flammable 

petroleum products. See GMP 6.

Fill the standard with water and then transfer the water into the unknown 

prover establishing a wet down of the standard. Wait 30 s after the cessation 

of the main flow on the standard before closing the drain valve. 

Wet down the unknown test measure or prover to its nominal level and 

empty it following the applicable procedure. 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 degree angle from vertical, touching off any adhering 

drop from the neck, during calibration and use. Provers are emptied by 

gravity drain, followed by a 30 s final drain after the cessation of flow.

Note: Filling the vessel from the slicker-plate type standard following the 

instructions in steps 2.5.1, 2.5.2, and 2.5.3 will ensure that both the standard 

and vessel are properly “wetted-down”.

Run 1. Fill the slicker-plate standard with water by placing the fill tube or 

hose inside the standard near the bottom to minimize air bubbles. Fill the 

standard to slightly higher than the rim as the water level is raised by surface 

tension. Ensure that the reference standard is level to ensure consistent drain 

and retention characteristics. Measure and record the temperature in the 

standard. Use the slicker plate to strike off excess water, checking to see 

that no air bubbles are entrained in the water during this process. 
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Position the unknown vessel beneath the slicker-plate standard drain valve 

or piping to be sure no water is lost. Open the valve at the base of the 

standard at the same time as removing the slicker plate from the top of the 

standard to transfer water from the standard to the wet-down vessel. Allow 

a 30 s drain period after cessation of the main flow on the standard. Close 

the valve on the slicker-plate standard and then move the unknown vessel 

from beneath the drain valve or piping to prevent additional water transfer. 

Level the test measure or prover and verify that the neck of the unknown 

test measure is vertical, or perpendicular to a level horizontal plane, by 

placing a level on the side of the neck as specified in NIST Handbook 105-

3 (or suspend by its handle, if appropriate). Read and record the gauge 

reading. Measure and record the temperature of water in the unknown 

measure. If the temperature change between the standard and unknown test 

measure exceeds the allowable limits shown in Table 1, use SOP 19.

Adjust the graduated scale of the vessel as described in 3.3 if needed. Seal 

the scale adjustment device. 

Run 2. Make a duplicate determination. 

Replicate runs of the test measure or prover must agree within ± 0.02 % of 

the nominal volume during calibration at the reference temperature (± 

0.2 in3 for 5 gal test measure), pass the F-test, or be within the limits on the 

standard deviation or range charts (as applicable). Agreement of 0.02 % is 

calculated as the difference between two consecutive runs, divided by the 

nominal volume, and then multiplied by 100.

Note: If excess disagreement between the runs is observed, check all vessels 

for cleanliness, leaks, or other damage, identify and correct any problems. 

Agreement problems may be due to temperature changes, contamination or 

lack of cleanliness, or poor field conditions, such as when calibration is 

conducted in an unstable environment. Agreement problems must be 

corrected before calibration can be completed.

3 Calculations 

When the water temperature is reasonably close to 60 °F, the coefficients of

expansion of the standard and the test vessel are sufficiently close together, and the 

deliveries and readings are made over a short period of time, temperature 

corrections are not made in this procedure. When conditions are not reasonably 

close to 60 oF and temperature corrections are needed, use SOP 19. Calculations 

provided in SOP 19 may be required for reporting volumetric results. 

Within the accuracy requirements of Table 1, no temperature corrections arising 

from dissimilarities of the standard and vessel are used. If differences are suspected

or observed, use SOP 19.
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The reading of Run 1 is used to adjust the scale of the vessel, if necessary, to the 

correct reading, which is set at the calibrated volume of the slicker-plate standard 

at 60 °F. Record the adjusted value as the “as left” value. Run 2 will validate the 

setting. Alternatively, the average of Run 1 and Run 2 may be used with the 

adjustment made after Run 2. In that case, a validation run should be conducted to 

ensure correct setting of the gage plate. 

Note: When the accuracy requirements necessitate a temperature correction, the 

temperature of the water that has been recorded in both the standard and the 

unknown should be used to perform calculations according to the procedure given 

in SOP 19.

Determine and report the volume of the test vessel as follows:

Prover volume gauge reading S V= − Eqn. (1)

where: VS = Volume of the slicker-plate standard at the appropriate reference 

temperature (usually 60 °F).

Calculate the within process standard deviation, sw, for the replicate runs. The 

degrees of freedom for this SOP is one. This value is plotted on the standard 

deviation chart and may be used to incorporate an F-test as well.

If the F-test is used, calculate the F statistic to compare the observed within process 

standard deviation, sw, to the accepted (pooled) within process standard deviation 

for the measurement process. (See NISTIR 6969, Sections 8.4 and 8.9.2, for more 

information on pooling standard deviations and F-tests).

