Article Ref: D-022

Weights and Measures Connection, Vol. 5, Issue 7 Page 1

December 10, 2014

Testing Compressed Natural Gas Retail Motor-Fuel Dispensers 

ARRIVING PREPARED TO TEST 

Byline: Rick Harshman

As the developing infrastructure for fueling vehicles that use compressed natural gas (CNG) as an engine 

fuel continues to unfold, some weights and measures jurisdictions are raising questions about the 

equipment needed to test the retail motor-fuel dispensers (RMFDs) that dispense CNG. The testing of 

CNG RMFDs necessitates some advance planning to ensure that all of the proper test equipment is 

available on the day that testing is to take place. There is information that officials can gather 

beforehand that will allow them to make a preliminary determination of whether or not the equipment 

made available to them for testing will be adequate and improve the likelihood that a complete test, as 

outlined in NIST EPO No. 28 Examination Procedure Outline for Compressed Natural Gas (CNG) Retail 

Motor Fuel Dispensers, can be performed on the date that testing is scheduled. This article identifies 

the information that needs to be known about the test equipment as well as the RMFD to be tested and 

explains how this information is used to make this determination.

Note: The word “preliminary” prefaces the word “determination” in the above paragraph for good 

reason. CNG RMFDs are tested gravimetrically, which requires the use of a suitable reference scale that 

has been proven to be sufficiently accurate and capable of repeating its indications. The reference scale 

is tested onsite, immediately prior to being used as a transfer standard in testing the CNG RMFDs. 

Testing of the reference scale typically takes place outdoors, under existing weather conditions, and in 

close proximity to where the scale will be used to weigh test drafts of CNG. The results from such onsite 

testing determine whether or not the scale performs well enough to be used as a standard in testing 

another device; in this case, a CNG RMFD. For this reason, even when a preliminary determination has 

been made that the reference scale being made available for testing is adequate (i.e., based on 

elements of its design), it isn’t until the results of testing the reference scale onsite are known that a 

final determination can be made relative to its adequacy. 

Dispenser Information

There are two things that officials need to find out about a CNG RMFD that they are planning to test. 

The first is the minimum measured quantity (MMQ) marked on the device. The second is how mass 

units are accessed.

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

and Measuring Devices, Section 3.37. Mass Flow Meters Code paragraphs S.5. Markings and 

N.1. Minimum Measured Quantity requires that the MMQ be specified and marked on the device by the 

manufacturer.
Article Ref: D-022

Weights and Measures Connection, Vol. 5, Issue 7 Page 2

NIST Handbook 44, Section 3.37. Mass Flow Meters Code Paragraphs 

S.5. Markings and N.1. Minimum Measured Quantity

The MMQ provides the critical piece of information needed to determine whether or not the value of 

the scale division (d) of the reference scale will be small enough for the scale to be used as a standard in 

testing the RMFD. To be small enough, the value of “d” must be less than or equal to one-tenth the 

smallest tolerance to be applied to the RMFD under test. The smallest applied tolerance on a CNG 

RMFD is the tolerance that gets applied to the smallest test draft; which in this case, is a test draft equal 

to the MMQ. The variables used to determine the maximum allowable division size for the reference 

scale are expressed in formula (1). 

Reference scale division (d) ≤ smallest test load x tolerance ÷ 10 (1)

Rearranging some of the variables in this equation also makes possible a determination of the smallest 

test load of CNG to be delivered into the test cylinder during testing. Formula (2) establishes the 

smallest permissible test load or test draft size.

Reference scale division (d) x 10

Tolerance of device under test

Note: Error weights or an expanded scale resolution feature, providing the scale is equipped with one, 

may be used to expand the readability of a scale to a value smaller than “d.” If readability can be 

expanded, it is the value to which a scale can be read that should be inserted for the value of the scale 

division “d” in formulas (1) and (2) shown above. For example, in formula (1), if d = 0.02 lb and the scale 

can be put into an expanded resolution and read to 0.002 lb, then it is 0.002 lb, rather than 0.02 lb, that 

is to be less than or equal to one-tenth the smallest applied tolerance. 

