Handbook 44 – 2020 3.37. Mass Flow Meters

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Table of Contents

Page

Section 3.37. Mass Flow Meters....................................................................................... 3-107

A. Application .................................................................................................................................................3-107

A.1. Liquids. .............................................................................................................................................3-107

A.2. Vapor (Gases). ..................................................................................................................................3-107

A.3. Additional Code Requirements.........................................................................................................3-107

S. Specifications..............................................................................................................................................3-107

S.1. Indicating and Recording Elements. .................................................................................................3-107

S.1.1. Indicating Elements. ...........................................................................................................3-107

S.1.2. Compressed Natural Gas and Liquefied Natural Gas Dispensers.......................................3-107

S.1.3. Units....................................................................................................................................3-107

S.2. Operating Requirements. ..................................................................................................................3-108

S.2.1. Return to Zero.....................................................................................................................3-108

S.2.2. Indicator Reset Mechanism.................................................................................................3-108

S.2.3. Non-resettable Indicator......................................................................................................3-108

S.2.4. Provisions for Power Loss. .................................................................................................3-108

S.2.5. Display of Unit Price and Product Identity. ........................................................................3-109

S.2.6. Money-Value Computations...............................................................................................3-109

S.2.7. Recorded Representations, Point-of-Sale Systems..............................................................3-110

S.2.8. Indication of Delivery. ........................................................................................................3-110

S.2.9. Automatic Timeout – Pay-At-Retail Motor-Fuel Devices .....................................................3-110

S.3. Measuring Elements and Measuring Systems...................................................................................3-110

S.3.1. Maximum and Minimum Flow-Rates.................................................................................3-110

S.3.2. Adjustment Means. .............................................................................................................3-110

S.3.3. Air/Vapor Elimination. .......................................................................................................3-110

S.3.4. Maintenance of Liquid State...............................................................................................3-111

S.3.5. Provision for Sealing...........................................................................................................3-111

S.3.6. Automatic Density Correction. ...........................................................................................3-112

S.3.7. Pressurizing the Discharge Hose.........................................................................................3-112

S.3.8. Zero-Set-Back Interlock, Retail Motor-Fuel Devices. ........................................................3-113

S.4. Discharge Lines and Valves..............................................................................................................3-113

S.4.1. Diversion of Measured Product. .........................................................................................3-113

S.4.2. Pump-Discharge Unit..........................................................................................................3-113

S.4.3. Directional Flow Valves. ....................................................................................................3-113

S.4.4. Discharge Valves. ...............................................................................................................3-113

S.4.5. Antidrain Means. ................................................................................................................3-113

S.4.6. Other Valves. ......................................................................................................................3-113

S.5. Markings. ..........................................................................................................................................3-114

S.5.1. Location of Marking Information; Retail Motor-Fuel Dispensers......................................3-114

S.5.2. Marking of Equivalent Conversion Factors for Compressed Natural Gas..........................3-114

S.5.3. Marking of Equivalent Conversion Factor for Liquefied Natural Gas................................3-114

S.6. Printer. ..............................................................................................................................................3-115

S.6.1. Printed Receipt....................................................................................................................3-115

S.7. Totalizers for Retail Motor-Fuel Devices. ........................................................................................3-115

N. Notes............................................................................................................................................................3-115

N.1. Minimum Measured Quantity...........................................................................................................3-115

N.2. Test Medium.....................................................................................................................................3-115

N.2.1. Liquid-Measuring Devices..................................................................................................3-115
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N.2.2. Vapor-Measuring Devices. .................................................................................................3-115

N.3. Test Drafts.........................................................................................................................................3-115

N.4. Minimum Measured Quantity...........................................................................................................3-115

N.5. Motor-Fuel Dispenser. ......................................................................................................................3-115

N.6. Testing Procedures............................................................................................................................3-116

N.6.1. Normal Tests.......................................................................................................................3-116

N.6.2. Special Tests. ......................................................................................................................3-116

N.6.3. Repeatability Tests..............................................................................................................3-116