2

Accepted

2

Observed

w

w

s

s F = Eqn. (1)

Where used, the calculated F statistic must be less than the F value obtained from 

an F table at 95 % confidence level (Table 9.12, NISTIR 6969) to be acceptable. 

The F value is obtained from the F-table for one degrees of freedom for the 

numerator and the degrees of freedom for the denominator is equal to the number 

of degrees of freedom in the pooled within process standard deviation. If the 

repeatability evaluation fails and the source of the error cannot be determined 

conclusively, the measurement must be repeated (or possibly, the test measure 

must be rejected).

Calculate and report the mean volume of the volumetric standard at its applicable 

reference temperature. 

If adjustments were made during replicate runs, report the “as found” volume or 

the mean of “as found” volumes and also the “as left” volume or mean of “as left” 

volumes, as applicable, at the appropriate reference temperature. (I.e., do not 
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calculate a mean value by combining “as found” and “as left” values when 

adjustments are made).

4 Measurement Assurance

Duplicate the process with a suitable check standard. See SOP 17 or SOP 30. Plot 

the check standard volume and verify that it is within established limits. The mean 

of the check standard observations may be used to evaluate bias and drift over time 

when a reference value for the check standard is available. Check standard 

observations are used to calculate the standard deviation of the measurement 

process which contributes to the Type A uncertainty components.

A t-test may be incorporated to check the observed value against an

accepted value.

( ) p

c c

s

S S

t − = Eqn. (2)

The t-statistic is evaluated using Eqn. 2 with a 95 % confidence level. All 

values must be entered in the control chart, even if failing this statistic to 

ensure the variability obtained for the process is truly representative of the 

process and not unduly reduced over time. The observed value of the check 

standard is compared to the accepted mean value of the check standard and 

divided by the standard deviation for the check standard observations over 

time. The limits for the t-test are based on applicable warning and action 

limits on the control chart. The applicable limits are shown in SOP 20 and 

based on the number of replicates.

A standard deviation chart is also used for measurement assurance through the 

evaluation of replicate measurements. The standard deviation of each combination 

of Run 1 and Run 2 is calculated and the pooled (or average) standard deviation 

over time may be used to estimate the short-term variability in the measurement 

process. The short-term standard deviation may be used with an F-test (observed 

vs. accepted) to assess the measurement process. A standard deviation chart for 

unknown provers represents the variability in condition of test measures submitted 

for calibration as well as the short-term repeatability of the measurement process

but does not monitor the stability of the reference standard.

For unknown standards that are adjusted, do not combine an “as found” value with 

an “as left” value for the two runs recorded in the chart; use the adjusted value from 

Run 1 and the value from Run 2, both at the applicable reference temperature, when 

entering values in a standard deviation or range chart.

5 Assignment of Uncertainties

The limits of expanded uncertainty, U, include estimates of the standard uncertainty of the 

laboratory volumetric standards used, us, plus the standard deviation of the process, sp, and the 

additional items noted below and in the uncertainty budget table, Table 3, at an approximate 95 % 
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level of confidence. See SOP 29 for the complete standard operating procedure for calculating the 

uncertainty. 

The standard uncertainty for the standard, us, is obtained from the calibration 

certificate. The combined standard uncertainty, uc, is used and not the expanded 

uncertainty, U, therefore the reported uncertainty for the standard will usually need 

to be divided by the coverage factor k. 

The standard deviation of the measurement process, sp, is obtained from a control 

chart for the check standard so that it reflects performance over time (See SOP 17 

or 20, and SOP 30). The larger of the value from the standard deviation over time 

for a check standard from Section 4.1 or from the standard deviation chart taken 

from Section 4.2 is used in the uncertainty calculations. Where multiple staff 

members perform this calibration and obtain values for the check standard, the 

standard deviation for a check standard may represent variability and systematic 

errors associated with the meniscus reading. When a standard deviation chart is 

used alone, the repeatability represents the process only and does not represent 

potential systematic differences in meniscus readings. In a laboratory with only one 

staff member performing calibration and entering data for a check standard, 

additional uncertainties for meniscus readings are needed per Section 5.3 and GMP 

3. 

Other standard uncertainties usually included at this calibration level primarily 

include 1) uncertainties associated with the ability to read the meniscus, only part 

of which is included in the process variability due to parallax and visual capabilities

(See GMP 3), and 2) uncorrected temperature corrections related to the cubical 

coefficient of expansion for the prover under test, use of proper temperature 

corrections, and the accuracy of temperature measurements. Additional factors that 

might be included are: round robin or proficiency testing data showing bias,

environmental variations over time, and drift of the standard as noted in control 

charts. 

