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SOP 16

Standard Operating Procedure for

Calibration of Measuring Flasks

Volume Transfer Method

1 Introduction

Purpose of Test 

This procedure describes a method for volume transfer calibration of the “To 

Deliver” (TD) volume of measuring flasks from calibrated volumetric standards. 

The test items are then used as volumetric measurement standards, often for 

packaged commodity verification. If “To Contain” (TC) calibrations are to be 

performed, appropriate modifications are needed in the procedure to ensure 

complete drying of the flask between calibration runs. Detailed measurement 

ranges, standards, equipment, and uncertainties for this SOP are generally compiled 

in a separate document in the laboratory.

Conformity Assessment

Standards that are calibrated for use in legal weights and measures applications 

should be evaluated for conformance to the appropriate specifications and 

tolerances that apply. Where compliance is required by law, conformity evaluations 

should be conducted prior to performing calibrations. See Section 6.2 for reporting 

results.

Prerequisites 

Verify that valid calibration certificates are available for all the standards 

used in the calibration. All standards must have demonstrated metrological 

traceability to the international system of units (SI), which may be to the SI 

through a National Metrology Institute such as NIST. Where reference 

standards do not have suitable calibration values and uncertainties, SOP 14 

is preferred for calibration of measuring flasks.

Verify that the standards to be used have sufficiently small standard 

uncertainties for the intended level of calibration.

Verify that the standard burets and pipets are clean, free of leaks, and in 

good operational condition.

Verify the availability of an adequate supply of pure distilled or deionized 

water (See GLP 10) of sufficient quality and quantity for the calibration. Do 

not use tap water for this procedure to ensure the reference glassware is 

maintained with valid reference values!
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Verify that the operator has had specific training and is proficient in SOP 

16, GMP 3, GMP 7, and is familiar with the operating characteristics and 

conditioning of pipets and burets.

Verify that the laboratory facilities meet the minimum conditions shown in 

Table 1 to meet the expected uncertainty that is achievable with this 

procedure.

Table 1. Laboratory environmental conditions.

Procedure Temperature Relative Humidity

Volume 

Transfer

18 °C to 27 °C, 

Stable to ± 1 °C / 1 h during the calibration

40 % to 60 %

Stable to ± 20 % / 4 h

2 Methodology

Scope, Precision, Accuracy

The method is applicable for the calibration of any size of measuring flask for 

which standard pipets of comparable volume are available. Typical flasks have 

volumes in the range of 100 mL to 5 L (1 gill to 1 gal). The precision of calibration 

depends on the care exercised in adjusting the various volumes and strict adherence 

to the various steps of the procedure. The accuracy will depend on the accuracy of 

calibration of the volumetric standards used, accuracy in reading the meniscus, 

together with the precision of the comparison. Clean glassware and strict adherence 

to the drainage instructions are essential for precise and accurate results. 

Summary

The flask to be calibrated is given an initial wet-down using the correct drain 

procedure, then nearly filled with water delivered from a calibrated pipet. (This 

procedure relies on calibrated pipets that deliver slightly less than the full nominal 

volume). Additional water is added to the test flask from a calibrated buret until the 

meniscus in the test item coincides with the calibration graduation mark. The sum 

of the volumes delivered into the wetted flask is equivalent to its “To Deliver” (TD)

volume. The process described assumes that the flask is marked with a 10-second 

drain time. If the flask is marked with a drain time other than 10 s, the specified 

drain time should be used rather than the 10 s drain time described. A 30-second 

drain time should be used when a flask has no drain time specified.

Standards and Equipment 

Calibrated standard pipet(s) of suitable volumes. These standards are 

typically made of borosilicate glass.
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Calibrated standard buret(s) of 10 mL (or 120 minim1) capacity. These 

standards are typically made of borosilicate glass.

Calibrated thermometers, with resolution and uncertainty to ≤ 0.1 °C to 

determine water temperature.

Meniscus reading device (See GMP 3).

Stopwatch or another suitable timing device (does not need to be 

calibrated).

Procedure 

Cleanliness check 

Verify that all glassware, including the standards used and the vessels to be 

calibrated, are internally clean, as evidenced by uniform drainage of water. 

No water droplets should remain on the internal surfaces as the water drains 

from the vessels. If this occurs, the glassware must be cleaned with suitable 

agents and rinsed with pure water until uniform drainage is obtained. (See 

GMP 7).

Wet-down of pipet and flask

2.4.2.1 Fill the standard pipet until water flows from the top tip, then 

completely drain into an empty vessel. Check for uniform drainage. 

Touch off the pipet outlet tip against the container wall to remove 

excess droplets and to establish a constant tip retention volume. The 

pipet and bore of the delivery side of the stopcock should appear to 

be "empty" and the delivery tip should contain a small volume of 

water retained in it. This is the "wet-down" condition of the pipet. It 

must be done at the beginning of each test sequence. Ensure that the 

outside of the standard pipet is free of water droplets that may fall 

into the unknown flask.

