Article Ref: B-017 

June 1, 2011

The Proper Use of Substitute Test Fluids

Byline: Marc Buttler

Liquid-measuring devices are used in many different commercial applications. The vast array of 

liquid products that are measured includes some products that may pose difficulties in testing. 

NIST Handbook 44, Specifications, Tolerances, and Other Technical Requirements for Weighing 

and Measuring Devices, allows for substitution of fluids during testing in place of the actual 

fluids that will be used in commerce to mitigate some of these issues.

The most important consideration that will determine if one fluid can be used as a viable 

substitute for another is whether the results of the testing with the alternative fluid will 

accurately predict the performance of the device when measuring the fluid with which the 

device will normally be used. This article provides some guidance and considerations to help 

those who need to make decisions about whether or not to allow substitute fluids for testing.

Handbook 44 Notes on Test Liquids

Within Section 3.30. “Liquid-Measuring Devices” of NIST Handbook 44, paragraph N.1. “Test 

Liquid” makes the following statement: “The liquid used for testing of a liquid-measuring 

device shall be the type the device is used to measure, or another liquid with the same 

general physical characteristics.” The Handbook also states that “Following the completion of 

a successful examination of a wholesale device, the weights and measures official should 

attach a label or tag indicating the type of liquid used during the test.”

Similar statements also appear in other sections of the Handbook. For example, 

Section 3.31. “Vehicle-Tank Meters” and Section 3.3. “Milk Meters” include the above 

statements and both of these Sections further specify that, “A milk-measuring system shall be 

tested with the type of milk to be measured when the accuracy of the system is affected by the 

characteristics of the milk (e.g., positive displacement meters).” The latter example holds the 

key to understanding the phrase “another liquid with the same general physical 

characteristics.” The intent of this statement is to allow a fluid to be used as a valid substitute 

for another fluid during testing, so long as it is reasonable to expect that whatever differences 

do exist between the physical characteristics (properties) of the fluids will not have a significant 

impact on the meter’s performance and the conclusions from the testing. 

Justifications to Use a Substitute Fluid

Generally speaking, the most conclusive results can be obtained by testing with the same fluid 

that is to be sold commercially (hereafter referred to as the “primary fluid”). However, there 

Weights and Measures Connection, Vol. 2, Issue 3 Page 1
Article Ref: B-017 

are many reasons that may make it impractical to use the primary fluid for testing. When there 

is good reason to justify the use of a substitute fluid for testing, the suitability of the 

substitution fluid must be authorized by the regulatory official, and must also comply with any 

restrictions made by the meter manufacturer.

• In cases where handling of the product for the purpose of testing may result in risks to 

safety, a substitute fluid may well be justified. Testing methods often require greater 

exposure of the product to people and the environment than is present during normal 

operation. If testing with the primary fluid poses hazards, then a substitute fluid that is 

safer for the testing should be considered if one is available.

• Certain products may also pose a risk of damage to the integrity of the reference 

standards that are needed for a test. For example, a product that might result in 

corrosion, erosion, or fouling of an otherwise carefully maintained and valuable field 

test standard would clearly be something that the official would want to avoid.

• The cost and expense of testing will often be impacted by whether the product used for 

test drafts can be returned to inventory for later sale or must be discarded. It may be 

that the testing process renders the product unsalable or that there is unacceptable risk 

of contamination due to the test. In such cases, substitution of an acceptable 

alternative fluid can help to avoid excessive costs that would be prohibitive to a 

reasonable maintenance cycle interval.

Other reasons may arise to justify a substitute fluid and all should be considered on a case by 

case basis using good judgment to decide if they warrant the substitution of an alternate fluid. 

In any case in which a reason is deemed to be justified, the need still remains to also confirm 

that the proposed substitute fluid complies with the statement that the fluid be “another liquid 

with the same general physical characteristics.”

Substitute Fluids - The Four Key Factors

The suitability of a fluid for testing really depends on influence factors that affect the measuring 

device that is to be tested. If a particular fluid property (characteristic), such as density or 

viscosity, is known to have an influence on the performance of the device to be tested, then it is 

important that any proposed substitute fluid have properties (characteristics) similar to the 

primary fluid. Because different device technologies and designs are affected by different 

influence factors, the type of measuring device will determine the fluid characteristics that 

must be similar between the primary and the substitute fluids. For example, the performance 

of positive displacement meters is typically influenced by fluid viscosity, whereas conductivity 

of a fluid may influence the performance of a magnetic flow meter.