T. Tolerances...................................................................................................................................................3-116

T.1. Tolerances, General. .........................................................................................................................3-116

T.2. Tolerances.........................................................................................................................................3-116

T.3. Repeatability. ....................................................................................................................................3-117

T.4. Type Evaluation Examinations for Liquid-Measuring Devices........................................................3-117

UR. User Requirements ....................................................................................................................................3-118

UR.1. Selection Requirements. ...................................................................................................................3-118

UR.1.1. Discharge Hose-Length. .....................................................................................................3-118

UR.1.2. Minimum Measured Quantity.............................................................................................3-118

UR.2. Installation Requirements. ................................................................................................................3-118

UR.2.1. Manufacturer’s Instructions. ...............................................................................................3-118

UR.2.2. Discharge Rate....................................................................................................................3-118

UR.2.3. Low-Flow Cut-Off Valve. ..................................................................................................3-118

UR.3. Use of Device....................................................................................................................................3-118

UR.3.1. Unit Price and Product Identity for Retail Dispensers. .......................................................3-118

UR.3.2. Vapor-Return Line..............................................................................................................3-119

UR.3.3. Ticket Printer; Customer Ticket..........................................................................................3-119

UR.3.4. Printed Ticket......................................................................................................................3-119

UR.3.5. Ticket in Printing Device. ...................................................................................................3-119

UR.3.6. Steps After Dispensing. ......................................................................................................3-119

UR.3.7. Return of Indicating and Recording Elements to Zero. ......................................................3-119

UR.3.8. Return of Product to Storage, Retail Compressed and Liquefied Natural Gas Dispensers......

............................................................................................................................................3-120
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Section 3.37. Mass Flow Meters

A. Application

A.1. Liquids. – This code applies to devices that are designed to dynamically measure the mass, or the mass and 

density of liquids. It also specifies the relevant examinations and tests that are to be conducted.

(Amended 1997)

A.2. Vapor (Gases). – This code applies to devices that are designed to dynamically measure the mass of 

hydrocarbon gas in the vapor state. Examples of these products are propane, propylene, butanes, butylenes, ethane, 

methane, natural gas and any other hydrocarbon gas/air mix.

A.3. Additional Code Requirements. – In addition to the requirements of this code, Mass Flow Meters shall meet 

the requirements of Section 1.10. General Code.

S. Specifications

S.1. Indicating and Recording Elements.

S.1.1. Indicating Elements. – A measuring assembly shall include an indicating element. Indications shall be 

clear, definite, accurate, and easily read under normal conditions of operation of the instrument.

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

price contract sales, a compressed or liquefied 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) (Amended 2016)

S.1.3. Units.

S.1.3.1. Units of Measurement. – Deliveries shall be indicated and recorded in grams, kilograms, metric 

tons, pounds, tons, and/or liters, gallons, quarts, pints and decimal subdivisions thereof. The indication of a 

delivery shall be on the basis of apparent mass versus a density of 8.0 g/cm3

. The volume indication shall be 

based on the mass measurement and an automatic means to determine and correct for changes in product 

density.

(Amended 1993 and 1997)

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 gallon equivalent units

(GGE)” or “diesel gallon equivalent units (DGE),” or in mass. (Also see Appendix D. Definitions.)

(Added 1994) (Amended 2016)

S.1.3.1.2. Liquefied Natural Gas Used as an Engine Fuel. – When liquefied natural gas is 

dispensed as an engine fuel, the delivered quantity shall be indicated in diesel gallon equivalent units 

(DGE) or in mass. (Also see Appendix D. Definitions.)

(Added 2016)
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S.1.3.2. Numerical Value of Quantity-Value Divisions. – The value of a scale interval shall be equal to:

(a) 1, 2, or 5; or

(b) a decimal multiple or submultiple of 1, 2, or 5.

