Gas Flowmeter Calibrations with the Working Gas Flow Standard

NIST Special Publication 250-80

John D. Wright, Jean-Philippe Kayl, Aaron N. Johnson, and Gina M. Kline

November 23, 2009

Fluid Metrology Group

Process Measurements Division

Chemical Science and Technology Laboratory

National Institute of Standards and Technology

U. S. Department of Commerce
ii

Table of Contents

Gas Flowmeter Calibrations with the Working Gas Flow Standard .......................... i

Abstract ..................................................................................................................... 1

1 General Description of Gas Flow Calibration Services.......................................... 1

2 Procedures for Submitting a Flowmeter for Calibration ....................................... 3

3 Overview................................................................................................................ 3

4 The WGFS with Critical Nozzle Working Standards............................................. 5

5 Data Acquisition and Control System.................................................................... 7

6 Nozzle Flow Calculations ...................................................................................... 8

7 The WGFS with a Laminar Flowmeter Working Standard.................................. 10

8 Laminar Flow Calculations.................................................................................. 13

9 Uncertainty Overview .......................................................................................... 14

10 Uncertainty Analysis for Nozzle Working Standards ........................................ 15

11 Uncertainty Analysis for Laminar Flowmeter Working Standards.................... 19

12 Summary ............................................................................................................ 21

Appendix: Sample Calibration Report .......................................................................................1
1

Abstract

The Working Gas Flow Standard (WGFS) uses critical venturis, critical nozzles, or 

laminar flowmeters as working standards to calibrate customer flowmeters. The working 

standards are periodically calibrated with primary standards: the 34 L, 677 L, or 26 m3

PVTt standards, or a static gravimetric standard. The WGFS is used to calibrate 

flowmeters with low pressure drop, in dry air, at flows from 0.001 L/min1to 

70 000 L/min with an uncertainty of 0.15 % or less. At flows less than 2000 L/min, 

calibrations in other non-hazardous and non-corrosive gases are available.

In this document, we provide an overview of the gas flow calibration service and the 

procedures for customers to submit their flowmeters to the National Institute of Standards 

and Technology (NIST) for calibration. We also document the flow calculation 

algorithms and uncertainties of the WGFS.

Key words: calibration, critical nozzle, critical venturi, laminar, flow, flowmeter, gas 

flow standard, uncertainty, working standard.

1 General Description of Gas Flow Calibration Services

Customers should consult the web address 

http://ts.nist.gov/MeasurementServices/Calibrations/mechanical_index.cfm to find the 

most current information regarding our calibration services, calibration fees, technical 

contacts, and flowmeter submittal procedures. 

NIST uses the Working Gas Flow Standard described herein to provide gas flowmeter 

calibrations for flows between 0.001 L/min and 70 000 L/min. The gases available for 

calibrations in the WGFS are dry air, nitrogen, carbon dioxide, argon, and helium. The 

source of air, at pressures up to 1.7 MPa, is an oil-free reciprocating compressor and a 

refrigeration drier. The dew point temperature of the dried air is 256 K so the mole 

fraction of water in the air is 0.14 %. Nitrogen (at pressures up to 800 kPa and purity of 

99.998 %) is supplied by liquid nitrogen dewars. Higher pressures of nitrogen as well as 

argon, carbon dioxide, and helium gas can be supplied from compressed gas cylinders. 

Other non-toxic, non-corrosive gases can be accommodated upon customer request. 

While other gases are certainly feasible in the 677 L PVTt standard, in practice, test gases 

are limited to air from the compressor and nitrogen from dewars because a very large 

number of gas cylinders would be necessary to provide gas at 2000 L/min. The gas 

temperatures are nominally room temperature.

Readily available fittings for the installation of flowmeters in the 34 L and 677 L PVTt

standards are Swagelok (1/8 in to 1 in), A/N 37 degree flare (1/4 in to 1 in), national pipe 

thread or NPT (1/8 in to 3 in), VCR (1/4 in and 1/2 in), and VCO (1/2 in and 1 in). 

 

1 Reference conditions of 293.15 K and 101.325 kPa are used throughout this document for volumetric 

flows.

Certain commercial equipment, instruments, or materials are identified in this paper to foster 

understanding. Such identification does not imply recommendation or endorsement by the National 

Institute of Standards and Technology, nor does it imply that the materials or equipment identified are 

necessarily the best available for the purpose. VCR and VCO are registered trademarks of Swagelok.
2

Meters can be tested if the flow range, gas, and piping connections are suitable, and if the 

system to be tested has precision appropriate for calibration with the NIST flow 

measurement uncertainty. The vast majority of flowmeters calibrated in the gas flow 

calibration service are critical flow venturis (CFVs), critical nozzles, or laminar 

flowmeters because these are presently regarded as the best candidates for transfer and 

working standards by the gas flow metrology community. Occasionally we have tested 

positive displacement meters, roots meters, rotary gas meters, thermal mass flowmeters, 

and turbine meters. Meter types with calibration instability significantly larger than the 

primary standard uncertainty should not be calibrated with the NIST standards for 

economic reasons. For example, a rotameter for which the float position is read by the 

operator’s eye normally cannot be read with precision any better than 1 %. It is not wise

to obtain 0.05 % or less uncertainty flow data from NIST for such a flowmeter when 

0.5 % data is perfectly adequate and available from other laboratories at significantly 

lower cost.

A normal flow calibration performed by the NIST Fluid Metrology Group consists of five 

flows spread over the range of the flowmeter. A laminar flowmeter is normally calibrated 

at 10 %, 25 %, 50 %, 75 %, and 100 % of the meter full scale. At each of these flow set 

points, three (or more) flow measurements are made with the WGFS. The same meter is

tested on a second occasion, with flows in decreasing order instead of the increasing 

order of the first set. Therefore, the final data set consists of six (or more) primary flow 

measurements made at five flow set points, i.e., 30 individual flow measurements. The 

sets of three measurements can be used to assess repeatability, while the sets of six can be 

used to assess reproducibility. For further explanation, see the sample calibration report 

that is appended to this document. Variations on the number of flow set points, spacing of 

the set points, and the number of repeated measurements can be discussed with the NIST 

technical contacts. However, for quality assurance reasons, we rarely conduct calibrations 

involving fewer than three flow set points and two sets of three flow measurements at 

each set point.

The Fluid Metrology Group prefers to present flowmeter calibration results in a 

dimensionless format that takes into account the physical model for the flowmeter type. 

The dimensionless approach facilitates accurate flow measurements by the flowmeter 

user even when the conditions of usage (gas type, temperature, pressure) differ from the 

conditions during calibration. Hence for a laminar flowmeter, a report presents the 

viscosity coefficient and the flow coefficient as defined in section 8. In order to calculate 

the uncertainty of these flowmeter calibration factors, we must know the uncertainty of 

the standard flow measurement as well as the uncertainty of the instrumentation 

associated with the meter under test (normally absolute pressure, differential pressure, 

and temperature instrumentation). We prefer to connect our own instrumentation 

(temperature, pressure, etc.) to the meter under test because they have established 

uncertainty values based on calibration records that we rarely have for the customer’s 

instrumentation. In some cases, it is impractical to install our own instrumentation on the 

meter under test and the meter under test reads the flow directly. In these cases, we 

provide a table of flow indicated by the meter under test, flow measured by the NIST 

standard, and the uncertainty of the NIST flow value.
3

2 Procedures for Submitting a Flowmeter for Calibration 

Customers should consult the web address 

http://ts.nist.gov/MeasurementServices/Calibrations/ to find the most current information 

regarding our calibration services, calibration fees, technical contacts, turn around times, 

and instrument submittal procedures. The instructions for domestic customers have the 

sub-headings: A.) Customer Inquiries, B.) Pre-arrangements and Scheduling, C.) 