There are four key factors to consider while assessing the acceptability of one fluid as a 

substitute for another:

Weights and Measures Connection, Vol. 2, Issue 3 Page 2
Article Ref: B-017 

• the DEVICE TYPE to be tested,

• the difference in the CRITICAL FLUID CHARACTERISTIC for that device,

• whether or not the two fluids are in the same PRODUCT CATEGORY TEST 

REQUIREMENT GROUP for that device, and

• the difference in OTHER FLUID CHARACTERISTICS IDENTIFIED BY THE MANUFACTURER 

as important for that device.

Device Type. The reason that the type of device is such an important consideration is that 

some fluid characteristics affect different device types differently than they will other device 

types. A change in one particular fluid characteristic that has no effect at all on one type of 

device may, in contrast, have a significant effect on another type of device. For example, 

viscosity has two effects that are unique to a turbine flow meter. Viscosity affects the flow 

profile and the boundary layer thickness as the fluid flows over the turbine blades, resulting in 

non-linear performance. Viscosity also creates mechanical drag on the turbine bearing, which 

impacts the resistance of the rotor to turn freely. Comparatively, magnetic and Coriolis flow 

meters are less subject to viscosity changes, but instead have other unique influence factors 

related to other fluid characteristics.

Critical Fluid Characteristic. Device types have a single “Critical Fluid Characteristic”, which is 

specific to each device type as defined by the policies used for National Type Evaluation 

Program (NTEP) evaluations. NTEP policies for grouping products according to key 

characteristics were developed with input from State and Federal government officials, device 

manufacturers, and other experts. These policies take into account meter type and the effect 

of the different product characteristics on performance. For example, for mass flow meters, 

the critical fluid characteristic is “specific gravity” because some mass flow meters use 

automatic density correction to correct for changes in product density. A fluid with a 

significantly different specific gravity or density than the primary fluid would be a poor 

substitute to use for testing. 

National Conference on Weights and Measures (NCWM) Publication 14 for Measuring Devices 

contains a valuable tool to help establish whether or not one fluid should be substituted for 

another. The “Table of Product Families for Meters” can be found in Part C of the Technical 

Policy in Publication 14. This table is organized to show what fluid products share the same 

“Product Category” for a given type of meter. The table also shows, in the definitions of the 

various tests, what the critical fluid characteristic is for a meter type.

The “Critical Fluid Characteristics” for four different meter types are shown in Table 1. 

Weights and Measures Connection, Vol. 2, Issue 3 Page 3
Article Ref: B-017 

Table 1. 

Critical Fluid Characteristics by Meter Type

as Determined by NCWM Publication 14 for Measuring Devices

Mass Flow 

Meters

Magnetic Flow 

Meters

Positive 

Displacement 

(PD) Meters

Turbine 

Meters

Critical Fluid 

Characteristic

Specific Gravity 

(Density)

Conductivity Dynamic 

Viscosity

Kinematic 

Viscosity

The critical fluid characteristic can also be identified by consulting the NTEP Certificate of 

Conformance for a given meter or another meter of the same type. The “Applications” section 

of the Certificate of Conformance should state the range values for the critical fluid 

characteristic that are approved for a device based on previous NTEP testing and the 

application of Technical Policy C.

Product Category Test Requirement Group. Product categories are defined as groups of 

products that have been shown to produce similar performance results for a given meter type. 

These categories were developed to minimize the amount of testing required during NTEP 

evaluations intended to demonstrate meter performance with multiple products. Examples of 

product categories include fertilizers, crop chemicals, compressed gases, compressed liquids, 

water, chemicals, fuels, solvents, and heated products. 

Product categories are combined together in NCWM Publication 14 into groupings that are 

defined by the type and extent of testing required. These groupings are identified by the 

designation of the “test requirements.” There are six different test requirements named “Test 

A” through “Test F.” Some test requirements apply only to certain device types. “Test F” 

requirements apply only to magnetic flow meters, “Test C” requirements apply only to positive 

displacement meters, “Test B” requirements apply only to mass flow meters, and “Test E” 

requirements apply only to turbine meters. Other test requirements apply to multiple device 

types. “Test D” requirements apply to all device types and “Test A” requirements apply to 

either turbine or positive displacement meters. The specific test requirements that apply in any 

given case will depend on both the device type and the product.