S.1.3.3. Maximum Value of Quantity-Value Divisions. - The maximum value of the quantity-value 

division shall not exceed the following:

(a) For compressed natural gas dispensed as an engine fuel:

(1) 0.001 for gasoline gallon equivalent (GGE) units; or

(2) 0.001 diesel gallon equivalent (DGE) units; or

(3) 0.001 kg or 0.001 lb for mass units.

(b) For liquefied natural gas dispensed as an engine fuel:

(1) 0.001 for diesel gallon equivalent (DGE) units; or

(2) 0.001 kg or 0.001 lb for mass units.

(c) For all liquids other than liquefied natural gas dispensed as an engine fuel a maximum value not greater 

than 0.2 % of the minimum measured quantity.

(Amended 1994 and 2019)

S.1.3.4. Values Defined. – Indicated values shall be adequately defined by a sufficient number of figures, 

words, symbols, or combinations thereof. A display of “zero” shall be a zero digit for all displayed digits to 

the right of the decimal mark and at least one to the left.

S.2. Operating Requirements.

S.2.1. Return to Zero. – Except for measuring assemblies in a pipeline:

(a) One indicator and the primary recording elements, if the device is equipped to record, shall be provided 

with a means for readily returning the indication to zero either automatically or manually.

(b) It shall not be possible to return primary indicating elements, or primary recording elements, beyond the 

correct zero position.

(Amended 1993)

S.2.2. Indicator Reset Mechanism. – The reset mechanism for the indicating element shall not be operable 

during a delivery. Once the zeroing operation has begun, it shall not be possible to indicate a value other than the 

latest measurement, or “zeros” when the zeroing operation has been completed.

S.2.3. Non-resettable Indicator. – An instrument may also be equipped with a non-resettable indicator if the 

indicated values cannot be construed to be the indicated values of the resettable indicator for a delivered quantity.

S.2.4. Provisions for Power Loss.

S.2.4.1. Transaction Information. – In the event of a power loss, the information needed to complete 

any transaction in progress at the time of the power loss (such as the quantity and unit price, or sales price) 

shall be determinable for at least 15 minutes at the dispenser or at the console if the console is accessible to 

the customer.

(Added 1993)
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S.2.4.2. User Information. – The device memory shall retain information on the quantity of fuel 

dispensed and the sales price totals during power loss.

(Added 1993)

S.2.5. Display of Unit Price and Product Identity.

S.2.5.1. Unit Price. – A computing or money-operated device shall be able to display on each face the 

unit price at which the device is set to compute or to dispense.

(Added 1993)

S.2.5.2. Product Identity. – A device shall be able to conspicuously display on each side the identity of 

the product being dispensed.

(Added 1993)

S.2.5.3. Selection of Unit Price. – Except for dispensers used exclusively for fleet sales, other price 

contract sales, and truck refueling (e.g., truck stop dispensers used only to refuel trucks), when a product or 

grade is offered for sale at more than one unit price through a computing device, the selection of the unit 

price shall be made prior to delivery using controls on the device or other customer-activated controls. A 

system shall not permit a change to the unit price during delivery of a product.

[Nonretroactive as of January 1, 1998]

(Added 1997)

S.2.5.4. Agreement Between Indications. – When a quantity value indicated or recorded by an auxiliary 

element is a derived or computed value based on data received from a retail motor-fuel dispenser, the value 

may differ from the quantity value displayed on the dispenser, provided the following conditions are met:

(a) all total money-values for an individual sale that are indicated or recorded by the system agree; 

and

(b) within each element the values indicated or recorded meet the formula (quantity x unit price = total 

sales price) to the closest cent.

[Nonretroactive as of January 1, 1998]

(Added 1997)

S.2.6. Money-Value Computations. – A computing device shall compute the total sales price at any 

single-purchase unit price (i.e., excluding fleet sales, other price contract sales, and truck stop dispensers used 

only to refuel trucks) for which the product being measured is offered for sale at any delivery possible within 

either the measurement range of the device or the range of the computing elements, whichever is less.

(Added 1993)

S.2.6.1. Auxiliary Elements. – If a system is equipped with auxiliary indications, all indicated moneyvalue and quantity divisions of the auxiliary element shall be identical with those of the primary element.