Purchase Orders, D.) Shipping, Insurance, and Risk of Loss, E.) Turnaround Time, and 

F.) Customer Checklist. There are also special instructions for foreign customers.

3 Overview

NIST offers calibrations of gas flowmeters in order to provide traceability to flowmeter 

manufacturers, secondary flow calibration laboratories, and flowmeter users. For a fee, 

NIST calibrates a customer’s flowmeter and delivers a calibration report that documents 

the calibration procedure, the calibration results, and their uncertainty. The flowmeter and 

its calibration results may be used in different ways by the customer. The flowmeter is 

often used as a transfer standard to compare the customer’s primary standards to NIST’s

primary standards so that the customer can establish traceability, validate his / her

uncertainty analysis, and demonstrate proficiency. Customers with no primary standards 

use their NIST calibrated flowmeters as working standards or reference standards in their 

laboratories to calibrate other flowmeters.

The Fluid Metrology Group of the Process Measurements Division (part of the Chemical 

Science and Technology Laboratory) at NIST provides gas flow calibration services 

spanning the range from 0.001 L/min to 78000 L/min. Figure 1 presents the flow ranges 

covered by the primary gas flow standards in the Fluid Metrology Group. Flows from 

900 L/min to 77 600 L/min can be measured with a 26 m3 PVTt standard that was built in 

the late 1960’s and has been upgraded several times.

2, 3, 4

It presently has an expanded 

uncertainty (approximately 95 % confidence level or k = 2) of 0.09 %. Flows from 

2000 L/min down to 0.010 L/min can be measured with the 34 L and 677 L PVTt

standards that have expanded uncertainty of 0.025 %.

5

Flows of 5 L/min to 0.001 L/min

can be calibrated by with a static gravimetric flow standard.6

 

2 Olsen, L. and Baumgarten, G., Gas Flow Measurement by Collection Time and Density in a Constant 

Volume, Flow: Its Measurement and Control in Science and Industry, Instrument Society of America, 

Pittsburgh, PA, USA, pp. 1287–1295, 1971.

3

Johnson, A. N., Wright, J. D., Moldover, M. R., and Espina, P. I., Temperature Characterization in the 

Collection Tank of the NIST 26 m3 PVTt Gas Flow Standard, Metrologia, 40, 211–216, 2003.

4 A.N. Johnson and J.D. Wright, Revised Uncertainty Analysis of NIST 26 m3 PVTt Flow Standard, 

Proceedings of the International Symposium on Fluid Flow Measurement, Queretaro, Mexico, (2006).

5

J. D. Wright, A. N. Johnson, M. R. Moldover, and G. M. Kline Gas Flowmeter Calibrations with the 34 L 

and 677 L PVTt Standards, NIST SP 250-63 (Gaithersburg, MD: NIST 2004).

6 Berg, R. F. and Tison, S. A., Two Primary Standards for Low Flows of Gases, J. Res. Natl. Inst. Stand. 

Technol., 109, 436-450, 2004.
4

Figure 1. Flow ranges for gas flow standards in the NIST Fluid Metrology Group.

PVTt systems are most readily applied to critical nozzles because nozzles are largely 

immune to the unavoidable pressure changes imposed by a PVTt standard as the 

collection tank fills whereas other flowmeter types would be adversely affected. Hence, 

we have followed the design of Olsen and Baumgarten,

2

the Colarado Engineering 

Experiment Station,

7

and others: we calibrate critical nozzles with the PVTt standard and 

subsequently use the critical nozzles as working standards to calibrate other flowmeter 

types. The WGFS is comprised of a gas source, pressure regulators, piping, critical 

nozzles (or Molblocs for flows < 10 L/min), pressure and temperature instrumentation, 

and a computer for data acquisition and control.

Critical nozzles have been used in this manner for more than 30 years. Their calibration 

stability is well established and expected due to their lack of moving parts. Also they 

require only a temperature measurement, a pressure measurement, and an a priori flow 

calibration to calculate flow. They are largely immune to influences of installation effects 

if the 4 to 1 approach pipe diameter to critical nozzle throat diameter recommended by 

international standards is followed.

8 Their physical model is well known, so that a user 

can accurately predict their sensitivity to gas temperature, room temperature, and gas 

species.

9

The PVTt standards are ideally suited for the calibration of critical nozzles but are not so 

well suited to calibrations of other meter types for the following reason. Flowmeter 

calibrations are normally performed at steady state conditions of flow, pressure, and 

temperature to avoid mixing meter calibration errors with meter time constant errors. 

PVTt standards fill a collection tank from near vacuum to normally 100 kPa. The change 

 

7 Kegel, T. M., Uncertainty Analysis of a Sonic Nozzle Based Flowmeter Calibration, NCSL Workshop and 

Symposium, Chicago, IL, July, 1994.

8

International Organization for Standardization, ISO 9300:1990 (E), “Measurement of gas flow by means 

of critical flow Venturi nozzles,” ISO/TC 30, Measurement of fluid flow in closed conduits.

9 Wright, J. D., What Is the “Best” Transfer Standard for Gas Flow?”, FLOMEKO, Groningen, the 

Netherlands, May, 2003.
5

in pressure also occurs at the meter under test, giving non-steady state conditions. Critical 

nozzles are essentially immune to downstream pressure changes if the ratio of upstream 

to downstream pressures is greater than approximately 2, so controlling the upstream 

pressure and temperature gives steady state conditions for critical nozzles. 

In recent years we performed tests that demonstrate that if a throttling valve is used 

between a laminar flowmeter and the collection tank to reduce pressure changes at the 

meter under test, and if the PVTt fill time is long compared to the time constant of the 

meter under test, accurate calibrations of laminar flowmeters can be made directly on a

PVTt standard. This topic is covered in a later section of this document. Unfortunately, at 

low flows, a measurement with our existing PVTt flow standards is time consuming: a 

single flow measurement at 0.010 L/min requires 28 h in the 34 L PVTt standard. 

Therefore, we calibrate working standard laminar flowmeters (Molblocs) and use them to 

calibrate customer flowmeters because we can collect 5 calibration points in 30 min or 

less.

In the following sections we will describe the WGFS with critical venturis or critical 

nozzles used as the working standards. In the latter portions of this document, the WGFS 

with laminar flowmeters (Molblocs) as the working standards will be described.

4 The WGFS with Critical Nozzle Working Standards

A schematic of the WGFS with a nozzle as the working standard is shown in Fig. 2 and a 

photograph of the system is shown in Fig. 3. Gas flows from a high-pressure gas source 

through one or two stages of manual pressure regulation, then to a computer controlled, 

PID tuned pressure regulator. 

The WGFS can be operated in manual or automatic modes. In automatic, the computer 

increments pressure on the working standard flowmeter (and hence flow) through preset 

values read from a “recipe” file. Instrumentation outputs, working standard flow, and 

outputs from the meter under test are written to log and average files for analysis and 

report writing by the operator.