To properly evaluate uncertainties and user requirements (tolerances), assessment 

of additional user uncertainties may be required by laboratory staff. Through proper 

use of documented laboratory and field procedures, additional uncertainty factors 

may be minimized to a level that does not contribute significantly to the previously 

described factors. Additional standard uncertainties in the calibration of field 

standards and their use in meter verification may include: how the prover level is 

established, how delivery and drain times are determined, the use of a proper “wetdown” prior to calibration or use, the cleanliness of the prover and calibration 

medium, prover retention characteristics related to inside surface, contamination or 

corrosion, total drain times, and possible air entrapment in the water. 

Example components to be considered for an uncertainty budget table are shown in 

Table 3. 
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Table 3. Example uncertainty budget table.

Uncertainty Component Description Symbol Source Typical 

Distribution

Uncertainty of the standard (5.1) us

Calibration 

certificate

Expanded divided 

by coverage factor

Accepted standard deviation of the process

(5.2) sp

Control chart, 

standard deviation 

chart

Normal

Ability to read the Meniscus in S (5.3) um

None if using a 

slicker-plate type 

standard

Triangular

Ability to read the Meniscus in X (5.3) um GMP 3 Triangular

Uncorrected temperature errors (5.3) ute

Calculated based 

on limits and SOP 

19 equations

Rectangular

Uncertainty of bias or drift of standards (5.3) ub From control chart Rectangular

Water temperature (S) uts

Consider 

accuracy, 

resolution, and 

gradients

Rectangular

Water temperature (X) utx

Consider 

accuracy, 

resolution, and 

gradients

Rectangular

Cubical Coefficient of Expansion (CCE) on 

S uCCE

5 % to 10 % of the 

CCE (EURAMET 

CG-21)

Rectangular

Cubical Coefficient of Expansion (CCE) on 

X uCCE

5 % to 10 % of 

the CCE 

(EURAMET CG21)

Rectangular

Uncertainty of drain time (insignificant if 

closely following the procedure) ud

From 

experimental data Rectangular

Uncertainty Evaluation

Where applicable, uncertainties for volume calibrations that are assessed for 

conformity must be less than the tolerances specified in Handbook 105-3 and used 

for decision rules.

6 Calibration Certificate

Report results as described in NISTIR 6969, SOP 1, Preparation of Calibration

Certificates, with the addition of the following:

“To Contain” or “To Deliver” volume, reference temperature, uncertainty, material, 

thermal coefficient of expansion (assumed or measured), construction, any 
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identifying markings, tolerances (if appropriate), laboratory temperature, water 

temperature(s) at time of test, barometric pressure, relative humidity, and any out–

of-tolerance conditions.

Conformity Assessment

Evaluate compliance to applicable tolerances as needed or required by the customer 

or by legal metrology requirements. Compliance assessments must note the 

applicable documentary standard and which portions of the standard were or were 

not evaluated. The uncertainty for volume calibrations must be less than the 

tolerances published in the applicable documentary standards. For volume 

calibrations where the unknown standard can be adjusted, it is standard practice to 

adjust the standard or leave the scale plate in a position close enough to its nominal 

volume to ensure that the absolute value of the measurement result plus the 

uncertainty is less than the applicable tolerance. Where the unknown standard 

cannot be adjusted, and a portion of the uncertainty band from the error exceeds 

tolerance limits, it is not appropriate to state compliance with the tolerances unless 

additional decision rules are communicated with and agreed to by the end user. 

Correction values (measurement results) may need to be used by the end user in 

such cases.
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Appendix A

Volume Transfer Data Sheet

Laboratory data and conditions:

Vessel Owner Operator

Vessel Identification Date

Material Air Temperature

Cubical Coefficient of 

Expansion Relative Humidity

Standard deviation of the 

process, sp

Reference temperature of 

unknown prover

sp from Control Chart or 

Standard Deviation Chart

Degrees of Freedom

Volume standard(s) data:

Identification (ID)

(Note ID of Standards) Nominal Volume/Correction Expanded Unc:

From cal. report

Unc:

k factor

Cubical 

Coefficient of 

Expansion

S

S

S

S

Observations:

Run

Standard 

Material 

(MS/SS)

Unknow

n 

Material

(MS/SS)

Water 

Temp in 

Standard 

(°C)

Unknown 

Gauge 

Reading (in3

)

Water 

Temp in 

Unknown 

(°C)

Adjustment 

Made?

Temperature 

Difference

Acceptable for 

SOP 18?

1 ___Yes ___No ___Yes ___No

2 ___Yes ___No ___Yes ___No

3 ___Yes ___No ___Yes ___No

Temperature Difference Evaluation:

If Standard is stainless steel (SS) and Unknown is stainless steel, the temperature of the water between the standard 

and unknown must not change more than 0.5 oC during the calibration. 

If the standard is stainless steel and the unknown test measure is mild steel (MS), the change in water temperature 

between standard and unknown must be less than 0.2 oC during the calibration. 

If these limits are exceeded, use SOP 19 calculations to determine the volume of the unknown volumetric measure. 
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