2.4.2.2 Refill the pipet and use it to fill the flask to be calibrated. Empty the 

flask using a gentle pour in a 30 s ± 5 s period by gradually inclining 

the flask to avoid splashing of the walls as much as possible. When 

the main drainage stream has ceased, the flask will be nearly 

vertical. Allow an additional 10 second drainage after discharge of 

the flask contents while holding the flask at a 10° to 15° angle from 

vertical, then touch off the rim of the flask to remove any drops 

adhering to it. At the same time, check that uniform drainage has 

been achieved. This establishes the “To Deliver” (TD) condition of 

 1 1 [US] minim = 0.061615496 milliliter (mL). 120 minim standards were provided to the States as reference 

standards for use in volume transfer calibrations. These standards have higher resolution than metric standards 

provided at the same time.
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the flask. Note that some small amount of water will remain in the 

flask. Each flask to be calibrated must be given this "wet-down" 

treatment immediately before the start of the test.

Conditioning the buret 

2.4.3.1 Fill the standard buret to overflow and drain several times to verify 

uniform drainage. Refill with water. Note that the stopcock bore and 

delivery tip should be filled with water at all times, in contrast to the 

condition for the pipet.

2.4.3.2 Touch off the delivery tip against the wall of the receiving vessel, to 

remove any droplet adhering to the external surface of the tip. This 

is an operation that must be done every time a measured volume of 

water is delivered from the buret to ensure consistent volume 

delivery.

Calibration 

2.4.4.1 Run 1. Fill the standard pipet to overflow. The delivery bore and the 

delivery tip will be empty, except for the small volume retained in 

the tip. Place the inside of the flask neck in contact with the tip of 

the pipet or buret, to avoid splashing but in a manner that does not 

block the flow of water. 

2.4.4.2 Deliver the contents of the pipet into the "wet-down" flask. Repeat 

as necessary to nearly fill the flask to the calibration line. The value 

recorded for the standard pipet will be the total of the deliveries 

used.

2.4.4.3 Add water to the flask from the buret until the meniscus coincides 

with the calibration mark. (See GMP 3 for instructions on how to 

read a meniscus). Read the volume of water delivered from the buret

and record on a suitable data sheet such as the one in the Appendix. 

Note: If multiple deliveries of the complete buret contents are 

required, ensure that at no time the buret water level drops below the 

lowest graduation mark, or the entire process must be started over. 

The value recorded for the standard buret will be the sum of the 

delivered volumes.

2.4.4.4 Empty the measuring flask as described in 2.4.2.2 to re-establish 

"wet-down."

2.4.4.5 Record all data using the form given in the Appendix or a similar 

format.

2.4.4.6 Run 2. Replicate Measurement. Repeat the procedure described in 

2.4.4. The test measure must be capable of repeating to 0.01 % of 
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the test volume during calibration. Repeatability problems may be 

due to contamination, poor meniscus readings, lack of cleanliness, 

or conduct of the calibration in an unstable environment. 

Repeatability problems must be corrected before calibration can be 

completed.

2.4.4.7 Calibration Note: If the test vessel has a graduated neck (in addition 

to the nominal mark) additional graduations should be inspected for 

proper marking and calibrated as well.

3 Calculations 

Compute the individual “To Deliver” (TD) volumes, VTD, for Run 1 and Run 2

using Eqn. 1. 

V VV TD P B = + Eqn. (1)

Table 2. Variables for volume equation.

Variable Description

VTD “To deliver” Volume 

VP "To deliver" calibrated volume of the standard pipet

VB Volume of water delivered from the buret, corrected for any calibration values

Temperature correction

For calibration of glassware that is not made of borosilicate glass, not only should 

the above temperature conditions be realized, but the water temperature must also 

be known. The water used must be stored in the laboratory until its temperature is 

equilibrated with its surroundings, and the temperature of the discharged water is 

measured using the calibrated thermometer. The "To Deliver" volume of the flask, 

corrected to the reference temperature of 20 °C, VTD20, is computed, using Eqn. 2:

VV t TD TD 20 = +− −   1 (α β )( 20)   Eqn. (2)

Table 3. Variables for volume temperature correction equation.

Variable Description

VTD observed delivered volume as computed in 3.1

VTD20 “To deliver” volume at the reference temperature of 20 °C

t Temperature of water at time of calibration

α

Cubical coefficient of expansion of glass of standard (e.g., 0.000010 °C-1 for 

borosilicate glass)

β Cubical coefficient of expansion of glass of flask (e.g., 0.000025 °C-1 for sodalime glass)
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The volumetric standards are typically made of borosilicate glass. If borosilicate 

flasks are calibrated, no temperature correction is required. This procedure relies 

on the fact that since deliveries are made over a short time, the temperature remains 

the same in the standard as in the unknown. When standards and flasks are not of 

the same material, the water temperature change must be less than 0.5 °C while the

water is in the standards and the flask. 