Figure 2. Schematic of the Working Gas Flow Standard.

To set up the WGFS, a working standard is selected that will achieve the desired range of 

flows (see Tables 1 or 2). The nozzle is installed in the approach and exit tubing and the 

meter under test is connected to that assembly. Pressure and temperature sensors are 
6

installed upstream from the nozzle for the calculation of flow. Pressure, differential 

pressure, frequency, or temperature instrumentation may be selected to acquire outputs 

from the meter under test. Overlapping ranges are used to check the working standard 

nozzles against each other.

Figure 3. A photograph of the WGFS with a critical flow venture as the working standard 

and a Molbloc as the meter under test.

The WGFS uses a set of eight CFVs with throat diameters between 0.29 mm and 

6.35 mm or a set of nine critical nozzles (i.e. no diverging section downstream from the 

throat) available with throat diameters between 3.5 mm and 33 mm. These two sets of 

working standards (generically both referred to as nozzles herein) cover flows over the 

range of 1 L/min to 70 000 L/min. The CFVs have 25 mm fittings with o-ring seals. 

These CFVs cover flows from 1 L/min to 2700 L/min (see Table 1) and they are 

calibrated with the 34 L and 677 L PVTt flow standards. The nine critical nozzles were 

first calibrated in the late 1960’s.2 They can be used at flows from 250 L/min to 70 000 

L/min (see Table 2) and they are calibrated with the 26 m3 PVTt standard. The critical 

nozzles are designed for installation between two 4 inch ASA 150 lb flanges. 

Table 1. CFVs and the ranges of air flow they provide for pressures between 200 kPa and 

700 kPa.

d

(inches)

d

(mm)

Mass Flow

(g/s)

Volumetric Flow

(L/min)

min max min max

0.0112 0.2844 0.032 0.111 1.63 5.71

0.0155 0.3937 0.058 0.201 2.96 10.4

0.0255 0.6477 0.156 0.545 8.02 28.1

0.044 1.1176 0.464 1.624 23.9 83.6

0.063 1.6002 0.951 3.328 49.0 171

0.125 3.175 3.744 13.10 193 675

0.190 4.826 8.650 30.27 445 1559

0.250 6.35 14.97 52.41 771 2699

P and T

P controller Nozzle Instrumentation Meter under test
7

Table 2. Critical nozzles and the ranges of air flow they provide for pressures between 

200 kPa and 700 kPa.

d

(inches)

d

(mm)

Mass Flow

(g/s)

Volumetric Flow

(L/min)

min max min max

0.14234 3.615 4.85 17.0 250 875

0.20938 5.318 10.5 36.8 541 1893

0.28589 7.262 19.6 68.5 1009 3530

0.4004 10.170 38 134 1978 6924

0.56222 14.280 76 265 3901 13652

0.56402 14.326 76 267 3926 13739

0.79787 20.266 153 534 7856 27494

1.12602 28.601 304 1063 15646 54761

1.29841 32.979 404 1414 20803 72812

The nozzles have considerable overlap in their flow range so that they can be compared 

against one another. This is done in two ways: 1) the nozzles can be installed in series 

and the flow reported by each compared directly and 2) the nozzles can be used 

individually to calibrate a meter under test at the same flow and compared indirectly. 

Both of these tests are regularly used to assure the quality of calibration results. The tests 

will identify problems caused by incorrect nozzle calibration coefficients, leaks, and not 

attaining the critical pressure ratio at the nozzle working standard.

A second pressure sensor is desirable immediately downstream from the nozzle to assess 

whether the nozzle is operating under critical flow conditions. Critical flow conditions 

are normally achieved for a critical flow venturi when P1 >1.3 P2 and for a critical nozzle 

when P1 >2 P2, however the actual value will depend on the specific heat ratio of the gas 

and the shape of the nozzle outlet. The upstream pressure necessary to reach critical flow 

depends on the meter under test. One with a large pressure drop will increase the pressure 

at the nozzle outlet (P2) and increase the minimum pressure and flow at which that nozzle

can be used. We do not require a downstream pressure sensor in the WGFS, but the NIST 

operators are aware of the potential for erroneous flows at lower nozzle pressures and use 

a second, smaller nozzle (that operates at the same flow but a higher pressure) to assess 

that the flow calibrations are correct.

5 Data Acquisition and Control System

The control system for the WGFS utilizes a personal computer, a data acquisition card, a 

serial communication card and an IEEE-488 card. The interfaces permit communication 

with the necessary instrumentation, specifically pressure, temperature, differential 

pressure, and the pressure controller. A Labview program sets the upstream pressure 

based on a user defined recipe file, acquires readings from instrumentation selected by 

the operator, waits for flow stability criteria to be met, averages readings, and stores the 

averages in a file for later processing into a calibration report. Normally, the stability 

criterion is simply a 15 min timeout. After the timeout, the program averages data from 

all sensors, the nozzle, and the meter under test for 1 min and writes this data to the 

average file. This process is repeated 5 times at each pressure (or flow) set point. The 
8

program also writes all sensor readings to a “log” file every 10 s for the entire time that 

the program is running. The log file is useful for diagnosing problems after a test is over, 

plotting the stability of test conditions, etc.

The WGFS program calculates flow based on the nozzle pressure, temperature, throat 

diameter (d), and the gas species (all operator specified through the program front panel). 

The nozzle mass flow and volumetric flow (at a user specified reference temperature) are 

also written to the log and average files.

6 Nozzle Flow Calculations

The mass flow through a nozzle in the Working Gas Flow Standard,

m



, can be calculated 

using the following equation:

m Cd fit mth

  , (1)

where 

Cd

is a coefficient of discharge and the theoretical mass flow is calculated by:

*

0 R

th

0

P AC

m

RT

M

. (2) 

In Equation 2, 

P0

is the stagnation pressure upstream from the critical nozzle, 

4

2

A d

is the area at the throat of the critical nozzle, Ris the universal gas constant

(8314.471 [m2g]/[s2 K gmol]),

M

is the molecular mass, and 

T0

the stagnation 

temperature upstream from the critical nozzle. The variable 

* CR

is the real critical flow 

factor, also called the Johnson factor, and it is calculated via,

* *

* 0

R

0

a RT

C

P M

, (3)

where

*

and 

*

a

are the gas density and the sound velocity at the nozzle throat. To 

calculate

* CR

, isentropic and isoenergetic expansion is assumed from the stagnation 

conditions to the throat and an iterative solver is used along with a gas properties 

database. The Fluid Metrology Group uses the NIST properties database Refprop to 

calculate 

* CR

and the other gas properties (density and viscosity) needed for flowmeter 

calibrations.10

The critical pressure ratio 

Pr

allows one to evaluate whether the upstream pressure is 

sufficient to ensure sonic or critical flow. 

 

10 Lemmon, E.W., McLinden, M.O., and Huber, M.L., Refprop 23: Reference Fluid Thermodynamic and 

Transport Properties, NIST Standard Reference Database 23, Version 8.1, National Institute of Standards 

and Technology, Boulder, CO, April, 2007.
9

1

1

2

Pr , (4)

The critical pressure ratio can also be used to calculate the Mach number, Ma. The Mach 

number is needed to convert static pressure (P1) and temperature (T1) to stagnation 

values:

2

1

2

1

2

4

1 1

1

1

2

Ma

r

r r

P

d

D

P P

, (5)

where 

D

is the diameter of the upstream pipe and 

d

is the diameter of the nozzle throat. 