Each of the six test requirements explain the grouping of product categories by describing what 

products are covered by the test requirements. For example, “Test B” requirements state that 

“The Certificate of Conformance will cover all products in all product categories listed in the 

table under Test B within the specific gravity range tested.” Because the coverage under “Test 

B” includes all products in all product categories under “Test B”, then all “Test B” product 

Weights and Measures Connection, Vol. 2, Issue 3 Page 4
Article Ref: B-017 

categories fall into the same product category test requirement group. This “Test B” product 

category test requirement group for mass flow meters was referred to collectively as “Normal 

Liquids” in 2010 and earlier editions of NCWM Publication 14. It is valid to consider all the 

fluids in the “Test B” set of product categories as if they were in a single product category for 

consideration of substitute fluids. Similarly, all the fluids in the various product categories that 

fall under the “Test F” requirements should be considered as being in the same product 

category for magnetic flow meters when considering substitute fluids. As another example, 

“Test E” requirements state that “The Certificate of Conformance will cover all products in the

product category within the kinematic viscosity range tested.” Because the coverage under 

“Test E” includes all products in the product category, all “Test E” product categories represent 

individual and separate product category test requirement groups. Product category test 

requirement groups are separated by double lines in the Product Families Table in NCWM 

Publication 14 to help readily identify these groupings. The product category test requirement 

groups are structured such that the performance of a meter with a given product will predict 

the performance and give results similar to those of the meter with other products in that same 

product category test requirement group.

A product category test requirement group for one meter type will not necessarily contain all 

the same fluids as it would for another meter type because, as noted in “Device Type” above, 

different meter types are affected by different fluid characteristics. The relationships between 

different fluids that determine whether they are in the same product category for a given 

meter type or not are complex because they depend on similarities and differences in one or in 

many secondary characteristics. These secondary characteristics are other fluid properties, in 

addition to the critical fluid characteristic, that can potentially impact test results for a given 

meter type. Examples of secondary properties that help define product categories include 

temperature range (as in the case of heated and cryogenic products), lubricity, phase stability, 

coefficient of thermal expansion, vapor pressure, and solids content. The best way to ensure 

that test results will not be impacted by a secondary characteristic of a substitute fluid is to 

verify, using NCWM Publication 14, whether or not the two fluids share a common product 

category test requirement group for a particular device. 

Consider as an example a case where water is proposed as a substitute fluid for testing a device 

that has been installed to meter xylene. The owner of the device is reluctant to run xylene for 

the start-up tests because it is highly flammable and the testing arrangement will result in 

greater exposure and risk to safety than the normal system operation. NCWM Publication 14 

reveals in the Product Families Table that xylene is in the product category known as “Solvents 

General.” The “Solvents General” product category includes hexane, toluene, acetone, and 

some other solvents, but it does not include water. Water would not be a good substitute for 

xylene if the meter is either a positive displacement meter or a turbine meter because it is not 

Weights and Measures Connection, Vol. 2, Issue 3 Page 5
Article Ref: B-017 

in the same product category test requirement group as xylene. Although xylene is a “Test F” 

fluid for magnetic flow meters, water is not a “Test F” fluid, so water would also not be a good 

substitute for a magnetic flow meter. For a mass flow meter, both xylene and water are fluids 

in the same “Test B” product category test requirement group (formerly known as “Normal 

Liquids”), so they can be considered as if they were in the same product category, but only if 

the device were a mass flow meter. The critical fluid characteristic for mass flow meters is 

specific gravity (density). The specific gravity of xylene is typically 0.89 and that of water is 1.0, 

which are somewhat close together. If the NTEP certificate for the device confirms in the 

“Applications” section that the meter approval already covers a range of specific gravities that 

includes both 0.89 and 1.0, then the substitution may still be considered after consulting the 

manufacturer for their input.

Other Fluid Characteristics Identified by the Manufacturer. Some device designs have 

additional influence factors, identified by the meter manufacturer, that are related to certain 

particular fluid properties. In cases where the meter manufacturer has identified influence 

factors related to fluid characteristics beyond the critical fluid characteristic for that device 

type, these other fluid characteristics must also be considered when evaluating a fluid to use as 

a substitute for testing of that particular device. 

Once the fluid characteristics that can potentially influence the particular device type are 

identified, the next task is to determine the degree to which each of those characteristics may 

impact test results. The ideal scenario is for a substitute fluid to be equivalent to the primary 

fluid in every characteristic that is known to have an influence on the device performance, but 

in many cases, small differences will arise that will make it necessary to determine if the 

magnitude of that difference is likely to influence the outcome of the test.