In consideration of these two formulas, it is important to be mindful of the relationship between the 

value of “d” on the reference scale and the minimum test draft size. Increasing the size of the smallest 

test draft allows for the use of a reference scale with a higher scale division value. Stated differently, as 

the value of the scale division increases on the reference scale, so must the size of the smallest test 

draft. 

S.5. Markings. – A measuring system shall be legibly and indelibly marked 

with the following information:

(a) ...

minimum measured quantity; and

(b) ...

N.1. Minimum Measured Quantity. – The minimum measured quantity 

shall be specified by the manufacturer.

= Smallest test load (2)
Article Ref: D-022

Weights and Measures Connection, Vol. 5, Issue 7 Page 3

Accessing Mass Units. CNG RMFDs are required to indicate the delivered quantity in either gasoline 

gallon equivalent (GGE) units or gasoline liter equivalent (GLE) units as specified in Mass Flow Meters 

code paragraph S.1.3.1.1. Compressed Natural Gas Used as an Engine Fuel. Even though they must 

indicate in GGE or GLE units, the dispensers are required to display the mass measured for each 

transaction. The mass value may either be displayed continuously on an external or internal display 

accessible during the inspection and test of the dispenser, or the mass value may be accessed using 

controls on the device as specified in paragraph S.1.2. Compressed Natural Gas Dispensers. 

Section 3.37. Mass Flow Meters Code Paragraphs S.1.3.1.1. Compressed Natural 

Gas Used as an Engine Fuel and S.1.2. Compressed Natural Gas Dispensers

It is wise to determine how mass units are accessed in advance of arriving on site for a test so that if 

special assistance or some separate piece of equipment is needed to access the mass units, these can be 

made available at time of test. Some NTEP Certificates of Conformance (CCs) provide an indication of 

how mass units can be accessed. If not specified on the CC, the device owner should be able to provide 

this information. 

Test Equipment Information

Given the limited number of CNG refueling stations in some areas of the country, combined with the 

uncertainty of knowing just how many more might be added in the future, some states are finding it 

difficult to justify the expense of purchasing the test equipment needed to test CNG RMFDs. As an 

alternative to purchasing the test equipment, some states are currently opting to require, under the 

provisions of NIST Handbook 44 paragraph G-UR.4.4. Assistance in Testing Operations, that the owner 

or operator of the dispensers supplies the specialized test equipment. 

S.1.3.1.1. Compressed Natural Gas Used as an Engine Fuel. – When 

compressed natural gas is dispensed as an engine fuel, the delivered quantity 

shall be indicated in “gasoline liter equivalent (GLE) units” or “gasoline gallon 

equivalent (GGE) units” (see definitions).

(Added 1994)

S.1.2. Compressed Natural Gas Dispensers. – Except for fleet sales and other 

price contract sales, a compressed natural gas dispenser used to refuel vehicles 

shall be of the computing type and shall indicate the quantity, the unit price, 

and the total price of each delivery. The dispenser shall display the mass 

measured for each transaction either continuously on an external or internal 

display accessible during the inspection and test of the dispenser, or display the 

quantity in mass units by using controls on the device.

(Added 1994)
Article Ref: D-022

Weights and Measures Connection, Vol. 5, Issue 7 Page 4

NIST Handbook 44 paragraph G-UR.4.4. Assistance in Testing Operations

Whether provided by the jurisdiction or the device owner, the following test equipment is needed to 

perform an official test on a CNG RMFD: 

• Test cylinder – the container (or vessel) that receives the test drafts of CNG that are dispensed 

during the testing of a CNG RMFD. 

• Reference scale – the scale used to determine the actual weight of the CNG delivered into the 

test cylinder for each test draft.