The stagnation temperature and pressure are obtained from,

T T Ma 1 RF

2

1

1

2

0 1 , (6)

with the recovery factor

RF 0.75

and 

1 2

0 1 Ma

2

1

P P 1 . (7)

The discharge coefficient can be calculated from previously correlated calibration results 

and the theoretical Reynolds number data using a second order polynomial:

th

2

th

1

d fit 0

Re

a

Re

a

C a , (8)

The polynomial coefficients, ai, are calculated by a least squares best fit of Cd versus 

1 Reth

data resulting from a calibration of the nozzle with the primary gas flow 

standard. Rethis the theoretical Reynolds number:

d

m

Re th

th

4 

. (9)
10

Here, is the gas dynamic viscosity. The discharge coefficient is calculated from the 

expression:

th

PVTt

d

m

m

C





. (10)

To obtain volumetric flow 

V



at the meter under test, the mass flow from Eq. 1 is divided 

by the gas density. The NIST properties database (Refprop) is used to calculate viscosity 

and density.

10

To summarize the process, we use the PVTt standards to calibrate nozzles (WGFS) and to 

obtain a polynomial that we can use to calculate the nozzle discharge coefficient. 

Subsequently, the polynomial is used to obtain the reference mass flow (and volumetric 

flow) from the nozzle to calibrate other flowmeters. In this way, nozzles are used to 

transfer the PVTt mass flow measurements to other flowmeter types. As will be explained 

in following sections, the major sources of uncertainty in this system are due to the 

nozzle pressure and temperature measurements, the PVTt flow measurements, and the 

stability of the nozzle discharge coefficient over time.

7 The WGFS with a Laminar Flowmeter Working Standard

At flows less than 1 L/min, laminar flowmeters are more practical working standards than 

critical nozzles.9 In the following, we discuss calibrations of laminar flowmeter working 

standards (in this case Molblocs) directly against our primary standards and we describe 

the differences in operation of the WGFS when using Molblocs instead of nozzles.

Laminar flowmeters are not normally calibrated directly on PVTt standards due to the 

difficulty in achieving steady state conditions at the meter under test. However, we have 

performed such calibrations and achieved agreement with other calibration approaches, 

and we conclude that if the collection times are long compared to the meter time constant, 

the average results are unaffected by the unsteady conditions.

We used the configuration shown schematically in Figure 4 to test the Molblocs directly 

on our 34 L PVTt standard. Two flow control valves were adjusted to obtain pressures of 

approximately 100 kPa immediately downstream from the Molbloc and 50 kPa 

downstream from the first flow control valve (the inventory volume). The 100 kPa

5 kPa pressure downstream from the Molbloc is designed to match the “downstream 

calibration” conditions set during its subsequent usage as a working standard. The first 

flow control valve, having about a 2 to 1 pressure ratio across it, serves as a crude critical 

flow device and gives partial isolation to the Molbloc from the pressure variations in the 

filling collection tank. Using the vacuum pump and the second flow control valve allows 

us to set the inventory volume pressure to 50 kPa and thereby maintain the mass 

cancellation technique described in our PVTt references while only filling the tank to half 

an atmosphere.
11

Figure 4. Schematic of the arrangement used to test a Molbloc laminar flowmeter on the 

PVTt standard, labeled with pressures at various locations.

980

985

990

995

1000

1005

1010

1015

1020

250 260 270 280

time (min)

flow (sccm)

90

92

94

96

98

100

102

104

106

250 300 350 400 450 500

time (min)

flow (sccm)

Figure 5. The effects of downstream pressure changes on the flow measured by a 

1 L/min full scale Molbloc during PVTt calibrations at flows of 1 L/min and 0.1 L/min.

Figure 5 illustrates that the upstream flow control valve only crudely isolates the 

flowmeter test section from the downstream pressure changes imposed by the filling 

collection tank. At 1 L/min, the flow increases approximately 2 % when the flow is 

diverted to the collection tank and at 0.1 L/min, the increase is about 12 %. As the tank 

fills, the pressure (and hence the flow) gradually return to the values present when the 

flow is directed through the bypass valve.

Despite these unsteady conditions at the meter under test, the PVTt calibration data for 

the Molbloc agrees extremely well with the calibration results from two other flow 

standards we used to calibrate the same meter. A 10 L/min full scale Molbloc was 

calibrated at 1.5 L/min by both the WGFS (using the 0.2921 mm nozzle working 

standard) and the 34 L PVTt standard, allowing us to use the Molbloc as a transfer 

standard between the two systems. The agreement in air was 0.03 % and in nitrogen was 

0.01 %.
12

A 1 L/min full scale Molbloc was calibrated on the 34 L PVTt standard and with our

gravimetric standard at 0.2 L/min and at 0.8 L/min. The agreement between the two 

primary standards was better than 0.011 % at both flows. The comparison tests between 

the three flow standards used in these calibrations show agreement well within the 

uncertainties of the standards. This demonstrates that both the PVTt and gravimetric flow 

standards can be used to establish calibrations for the working standard Molblocs. The 

laminar flowmeters used as working standards are listed in Table 3.

Table 3. Working standard laminar flowmeters and the flow ranges of air they provide.

Serial No. Mass Flow

(g/s)

Volumetric Flow

(L/min)

min max min max

2862 0.0000215 0.000215 0.001 0.01

1851 0.00011 0.00215 0.005 0.1

1857 0.0011 0.0215 0.05 1

1861 0.011 0.215 0.5 10

Figure 6. The WGFS using Molbloc working standards in a series arrangement with the 

meter under test (MUT).

When using Molblocs as working standards, the WGFS can be arranged in two 

configurations. The first is a series arrangement similar to that used for nozzles, but with 

the working standard in the downstream position (see Figure 6). A mass flow controller 

maintains steady state flow and pressure conditions for the downstream portions of the 

system. Positioning the working standard Molbloc downstream from the test section 

allows the working standard to be used at its normal “downstream Molbloc” pressures 

(100 kPa 5 kPa) at which it was calibrated. A throttling valve between the meter under 

test and the working standard is used to set the pressure to between 250 kPa and 525 kPa 

as required for some meters under test (e.g. an “upstream Molbloc”). Manual isolation 

valves are used before starting a test to prove that any leaks are smaller than 0.01 % of 

the smallest flow to be tested.

The second WGFS configuration is the “crossflow system”

11 and it is shown in Figure 7. 

In this system, gas at approximately 600 kPa is fed to a mass flow controller, through a 

“Y” Molbloc, and then alternately to a reference Molbloc or the meter under test. A 

computer data acquisition program changes the mass flow controller set point, opens and 

closes pneumatic valves to change the flow path, and records data from the three 

 

11 Bair, M., The Dissemination of Gravimetric Gas Flow Measurements through an LFE Calibration 

Chain, 1999 NCSL Workshop and Symposium, Charlotte, NC, July 13, 1999.
13

Molblocs. Leak tests are performed with the isolation valves. In this test, after each 

change in flow set point or flow path, we wait 15 min for steady state conditions and then 

record 30 s averages of the readings from the Y Molbloc and the Molbloc in use. For 

each flow setpoint, five averages for each flow path are recorded, and this is repeated on 

two occasions.

Figure 7. The WGFS / Molblocs crossflow system uses working standard Molblocs to 

calibrate another Molbloc meter under test (MUT).