Alternative Reference Temperatures

Reference temperatures other than 20 oC (68 oF) may occasionally be used. 

Common reference temperatures for other liquids follow:

Table 4. Reference temperatures for flasks.

Commodity Reference Temperature

Frozen food labeled by volume (e.g., fruit juice) - 18 oC (0 oF)

Beer 3.9 oC (39.1 oF)

Food that must be kept refrigerated (e.g., milk) 4.4 oC (40 oF)

Distilled spirits or petroleum 15.56 oC (60 oF)

Petroleum (International Reference) 15 oC (59 oF)

Wine 20 oC (68 oF)

Unrefrigerated liquids (e.g., sold unchilled, like soft drinks) 20 oC (68 oF)

Petroleum (Hawaii) 26.67 oC (80 oF)

When alternative reference temperatures are used, at the request of the user of the 

field standard, and when the glassware is not marked with a reference temperature, 

the calibrated value of the standard may be reported at a different reference 

temperature, tref, by using Eqn. 3.

V V TD ref TD t 1 (α β )(t tref ) = +− −     Eqn. (3)

Report the average volume VTD20 as the value for the flask after the temperature 

correction has been applied to each Run.

4 Measurement Assurance

Duplicate this process with a suitable check standard (Quality Assurance Reference 

Flask, QARF) or have a suitable range of check standards for the laboratory with 

similar size graduations and neck diameters. See SOP 17 and NISTIR 6969, SOP 

30.

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

Alternatively, a t-test may be incorporated into the process to check the observed 

value against an accepted reference value. (See NISTIR 6969, Section 8).

The mean of the check standard value is used to evaluate bias and drift over time.
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Check standard observations are used to calculate the standard deviation of the 

measurement process.

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, at 

the 95 percent level of confidence. See NISTIR 6969, SOP 29 for the complete standard 

operating procedure for calculating the uncertainty. 

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

certificates for the pipet and buret used for the calibration. The combined standard 

uncertainty, uc, from the calibration report must be used and not the expanded 

uncertainty, U. Therefore, the reported uncertainty for the standard will usually 

need to be divided by a coverage factor, k. Multiple standards are used in this 

procedure; however, they are typically calibrated independently. Multiple standard 

uncertainties are handled according to SOP 29.

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

chart performance (See SOP 17 or 20, and SOP 30) for a suitable Quality Assurance 

Reference Flask (QARF) using the same volume transfer procedures on flasks of 

comparable size and neck dimensions. 

Other standard uncertainties usually included at this calibration level include 

uncertainties associated with the ability to read the meniscus, only part of which is 

included in the process variability (See GMP 3), the cubical coefficient of 

expansion for the flask under test, use of proper temperature corrections, the 

accuracy of temperature measurementstaking into consideration potential gradients 

in the glassware during calibration, round robin or proficiency testing data showing 

reproducibility, environmental variations over time, and bias or drift of the 

standard.

Uncertainty Evaluation

Where applicable, uncertainties for volume calibrations that are assessed for 

conformity must be less than the tolerances specified in Handbook 105-2 and used 

for decision rules. Where decision rules cannot be met using this procedure, SOP 

14, gravimetric calibration is preferred.

6 Calibration Certificate

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

Results, with the addition of the following items:

“To Contain” or “To Deliver” volume, applicable reference temperature, 

uncertainty, material, thermal coefficient of expansion (assumed or measured), 

construction, any identifying markings, tolerances (if appropriate), laboratory 
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temperature, water temperature 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 flasks, 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. 

7 References

Bean, V. E., Espina, P. I., Wright, J. D., Houser, J. F., Sheckels, S. D., and Johnson, A. N., NIST 

Calibration Services for Liquid Volume, NIST Special Publication 250-72, National Institute of 

Standards and Technology, Gaithersburg, MD, (2009).
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Appendix A

Calibration of Measuring Flasks

(Volume Transfer Method)

Laboratory data and conditions:

Test Number Date

Vessel Identification Operator

Nominal Volume Before After

Coefficient of Expansion Temperature

Process standard deviation 

from control chart, sp

Pressure

Degrees of Freedom Relative Humidity

Volume standard(s) data:

Identification

(enter interval if 

needed)

Nominal Calibrated 

Volume 

Volume 

Correction

Coefficient of 

Expansion

Exp. 

Uncertainty k factor

Pipet

Buret

Buret

Buret

Observations:

Run 1 Run 2

Calibrated Pipet Volume: Vp

Buret Volume: 

Final Reading

Initial Reading

Difference

Calibration Correction (if required) 

Corrected Buret Volume, VB 

VTD = VP + VB

Water temperature at time of test (t)

VTD20 (Use Eqn. 2 for temp. correction).

Average VTD20
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