(Added 1993)

S.2.6.2. Display of Quantity and Total Price. – When a delivery is completed, the total price and quantity 

for that transaction shall be displayed on the face of the dispenser for at least 5 minutes or until the next 

transaction is initiated by using controls on the device or other user-activated controls.

(Added 1993)
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S.2.7. Recorded Representations, Point-of-Sale Systems. – The sales information recorded by cash registers 

when interfaced with a retail motor-fuel dispenser shall contain the following information for products delivered 

by the dispenser:

(a) the total volume of the delivery;

(b) the unit price;

(c) the total computed price; and

(d) the product identity by name, symbol, abbreviation, or code number.

[Nonretroactive as of January 1, 1986]

(Added 1993)

S.2.8. Indication of Delivery. – The device shall automatically show on its face the initial zero condition and 

the quantity delivered (up to the nominal capacity). However, the first 0.03 L (0.009 gal) of a delivery and its 

associated total sales price need not be indicated.

[Nonretroactive as of January 1, 1998]

(Added 1997)

S.2.9. Automatic Timeout – Pay-At-Retail Motor-Fuel Devices. ̶ Once a retail motor-fuel device has been 

authorized, it must deauthorize within two minutes if not activated. Reauthorization of the retail motor-fuel device must 

be performed before product is delivered. If the time limit to deauthorize the retail motor-fuel device is programmable, 

it shall not accept an entry greater than two minutes. 

[Nonretroactive as of January 1, 2020]

(Added 2019)

S.3. Measuring Elements and Measuring Systems.

S.3.1. Maximum and Minimum Flow-Rates.

(a) The ratio of the maximum to minimum flow-rates specified by the manufacturer for devices measuring 

liquefied gases shall be 5:1 or greater.

(b) The ratio of the maximum to minimum flow-rates specified by the manufacturer for devices measuring 

other than liquefied gases shall be 10:1 or greater.

S.3.2. Adjustment Means. – An assembly shall be provided with the means to change the ratio between the 

indicated quantity and the quantity of liquid measured by the assembly. A bypass on the measuring assembly 

shall not be used for these means.

S.3.2.1. Discontinuous Adjusting Means. – When the adjusting means changes the ratio between the 

indicated quantity and the quantity of measured liquid in a discontinuous manner, the consecutive values of 

the ratio shall not differ by more than 0.1 %.

S.3.3. Air/Vapor Elimination. – A measuring system shall be equipped with an effective air/vapor eliminator 

or other automatic means to prevent the measurement of air/vapor. Vent lines from the air/vapor eliminator shall 

be made of appropriate non-collapsible material.

(Amended 1999 and 2017)

S.3.3.1. Air/Vapor Elimination on Loading Rack Liquid Measuring Systems.
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(a) A loading rack measuring system shall be equipped with an effective air/vapor eliminator or other 

automatic means to prevent the passage of air/vapor through the meter unless the system is designed 

or operationally controlled by a means such that neither air nor vapor can enter the system.

(b) Vent lines from the air/vapor eliminator shall be made of appropriate non-collapsible material.

(Added 1995) (Amended 2017)

S.3.4. Maintenance of Liquid State. – A liquid-measuring device shall be installed so that the measured 

product remains in a liquid state during passage through the instrument.

S.3.5. Provision for Sealing. – For devices or systems in which the configuration or calibration parameters 

can be changed by use of a removable digital storage device, security shall be provided for those parameters as 

specified in G-S.8.2. Devices and Systems Adjusted Using Removable Digital Storage Devices. For parameters 

adjusted using other means, the following applies.

Adequate provision shall be made for an approved means of security (e.g., data change audit trail) or physically 

applying security seals in such a manner that no adjustment or interchange can be made of:

(a) any measuring or indicating element;

(b) any adjustable element for controlling delivery rate when such rate tends to affect the accuracy of 

deliveries;

(c) the zero-adjustment mechanism; and

(d) any metrological parameter that will affect the metrological integrity of the device or system.