The crossflow configuration allows both the working standard and the meter under test to 

be used under exit pressure conditions of 100 5 kPa. This is necessary because there are 

known to be small effects on the flow measurements when Molblocs are used at different 

pressures. The Y Molbloc is used to normalize flow data from the two downstream 

Molblocs in case flow supplied by the mass flow controller is not perfectly constant. The 

full scale flows of the A and B Molblocs should be identical so that the pressure 

conditions at the Y Molbloc are independent of the downstream flow path, else the 

pressure effects on flow will lead to errors in the normalization process.

8 Laminar Flow Calculations

The working standard Molblocs used in the WGFS are periodically calibrated using the 

static gravimetric method and the 34 L PVTt standard. Dimensional analysis shows that a 

quantity called the viscosity coefficient, VC, is the appropriate independent variable for 

the analysis of laminar flowmeter data:

2

2

L P

VC

(11)

where L is a length scale, is the gas density at the middle of the Molbloc, 

P

is the 

differential pressure, and is the gas viscosity. Property calculations are based on the 

Refprop database.10 The length scale L may be set to unity or to the measured length of 

the flow tubes, but it is important that the same value is used during calibration and usage 

of the flowmeter. A quantity called the flow coefficient, FC, is the dependent variable:

V

L P

FC 

3

, (12)
14

where 

V



is the actual volumetric flow at the middle of the Molbloc. A second order 

polynomial best fit is performed on FC versus VC calibration is calculated:

2

FCfit a0 a1 VC a2 VC . (13)

During subsequent use as a working standard, the differential pressure, temperature, and 

absolute pressure at the middle of the Molbloc are used to calculate VC and the 

polynomial gives a value for FCfit. This value is then used to calculate the mass flow.

fit

3

FC

L P

m V  . (14)

Actual volumetric flow at the meter under test can be calculated from the mass flow 

using the density at the meter under test, calculated from local pressure and temperature 

measurements and the gas equation of state.

9 Uncertainty Overview

As described in the references12, 13 consider a process that has an output, y, based on N

input quantities, xi. For the generic basis equation:

( , , , )

1 2 N

y y x x  x , (15)

if all the uncertainty components are uncorrelated, the standard uncertainties are 

combined by root-sum-square (RSS):

N

u x

x

y

u y

i 1

i

2

2

i

c

, (16)

where u(xi) is the standard uncertainty for each of the inputs, and uc(y) is the combined 

standard uncertainty of the measurand. The partial derivatives in Eq. 16 represent the 

sensitivity of the measurand to the uncertainty of each input quantity. 

In cases where correlated uncertainties are significant (as in the following analysis), the 

following expression should be used instead of Eq. 16:

N N N

u x u x r x ,x

x

y

x

y

u x

x

y

u y

i 1

1

i 1 j i 1

i j i j

i j

i

2

2

i

c 2 , (17)

 

12 International Organization for Standardization, Guide to the Expression of Uncertainty in Measurement, 

Switzerland, 1996.

13 Coleman, H. W. and Steele, W. G., Experimentation and Uncertainty Analysis for Engineers, John Wiley 

and Sons, 2nd edition, 1999.
15

where r(xi, xj) is the correlation coefficient, which ranges from –1 to 1, and equals zero if 

the two components are uncorrelated. As will be seen in the following analysis, some 

uncertainty components in the WGFS are correlated and this leads to a significant 

improvement in the uncertainty of the measurand. 

The calibration coefficient for a meter under test will have additional uncertainties not 

considered in the following analyses due to measurements associated with the meter 

under test. For instance, if the meter under test is a laminar flowmeter, uncertainties 

related to the temperature and pressure measurements at the meter must be included in 

the uncertainty of the calibration coefficients. Also, all uncertainties given herein are of 

type B. The type A uncertainties from the meter under test and the WGFS are assessed by 

taking the standard deviation of repeated calibration measurements for a particular 

calibration.

The uncertainties that customers experience when they use the calibrated meter to make 

flow measurements in their own laboratory will have additional uncertainties beyond 

those given in the calibration report. These include uncertainties due to the pressure and 

temperature instrumentation, the drift in the meter calibration constants after it was tested 

at NIST, environmental effects on the meter (e.g. temperature), the effects of differences 

in gas composition, leaks, etc.

The uncertainties discussed below are generally k = 1, standard, or 68 % confidence level 

uncertainties. At the conclusion of the uncertainty analysis, a coverage factor of 2 will be 

applied to give an expanded uncertainty for mass flow measurements with an 

approximate 95 % confidence level.

10 Uncertainty Analysis for Nozzle Working Standards

The equations utilized to calculate mass and volumetric flow from the WGFS have been 

discussed in prior sections. In Fig. 8, the nozzle flow calculation process is summarized 

in a diagram that shows the measurement chain used to calculate flow. At the top of the 

diagram are the outputs, mass flow and volumetric flow. The inputs to the calculation of 

m



are the theoretical mass flow and the discharge coefficient as presented in Eq. 1. To 

calculate the discharge coefficient, the polynomial coefficients and the theoretical 

Reynolds number are required. The polynomial coefficients are calculated from nozzle

calibration data obtained from the PVTt flow standard. The inputs to the theoretical mass 

flow are shown only once (at the bottom of the chart). The right side of Fig. 8 shows the 

quantities necessary to calculate density and volumetric flow, i.e. pressure, temperature, 

compressibility, the universal gas constant, and molecular weight.
16

Figure 8. Relationships between measured and calculated quantities used to obtain mass 

and volumetric flow from nozzle working standards.

Certain components of uncertainty are common to the nozzle calibration and usage, 

leading to reduced uncertainty in the WGFS flow measurement. The diagram in Fig. 8

has been divided into “nozzle calibration” and “nozzle usage” regions to highlight these 

correlated uncertainties. For example, the theoretical mass flow calculation is made 

during both calibration and usage (although the inputs are only shown on one of the two 

occasions). If an erroneous value for the nozzle throat diameter is used during the 

calibration process, it will not increase the uncertainty of the mass flow measurement 

made via the nozzle as long as the same erroneous throat diameter value is used again. 

The discharge coefficient not only corrects for the non-ideal behavior of the flow, but it 

also corrects for errors in measurement of the throat diameter. Correlations also exist for 

uncertainties in the universal gas constant, the molecular weight of the gas (as long as the 

gas is the same composition between calibration and usage), the critical flow factor, and 

viscosity. If the same pressure and temperature sensors were used during nozzle

calibration and usage, uncertainties in these sensors would be correlated as well. 

However, we do not always use the same pressure and temperature sensors between the 

two operations (and the portion of their uncertainty that is correlated is relatively small) 

and hence will not claim correlation for these components.
17

After eliminating these correlated uncertainties, the remaining significant components of 

uncertainty are: 1) the mass flow from the primary standard 

PS u m

, 2) the pressure 

measurements 

u P

, 3) the temperature measurements 

u T

, 4) the stability of the 

discharge coefficient over time and the varying conditions of usage 

d u C

. The pressure 

and temperature uncertainties occur twice, once for calibration, once for usage. 