• Test weights - physical standards that meet specifications and tolerances in NIST Handbook 105-

series handbook (or other suitable and designated standards) that are used to test and 

determine the adequacy of the reference scale prior to it being used as a transfer standard in 

the testing of a CNG RMFD. Test weights of smaller value, known as “error weights” are 

sometimes needed to reduce rounding errors and expand the readability of a digital electronic 

reference scale so that it can be read to a value less than its displayed scale division. 

Test Cylinder

NIST EPO N0. 28 Examination Procedure Outline (EPO) for Compressed Natural Gas (CNG) Retail Motor 

Fuel Dispensers provides a list of criteria officials need to verify concerning the test cylinder. The EPO 

indicates that the following should be considered when selecting a test cylinder: 

• rating must be equivalent to or greater than the service pressure* marked on the device under 

test as required by the ANSI/IAS NGV 4.1/CSA 12.5 “NGV Dispensing Systems,” Standard for 

Natural Gas Vehicle Dispensing Systems;

• compatible fittings that allow for the connection of the dispenser discharge nozzle;

• bleed valve;

• pressure gauge;

• drain hose; and

G-UR.4.4. Assistance in Testing Operations. – If the design, 

construction, or location of any device is such as to require a 

testing procedure involving special equipment or accessories 

or an abnormal amount of labor, such equipment, 

accessories, and labor shall be supplied by the owner or 

operator of the device as required by the weights and 

measures official.
Article Ref: D-022

Weights and Measures Connection, Vol. 5, Issue 7 Page 5

• means for grounding the cylinder prior to connecting to dispensing equipment such as a quick 

connect ground strap.

*Note: Service pressure is the settled pressure at a uniform gas temperature of 21 oC (70 oF) and 

full gas content. It is the pressure for which the equipment has been constructed under normal 

conditions. This is different from the maximum working pressure.

In addition to verifying that a test cylinder complies with all the criteria listed in the NIST EPO, the 

capacity of the test cylinder (i.e., the amount of CNG it will contain) is needed to determine whether or 

not it is of sufficient capacity to be considered suitable for use in testing the particular RMFDs that are 

planned. In order to be considered suitable, the test cylinder must be able to hold at least three times 

the value of the minimum measured quantity (MMQ) marked on the dispenser for the following 

reasons: 

• MMQ is an operational rating declared by the manufacturer of the equipment and provides an 

indication of smallest quantity that can be accurately measured by the device. This rating 

applies to both use and testing of a device. Thus, it would not be appropriate to dispense an 

amount less than the MMQ for any tests intended to verify accuracy of the device; nor would it 

be appropriate for an operator to dispense a quantity less than the MMQ. 

• A segment of the overall test to verify accuracy of a CNG RMFD, as outlined in EPO NO. 28, 

requires the completion of three consecutive test drafts, each equal to one-third test cylinder 

capacity. The first test draft is dispensed into the empty test cylinder. The second and third test 

drafts are initiated with product from the previous test draft(s) remaining in the test cylinder. 

The test cylinder should be full after completion of the third test. Since it would not be 

appropriate for any test draft to be less than the MMQ, the test cylinder needs to be able to 

hold at least three times the value of the MMQ to accommodate proper completion of these 

three consecutive, one-third capacity tests. 

Reference Scale

Aside from having to test a reference scale to prove its adequacy prior to using it as a transfer standard, 

there are four important questions relating to elements of scale design that need to be answered to 

determine whether or not a reference scale is adequate. 

1. Is it intrinsically safe? 

2. Is the value of the scale division small enough that rounding errors won’t use up too much of the 

tolerance to be applied to the device under test?

3. Is the capacity large enough to weigh a test cylinder full of product?

4. Does its design accommodate ease of application and proper positioning of the test cylinder 

during testing?