When applicable, the adjusting mechanism shall be readily accessible for purposes of affixing a security seal.

Audit trails shall use the format set forth in Table S.3.5. Categories of Device and Methods of Sealing.*

[*Nonretroactive as of January 1, 1995]

(Amended 1992, 1995, 2006, and 2019)
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Table S.3.5.

Categories of Device and Methods of Sealing

Categories of Device Methods of Sealing

Category 1: No remote configuration capability. Seal by physical seal or two event counters: one for

calibration parameters and one for configuration 

parameters.

Category 2: Remote configuration capability, but 

access is controlled by physical hardware.

The device shall clearly indicate that it is in the remote 

configuration mode and record such message if 

capable of printing in this mode or shall not operate 

while in this mode.

[The hardware enabling access for remote communication 

must be on-site. The hardware must be sealed using a 

physical seal or an event counter for calibration 

parameters and an event counter for configuration 

parameters. The event counters may be located either at 

the individual measuring device or at the system 

controller; however, an adequate number of counters must 

be provided to monitor the calibration and configuration 

parameters of the individual devices at a location. If the 

counters are located in the system controller rather than 

at the individual device, means must be provided to 

generate a hard copy of the information through an onsite device.]*

[*Nonretroactive as of January 1, 1996]

Category 3: Remote configuration capability access 

may be unlimited or controlled through a software

switch (e.g., password).

[Nonretroactive as of January 1, 1995]

The device shall clearly indicate that it is in the remote 

configuration mode and record such message if 

capable of printing in this mode or shall not operate 

while in this mode.

[Nonretroactive as of January 1, 2001]

An event logger is required in the device; it must include 

an event counter (000 to 999), the parameter ID, the date 

and time of the change, and the new value of the 

parameter. A printed copy of the information must be 

available on demand through the device or through 

another on-site device. The information may also be 

available electronically. The event logger shall have a 

capacity to retain records equal to 10 times the number of 

sealable parameters in the device, but not more than 1000 

records are required. (Note: Does not require 1000 

changes to be stored for each parameter.)

[Nonretroactive as of January 1, 1995]

(Table Added 1995) (Amended 1995, 1998, 1999, 2006, and 2016)

S.3.6. Automatic Density Correction.

(a) An automatic means to determine and correct for changes in product density shall be incorporated in any 

mass flow metering system that is affected by changes in the density of the product being measured.

(b) Volume-measuring devices with automatic temperature compensation used to measure natural gas as a 

motor vehicle engine fuel shall be equipped with an automatic means to determine and correct for 

changes in product density due to changes in the temperature, pressure, and composition of the product.

(Amended 1994 and 1997)

S.3.7. Pressurizing the Discharge Hose. – The discharge hose for compressed natural gas shall automatically 

pressurize prior to the device beginning to register the delivery.

(Added 1993)
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S.3.8. Zero-Set-Back Interlock, Retail Motor-Fuel Devices. – A device shall be constructed so that:

(a) after a delivery cycle has been completed by moving the starting lever to any position that shuts off the 

device, an automatic interlock prevents a subsequent delivery until the indicating elements, and 

recording elements if the device is equipped and activated to record, have been returned to their zero 

positions;

(b) the discharge nozzle cannot be returned to its designed hanging position (that is, any position where the 

tip of the nozzle is placed in its designed receptacle and the lock can be inserted) until the starting lever 

is in its designed shut-off position and the zero-set-back interlock has been engaged; and

(c) in a system with more than one dispenser supplied by a single pump, an effective automatic control valve 

in each dispenser prevents product from being delivered until the indicating elements on that dispenser 

are in a correct zero position.

(Added 1993)

S.4. Discharge Lines and Valves.

S.4.1. Diversion of Measured Product. – No means shall be provided by which any measured product can be 

diverted from the measuring instrument. However, two or more delivery outlets may be permanently installed 

and operated simultaneously, provided that any diversion of flow to other than the intended receiving receptacle 

cannot be readily accomplished or is readily apparent. Such means include physical barriers, visible valves, or 

indications that make it clear which outlets are in operation, and explanatory signs if deemed necessary.