An example of this would be the case where a device manufacturer has published a secondary 

influence factor relating to pressure. To determine if the effect of pressure on the device is 

likely to influence the outcome of the test, first ascertain whether the substitute fluid would be 

measured at a different pressure than the primary fluid. If this is the case, then use the formula 

stated by the manufacturer to estimate the magnitude of the effect based on the expected 

difference in pressure between the substitute fluid and the primary fluid. If the magnitude of 

this effect under the proposed test conditions with the substitute fluid would be significant in 

comparison with the allowed tolerance, then the proposed fluid would be a poor substitute.

Consult Device Manufacturers

Whenever possible, the manufacturer of the device should be consulted before approving a 

substitute test fluid. This can sometimes be achieved simply by referring to their published 

specifications. It is not uncommon for the published specifications of a measuring device to 

contain quantified influence factors of field effects. Unfortunately, these effects are not always 

Weights and Measures Connection, Vol. 2, Issue 3 Page 6
Article Ref: B-017 

published or even well understood by all manufacturers, so the absence of these types of 

specifications cannot be interpreted as an absence of any influence factors.

Ideally, the manufacturer should be contacted directly. If the manufacturer believes that the 

substitute fluid may have an effect on your test results, or is not sure of what the effects might 

be, then you should only proceed with extreme caution. In order to avoid any 

misunderstanding and possible wrong conclusions, the manufacturer should be provided with 

the names and known properties of both the substitute fluid and the primary fluid. 

Additionally, you should provide the manufacturer with as much information as is known about 

the test conditions and the normal operating conditions (e.g., pressure, temperature, flow, etc.) 

since some of these conditions may impact the characteristics of one or both of the fluids 

differently in a way that affects performance of the device.

One other valuable piece of information that either the equipment owner or the manufacturer 

may be able to provide is data from prior controlled testing with the primary fluid. If the device 

model has been proven to perform within tolerances using the primary fluid at least once, then 

confidence in future testing using a qualified substitute fluid is higher than if there is no 

historical data available involving the primary fluid.

Automatic Temperature-Compensating Systems

In certain applications, automatic temperature compensation may be in use. Automatic 

temperature compensation adjusts the indication and registration of a measured volume to the 

volume that the particular product would occupy if it were at a specified reference 

temperature, typically 15 °C or 60 °F for petroleum products.

Substitution of another fluid in the testing of systems that include automatic temperature 

compensation is generally not recommended. The ultimate measurement accuracy of these 

types of systems depends on both the accuracy of the primary flow measurement and the 

accuracy of the automatic temperature compensation system and its associated calculations. 

Because the automatic temperature compensation system and its calculations are directly 

linked to the coefficient of thermal expansion (CTE) of the primary fluid, it is highly likely that a 

substituted fluid with a different CTE will have a significant impact on test results.

Substitutions for Gaseous Products

The same reasons that might exist to justify substitution of one liquid for another during testing 

might also arise when metering gases. Since gases are more difficult to contain and control 

than liquids, this can be especially evident when the primary gas represents a safety hazard 

because it is either toxic or flammable.

As in the case of liquids, NIST Handbook 44 also recognizes the substitution of gases for testing. 

In both Section 3.33. “Hydrocarbon Gas Vapor-Measuring Devices,” paragraph N.1. “Test 

Weights and Measures Connection, Vol. 2, Issue 3 Page 7
Article Ref: B-017 

Medium,” and in Section 3.37. “Mass Flow Meters,” paragraph N.2., the Handbook states that 

“The device shall be tested with air or the product to be measured.”

As more high pressure gaseous fueling applications (e.g., hydrogen) emerge, further attention 

needs to be given when considering substitution of different gases during testing, primarily 

because of the highly dynamic nature of flows, pressures, and temperatures that are seen 

during high pressure gas fueling. The Joule-Thompson coefficient is a gas property that 

indicates whether and by how much a gas will heat up or cool down as pressure changes and it 

can have dramatic impacts on the temperature of a gas during test drafts. Different gases can 

behave entirely differently under otherwise similar conditions.

The Bottom Line – Who determines the suitability of a substitute liquid?

In the case of installation, start-up, and maintenance verifications of individual devices used in 

legal trade, the weights and measures official has full jurisdictional authority and the final 

decision on whether a certain substitute fluid can be used or not at their discretion. For Type 

Evaluation, any substitution of fluids is subject to the approval of the NTEP laboratories and the 

NTEP administrator.

For questions about this article or further information on product substitutions during testing 

of commercial measuring devices, contact Marc Buttler by e-mail at marc.buttler@nist.gov or 

by phone at (301) 975-4615.

Weights and Measures Connection, Vol. 2, Issue 3 Page 8