The following provides a discussion of these questions and explains their significance in determining the 

adequacy of a reference scale. 
Article Ref: D-022

Weights and Measures Connection, Vol. 5, Issue 7 Page 6

• Intrinsically safe? NIST EPO NO. 28 provides the following guidance on intrinsic safety as it 

relates to a reference scale: scale meets Underwriters Laboratories (UL) Area Classification 

Class I Division 2 Group D (scale equipment must be located outside of the classified area, which 

is five feet from the hose fueling connection to the dispenser). It’s important to confirm that 

the reference scale complies with the UL standard specified due to the high flammability of 

CNG.

• Is the value of “d” small enough? As explained earlier in the section of this article titled “The 

RMFD,” the value of “d” is small enough when the scale can be read to a value that is less than 

or equal to one-tenth the tolerance to be applied to the RMFD. Therefore, to determine if scale 

resolution is appropriate, one only needs to know the value to which the reference scale can be 

read; the tolerance to be applied to the RMFD during testing; and the MMQ marked on the 

dispenser. If the scale is to be setup and used outdoors, it’s also a good idea to determine if 

shielding will be made available to reduce the effect of wind. 

• Is the scale capacity large enough? The scale selected for use must have a capacity large 

enough to weigh a test cylinder full of product including the weight of any shims, cart, or other 

accessory items that will likely be applied to the scale when weighing the test cylinder during 

testing. Therefore, one must have knowledge of the approximate weight of the empty test 

cylinder selected for use in testing and also the approximate weight of the CNG that it will 

contain when full. When added together, the result equals the approximate weight of the test 

tank full of product. In some cases, the supplier of the test cylinder may be able to provide the 

approximate weight of the cylinder full of product. If not, he or she should be able to provide 

the approximate weight of the test cylinder empty and the volume capacity of the test cylinder 

in GGEs or GLEs. The volume capacity of the test cylinder can be converted into weight by 

multiplying the volume capacity value by its associated conversion factor (5.660 lb for a volume 

capacity specified in GGEs and 0.678 kg for a volume capacity specified in GLEs) to determine 

the capacity of the test cylinder in units of weight. The approximate weight of a test tank full of 

CNG may then be determined by adding the weight of the empty test cylinder to that of the 

CNG contained in a full test cylinder. 

• Does its design accommodate ease of loading and proper positioning of the test cylinder? 

Determine the approximate weight of the test cylinder in an empty state and when full of CNG 

to gain a better understanding of the amount of weight to be moved and hoisted onto the scale. 

Determine the type of scale selected for use and the ease with which the test cylinder can be 

safely applied to the load receiving element, both when empty and when full of CNG. Confirm 

that when the test cylinder is positioned on the scale for weighing, the load is distributed evenly 

over the main load supports of the scale and does not produce binding or off-center loading. A 

preliminary determination of whether or not scale design accommodates ease of loading and 

proper position of the test cylinders can be made based on a comparison of the type of 

reference scale used, size test cylinder and its weight when full of product. 
Article Ref: D-022

Weights and Measures Connection, Vol. 5, Issue 7 Page 7

Test Weights

Based on a NIST OWM recommendation that reference scales used in the testing of CNG RMFDs be 

tested to at least the weight of the largest test draft of CNG to be weighed, the weight of the test 

cylinder full of product must be known to determine the minimum amount of test weight that will be 

needed to test the reference scale. NIST Class F standards are generally of sufficient accuracy to test a 

reference scale that will be used to weigh test drafts of CNG dispensed from a RMFD. 

Summary: When planning for the test of a CNG RMFD, it is always a good idea to obtain the information 

necessary to determine whether or not the different test equipment made available for testing will be 

adequate. This article was developed to assist in identifying the information that is needed and explain 

how that information is used to make this determination. Verifying in advance that the equipment 

being supplied is suitable improves the likelihood that testing can be safely completed and with 

confidence at the time scheduled. For additional information regarding this article, please contact Rick 

Harshman at richard.harshman@nist.gov or by phone: (301) 975-8107.

Office of Weights and Measures (301) 975-4004, e-mail: owm@nist.gov