Therefore, expanded uncertainty Ue of a mass flow measurement made with the working 

standad nozzles can be expressed as:

2

d

2

2 2

e NOZ P S 2

1

U m 2 u m 2 u P 2 u T u C . (18)

Uncertainty related to “storage effects” have been assumed negligible based on the long 

wait for thermal and pressure stability (15 min), the slow rate of change of the 

environmental conditions in the room where the WGFS is used, and the short period over 

which data is averaged (1 min or less). Uncertainties due to leaks out of the system have 

also been neglected because leak checks are always performed prior to collecting data 

from the WGFS.

The uncertainty of the volumetric flow has another component that arises from the 

density of the gas at the meter under test and the measurements of temperature and 

pressure needed for its calculation.

The standard uncertainty (k = 1) of each of the significant uncertainty components will be 

considered in the following sections. More detail about many of these uncertainty values 

can be found in the references5due to shared traceability paths.

Primary Standard: The uncertainty of the 34 L and 677 L PVTt standardsis 0.0125 %.

14

The uncertainty of the 26 m3 PVTt standard is 0.045 %.

4 We will primarily use 0.0125 %

in the following analysis (the uncertainty for a 0.01 L/min to 2000 L/min flow), but we 

will also consider the uncertainty for nozzles calibrated with the 26 m3 PVTt standard in 

the text.

Pressure: The WGFS uses Paroscientific pressure transducers with full scale of 1400 kPa 

to measure the nozzle pressure. Calibration records and an uncertainty analysis of the 

pressure calibration process leads to a value of 0.02 % for pressures between 200 kPa and 

1400 kPa. 

Temperature: The standard uncertainty for the temperature sensors used in the WGFS is 

0.01 % as a result of sampling uncertainties. In other testing arrangements, sampling

uncertainties may be a much more significant source of uncertainty. For instance, when 

two nozzles are used in series, and if a heat exchanger is not used between them to 

thermalize the gas, the cold jet from the first nozzle may impinge on the second nozzle’s 

 

14 Wright, J. D. and Johnson, A. N., Lower Uncertainty (0.015 % to 0.025 %) of NIST’s Standards for Gas 

Flow from 0.01 to 2000 Standard Liters / Minute, Proc. of the 2009 Measurement Science Conference, 

Anaheim, CA, 2009.
18

thermister and cause an incorrectly low temperature measurement. The calibration 

uncertainties of the temperature sensors are much better: the uncertainty of the water bath 

temperature calibration procedure is less than 0.002 % and the drift observed for the four 

thermisters over a period of more than one year is less than 0.001 %. 

Stability of the Discharge Coefficient: The discharge coefficient of a nozzle may change 

over time due to dirt or scratches near the nozzle throat caused by mishandling. 

Experimental measurements of Cd over time will also show variation caused by changes 

in calibration of the pressure and temperature sensors used and changes in the primary 

standard. A third source of Cd drift is the influence of environmental conditions 

(particularly temperature) on the flow through the nozzle. First order effects of 

temperature on the nozzle are accounted for by the gas properties in equation 1, but 

second order effects are not. These include thermal expansion of the nozzle body and 

throat, the thermal boundary layer, and sampling errors caused by the temperature sensor 

being located upstream from the location where temperature is needed. Based on periodic 

calibrations performed over many years on the nozzles used in the WGFS, we find the Cd

stability to be 0.03 %.

Table 4. Uncertainty for the actual volumetric flow at the meter under test using nozzle 

working standards. The contribution percentage is calculated using the square of each 

component relative to the sum of the squares of all components.

34 L and 677 L PVTt 26 m3 PVTt

Uncertainty Category

Standard 

Uncertainty

(k = 1, %)

Contribution

(%)

Standard 

Uncertainty

(k = 1, %)

Contribution

(%)

Calibration 0.0125 6 0.045 46

Pressure ( 2) 0.02 32 0.020 18

Temperature ( 2) 0.01 8 0.010 5

Cd Stability 0.03 36 0.030 21

Density 0.021 18 0.021 10

Combined Uncertainty 

(k = 1) 0.050 0.066

Expanded Uncertainty 

(k = 2) 0.100 0.132

Density: The uncertainty of the density of air at the meter under test is due to the 

uncertainty in the measurement of pressure and temperature at the meter under test as 

well as the uncertainty of the equation of state (compressibility, molecular weight, and 

the universal gas constant). The uncertainty of the density of air using our 

instrumentation is 0.021 %.

Nozzle Working Standard Flow Uncertainty: The expanded uncertainty of a mass flow 

measurement of dry air made with the WGFS using nozzles calibrated in the 34 L or 

677 L PVTt standards is 0.09 % (k = 2). As shown in Equation 18, the nozzle pressure 

and temperature uncertainties contribute twice, and their uncertainty contribution takes 
19

this into account. If the 26 m3 PVTt system is the source of the nozzle flow calibration, 

then the WGFS mass flow uncertainty increases to 0.125 % (k = 2).

The sources of uncertainty for a volumetric flow measurement and their magnitude are 

listed in Table 4. For flows less than 2000 L/min it is 0.10 % (k = 2). At higher flows 

where the 26 m3 PVTt is the source of the nozzle calibration, the volumetric flow 

uncertainty is 0.132 % (k = 2).

11 Uncertainty Analysis for Laminar Flowmeter Working Standards

The standard uncertainty (k = 1) of each of the significant uncertainty components for a 

Molbloc used as a working standard will be considered in the following text. More detail 

about many of these uncertainty components can be found in the references.

5, 15

Primary Standard: The Molbloc working standards are calibrated with the 34 L PVTt

standard and the static gravimetric standard which both have standard uncertainty of 

0.0125 %. 

Flow Model and Gas Properties: Changes in the gas species used as well as variations in 

the pressure and temperature of the gas metered are corrected via the dimensionless 

quantities viscosity coefficient and flow coefficient. Uncertainties in the flow model 

exist, most likely due to errors in the viscosity values available in RefpropError! Bookmark not 

defined. and inadequacy of the physical model for the laminar flowmeter. If the Molblocs 

were calibrated and subsequently used in gas of the same composition at the same 

pressure and temperature, uncertainties due to properties would be correlated and 

negligible. For this reason, we use nitrogen of 99.998 % purity or better (zero grade 

cylinders or industrial grade liquid). 

The uncertainty of mass flow measurements by critical nozzles and laminar flowmeters 

due to moisture in air was analyzed in a prior publication.

9 We applied the analysis to the 

variations in dew point temperatures of the compressed, dried air used at NIST (dew 

point temperatures of -15 C or lower) and found that this leads to standard uncertainties 

of 0.01 %. It is important to note that the uncertainties caused by gas properties can be 

much larger if other versions of air, such as UZAM or air with more water vapor are 

used.

Gas property and flow model uncertainties are apparent in plots of FC versus VC for 

various gas species (see Figure 9). They are also the reason for the restrictions on 

pressure conditions for the working standard (100 kPa 5 kPa). The standard uncertainty

due to the flow model and gas properties is 0.03 %.

 

15 Bair, M., Uncertainty Analysis for Flow Measured by Molbloc-L and Molbloc-S Mass Flow Transfer 

Standards, DH Instruments Technical Note 2011TN06A, May, 18, 2004.
20

5720

5740

5760

5780

5800

5820

5840

5860

5880

5900

0 1 2 3 4 5

VC / 10 6 FC

N2

Air

CO2

grav N2

best fit

Figure 9. Flow coefficient versus viscosity coefficient for a 0.1 L/min working standard 

Molbloc (serial number 1851) in nitrogen, air, and carbon dioxide gases.

Differential Pressure: The differential pressure uncertainty is 0.03 % if taring procedures 

are followed and if the Molblocs are used at flows of more than 10 % of full scale. 

Calibration Stability: The stability of the calibration of a Molbloc is related to the 

geometrical stability of the laminar flow path within the flowmeter and the accumulation 

or removal of contaminants from the flow path surfaces. Experimental measurements of 

calibration stability by repeated, periodic calibrations will show variations due to pressure 

and temperature sensor drift, gas composition changes, primary standard drift, and 

environmental influences (esp. room temperature).

The influence of environmental temperature and the differences between the flowmeter 

body temperature and the inlet gas temperature will be considered as calibration stability 

uncertainties in this analysis. We conducted temperature sensitivity tests on a model 1E4 

Molbloc-L in 2003.16 These tests showed temperature sensitivity of the flow of 0.01 % or 

less per C of temperature difference between the environment and the inlet gas. 

Therefore, if we assume that the laboratory temperature is maintained within 1 C of the

normal 23.5 C, standard uncertainties due to environmental temperature differences are 

less than 0.01 %.

We performed periodic calibrations over more than one year on a set of 4 Molblocs under 

the varying environmental conditions actually occurring in the laboratory and found the 

standard deviation of repeated calibrations to be 0.04 % or less and we use this figure as 

the calibration stability for our working standard Molblocs.

 

16 Wright, J. D., What is the “Best” Transfer Standard for Gas Flow?, Proceedings of FLOMEKO, 

Groningen, Netherlands, (2003), see figure 6.
21

Density: The uncertainty of the density of air using our instrumentation and the equation 

of state is 0.021 %.

Laminar Flowmeter Working Standard Uncertainty: The uncertainty categories and their 

magnitudes are summarized and tallied in Table 5. The most significant contributors are 

differential pressure and calibration stability. The differential pressure components 

contribute twice (they are not considered correlated), once during calibration and again 

during usage. The density component also contributes twice, once at the working 

standard and a second time at the meter under test. They are not considered correlated 

because the largest sources of uncertainty to density are calibration drifts in the pressure 

and temperature sensors, not their common calibration traceability chains.

Table 5. Uncertainty for a actual volumetric flow at the meter under test using laminar 

flowmeter working standards.

Uncertainty Category Standard Uncertainty

(k = 1, %)

Contribution

(%)

Calibration 0.0125 3

Flow Model and Gas Prop. 0.030 34

Differential Pressure ( 2) 0.030 17

Calibration Stability 0.040 30

Density ( 2) 0.021 17

Combined Uncertainty 

(k = 1) 0.073

Expanded Uncertainty 

(k = 2) 0.146

12 Summary

The principles of operation and uncertainty analysis for a Working Gas Flow Standard 

based on critical nozzles and laminar flowmeters have been presented. Laminar 

flowmeters (Molblocs) cover flows from 0.001 L/min to 10 L/min with k = 2 expanded 

uncertainty of 0.15 %. Critical flow venturis and critical nozzles used in the WGFS cover 

flows between 1 L/min and 70 000 L/min and allow one to measure reference mass and 

volumetric flow with k = 2 expanded uncertainty of 0.10 % at flows less than 

2000 L/min and 0.13 % at flows larger than 2000 L/min. The traceability of the working 

standards is to a primary gravimetric flow standard and three PVTt primary flow 

standards (34 L, 677 L, and 26 m3). For certain flowmeter types and over certain flow 

ranges, the WGFS is more time efficient than the primary standards, however it does 

have larger uncertainty.
NIST Test Number: 836-123456-03-01 Page 1 of 6

Calibration Date: February 24, 2007

Appendix: Sample Calibration Report

REPORT OF CALIBRATION

FOR

A LAMINAR FLOW METER

March 30, 2007

Mfg.: LFE Manufacturing

Model No: asc124

Serial No: 1234

submitted by

Flowmasters, Inc.

Anywhere, AZ

Purchase Order No. A123 dated February 27, 2007

The flow meter identified above was calibrated by flowing filtered dry air using the 

critical flow venturis (CFV) in the NIST Working Gas Flow Standard. The expanded 

uncertainty (k = 2) of the mass flow measured with the Working Gas Flow Standard is 

0.1 % of reading. A 0.0255 in. (0.6477 mm) diameter CFV was used at a laminar flow 

meter differential pressures of 0.0783 kPa; a 0.063 in. (1.6002 mm) diameter CFV was 

used at laminar flow meter differential pressures of 0.6232 kPa to 1.2449 kPa; and a 

0.125 in. (3.175 mm) diameter CFV was used at laminar flow meter differential pressures 

of 1.2449 kPa to 2.4854 kPa.

The flow meter was calibrated at steady state conditions (following a 15 min wait), at five 

flows. Five (or more) 60 s averages of instrument data were gathered at each of these 

flows on two different occasions. As a result, the tabulated flow meter calibration data 

are averages of ten or more individual calibration measurements.

The laminar flow element (LFE) and sensors were installed as shown in Figure 1. During 

the gas collection period, the differential pressure, 

P

, across the LFE was measured, as 

well as the temperature which was measured downstream of the meter, and absolute 

pressure, 

Pup

, of the gas which was measured at the upstream pressure tap of the LFE. 

Differential pressure was measured with two transducers (in parallel) in order to give us 

redundant measurements. The downstream end of the LFE was vented to the atmosphere. 

Temperature, T, was measured downstream of the LFE. The previously mentioned 
REPORT OF CALIBRATION Gas Flow Meters

Flowmasters, Inc. Purchase Order No. A123

NIST Test Number: 836-274920-07 Service ID Number: 18010C

Calibrated March 26-28, 2007 by Gina Kline Page 2 of 6

quantities were measured with the following NIST sensors1: Chub E4 SN A27253, 

thermistor #3

(T)

, Paroscientific SN 80832

( ) Pup

, and Mensor SNs 360503 and 360504 

( P) .

Figure 1. Piping and instrumentation connections used during the flow meter calibration 

The calibration results can be found in Table 1 and in Figure 2. Table 1 lists the 

temperature and pressure at the LFE (

T

and 

Pup

), the differential pressure, 

P

, the 

volumetric flow at the inlet of the LFE, 

Vup



, and two dimensionless parameters, a 

viscosity coefficient, 

2 2 VC L P

, and a flow coefficient 

FC L P Vup



3

, 

where is the absolute viscosity, is the gas density at the LFE, and 

L

is a length 

scale for the LFE. The length, 

L

, used to calculate the viscosity and flow coefficients 

was 7.62 cm (3.00 in.), based on an approximate length of the laminar flow tubes. 

Density at the LFE was calculated from the pressure and temperature at the LFE. The gas 

density and viscosity were calculated using best-fit equations which are based on the 

NIST gas properties database.2, 3In January 2003, the correlation equation for viscosity 

used by the NIST Fluid Metrology Group was changed from an older reference to the one 

used in this report. The five previous sets of calibration data shown in Figure 2 were 

 

1 The instrument make and model is stated for completeness of the calibration record and to document the 

chain of calibration traceability and is not an endorsement of the product.

2 Lemmon, E. W., McLinden, M. O., and Huber, M. L., Refprop 23: Reference Fluid Thermodynamic and 

Transport Properties, NIST Standard Reference Database 23, Version 7, National Institute of Standards 

and Technology, Boulder, Colorado, 2002.

3 Wright, J., Gas Properties Equations for the NIST Fluid Flow Group Gas Flow Measurement 

Calibration Services, 2/04.
REPORT OF CALIBRATION Gas Flow Meters

Flowmasters, Inc. Purchase Order No. A123

NIST Test Number: 836-274920-07 Service ID Number: 18010C

Calibrated March 26-28, 2007 by Gina Kline Page 3 of 6

reprocessed using the new property correlations for comparison to the recent results 

because the old and new calculations of viscosity differ by as much as 0.7 %.

The viscosity and flow coefficients are calculated with consistent units for 

L , , P , 

, and 

Vup



so that the results are dimensionless. Table 1 includes the relative expanded 

uncertainty of the flow coefficient (

Ue

) for this calibration.

Figure 2 shows the viscosity and flow coefficients for the LFE under test along with error 

bars for the present calibration, which represent the uncertainty of the flow coefficient. 

Also shown in Figure 2 are the results of five previous calibrations performed in dry 

filtered air.

The calibration data is presented in the dimensionless form so that a graph of the flow 

coefficient versus the viscosity coefficient can be used to determine 

Vup



accurately when 

operational conditions during use differ from those during calibration (such as a different 

temperature, absolute pressure, or gas). Note: if the volumetric flow at the outlet of the 

LFE is desired, the upstream flow must be multiplied by the ratio of the upstream 

pressure to the downstream pressure and by the ratio of the downstream temperature to 

the upstream temperature.

Table 1: Calibration data for LFE SN 739190-F1.

T

[K]

Pup

[kPa]

P

[kPa]

Vup



[alm]4

VC

x10 -10

FC

x10-7

Ue

[%]

295.58 100.74 0.0783 9.1840 0.1600 1.2321 0.52

295.60 101.33 0.6232 71.7856 1.2810 1.2546 0.25

295.54 101.95 1.2449 139.9243 2.5761 1.2860 0.25

295.50 102.55 1.8647 204.1679 3.8829 1.3203 0.25

295.49 103.16 2.4854 265.0542 5.2064 1.3556 0.26

 

4

alm = actual liters per minute using LFE pressure and temperature.
REPORT OF CALIBRATION Gas Flow Meters

Flowmasters, Inc. Purchase Order No. A123

NIST Test Number: 836-274920-07 Service ID Number: 18010C

Calibrated March 26-28, 2007 by Gina Kline Page 4 of 6

1.20

1.25

1.30

1.35

1.40

0 2 4 6

VC*10-10

FC*10- 7

May 1993

July 1994

February 1996

August 1997

March 2002

March 2007

Figure 2. LFE SN 739190-F1 dimensionless calibration results.

An analysis was performed to assess the uncertainty of the results obtained for the meter 

under test.5, 6, 7 The process involves identifying the equations used in calculating the 

calibration result (measurand) so that the sensitivity of the result to uncertainties in the 

input quantities can be evaluated. The approximately 

67%

confidence level uncertainty 

of each of the input quantities is determined, weighted by its sensitivity, and combined 

with the other uncertainty components by root-sum-square to arrive at a combined 

uncertainty (

Uc

). The combined uncertainty is multiplied by a coverage factor of 2.0 to 

arrive at an expanded uncertainty (

Ue

) of the measurand with approximately 

95%

confidence level.

As described in the references, if one considers a generic basis equation for the 

measurement process, which has an output, 

y

, based on 

N

input quantities, 

i

x ,

 

5

International Organization for Standardization, Guide to the Expression of Uncertainty in Measurement, 

Switzerland, 1996 edition.

6

Taylor, B. N. and Kuyatt, C. E., Guidelines for Evaluating and Expressing the Uncertainty of NIST 

Measurement Results, NIST TN 1297, 1994 edition.

7 Coleman, H. W. and Steele, W. G., Experimentation and Uncertainty Analysis for Engineers, John Wiley 

and Sons, 2nd ed., 1999.
REPORT OF CALIBRATION Gas Flow Meters

Flowmasters, Inc. Purchase Order No. A123

NIST Test Number: 836-274920-07 Service ID Number: 18010C

Calibrated March 26-28, 2007 by Gina Kline Page 5 of 6 ( , , , )

1 2 N

y y x x  x

(1)

and all uncertainty components are uncorrelated, the normalized expanded uncertainty is 

given by,

N

i i

i

i

e c

x

u x

k s

y

U y

k

y

U y

1

2

2

(2)

In the normalized expanded uncertainty equation, the 

u( x )'s i

are the standard 

uncertainties of each input, and 

s ' s i

are their associated sensitivity coefficients, given 

by,

y

x

x

y

s

i

i

i

(3)

The normalized expanded uncertainty equation is convenient because it permits the usage 

of relative uncertainties (in fractional or percentage forms) and of dimensionless 

sensitivity coefficients. The dimensionless sensitivity coefficients can often be obtained 

by inspection because for a linear function they have a magnitude of unity.

For this calibration, the uncertainty of the flow coefficient has components due to the 

measurement of the actual volumetric flow at the meter under test 

u V



, the differential 

pressure 

U P

, the gas viscosity 

u

, and the reproducibility of the test 

u R

, all of 

which have sensitivity coefficients of 1. The uncertainty of the actual volumetric flow has 

uncertainty components from the mass flow measurement by the primary standard,

u m

 0.05% , as well as the pressure, 

u P 0.02%

, and temperature, 

u T 0.03%, 

measurements used to convert the mass flows to actual volumetric flows at the meter 

under test, all of which have sensitivity coefficients of 1. The RSS of these components 

gives the combined uncertainty for the volumetric flow at the meter under test of

0.06% .

The uncertainty of the differential pressure measurements is 0.1 % based on analysis of 

calibration records, except at the lowest flow where the differential pressure uncertainty 

was 0.25 %.

The present uncertainty analysis does not include uncertainty in the experimental 

measurements of viscosity found in the references, which can amount to 

1%

or more. To 

prevent errors due to viscosity, the user must use the same gas and viscosity expression 

used by NIST when using the results given in Table 1, or must use flow coefficients 

calculated with their preferred viscosity relationship. Flow measurements made with this 

LFE and a gas other than air will have greater uncertainty than that given in the present 

analysis due to uncertainty in the gas viscosity. Given these assumptions, the viscosity 

uncertainty depends primarily on the uncertainty of the gas temperature measurement 

(0.03 %).
REPORT OF CALIBRATION Gas Flow Meters

Flowmasters, Inc. Purchase Order No. A123

NIST Test Number: 836-274920-07 Service ID Number: 18010C

Calibrated March 26-28, 2007 by Gina Kline Page 6 of 6

To measure the reproducibility8of the test, the standard deviation of the flow coefficient 

at each of the nominal flows was used to calculate the relative standard uncertainty (the 

standard deviation divided by the mean and expressed as a percentage). Using the values 

given above results in the expanded uncertainties listed in the data table and shown as 

error bars in the figure.

For the Director,

National Institute of Standards and Technology

Dr. John D. Wright

Project Leader, Fluid Metrology Group

Process Measurements Division

Chemical Science and Technology Laboratories

Ms. Gina M. Kline

Physical Science Technician, Fluid Metrology Group

Process Measurements Division

 

8 Reproducibility is herein defined as the closeness of agreement between measurements with the flow 

changed and then returned to the same nominal value.