An outlet that may be opened for purging or draining the measuring system, or for recirculating product if 

recirculation is required in order to maintain the product in a deliverable state shall be permitted. Effective 

automatic means shall be provided to prevent the passage of liquid through any such outlet during normal 

operation of the measuring system and to inhibit meter indications (or advancement of indications) and recorded 

representations while the outlet is in operation.

(Amended 2002 and 2006)

S.4.2. Pump-Discharge Unit. – A pump-discharge unit for liquids equipped with a flexible discharge hose 

shall be of the wet-hose type.

(Added 1993)

S.4.3. Directional Flow Valves. – If a reversal of flow could result in errors that exceed the tolerance for the 

minimum measured quantity, a valve or valves or other effective means, automatic in operation (and equipped 

with a pressure limiting device, if necessary) to prevent the reversal of flow shall be properly installed in the 

system. (Also see N.1. Minimum Measured Quantity.)

S.4.4. Discharge Valves. – A discharge valve may be installed on a discharge line only if the system is a 

wet-hose type. Any other shutoff valve on the discharge side of the instrument shall be of the automatic or 

semiautomatic predetermined-stop type or shall be operable only:

(a) by means of a tool (but not a pin) entirely separate from the device; or

(b) by means of a security seal with which the valve is sealed open.

S.4.5. Antidrain Means. – In a wet-hose type device, effective means shall be provided to prevent the drainage 

of the hose between transactions.

S.4.6. Other Valves. – Check valves and closing mechanisms that are not used to define the measured quantity 

shall have relief valves (if necessary) to dissipate any abnormally high pressure that may arise in the measuring 

assembly.
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S.5. Markings. – A measuring system shall be legibly and indelibly marked with the following information:

(a) pattern approval mark (i.e., type approval number);

(b) name and address of the manufacturer or his trademark and, if required by the weights and measures 

authority, the manufacturer’s identification mark in addition to the trademark;

(c) model identifier or product name selected by the manufacturer;

(d) nonrepetitive serial number;

(e) the accuracy class of the meter as specified by the manufacturer consistent with Table T.2. Accuracy Classes 

for Mass Flow Meter Applications Covered in NIST Handbook 44, Section 3.37 Mass Flow Meters;*

[*Nonretroactive as of January 1, 1995]

(Added 1994)

(f) maximum and minimum flow rates in pounds per unit of time;

(g) maximum working pressure;

(h) applicable range of temperature if other than − 10 °C to + 50 °C;

(i) minimum measured quantity; and

(j) product limitations, if applicable.

S.5.1. Location of Marking Information; Retail Motor-Fuel Dispensers. – The marking information required 

in General Code, paragraph G-S.1. Identification shall appear as follows:

(a) within 60 cm (24 in) to 150 cm (60 in) from the base of the dispenser;

(b) either internally and/or externally provided the information is permanent and easily read; and

(c) on a portion of the device that cannot be readily removed or interchanged (i.e., not on a service access 

panel).

[Nonretroactive as of January 1, 2003]

The use of a dispenser key or tool to access internal marking information is permitted for retail liquid and 

compressed gas-measuring devices.

(Added 2006) (Amended 2019)

S.5.2. Marking of Equivalent Conversion Factors for Compressed Natural Gas. – A device dispensing 

compressed natural gas shall have either the statement “1 Gasoline Gallon Equivalent (GGE) means 5.660 lb of 

Compressed Natural Gas” or “1 Diesel Gallon Equivalent (DGE) means 6.384 lb of Compressed Natural Gas” 

permanently and conspicuously marked on the face of the dispenser according to the method of sale used.

(Added 1994) (Amended 2016)

S.5.3. Marking of Equivalent Conversion Factor for Liquefied Natural Gas. – A device dispensing 

liquefied natural gas shall have the statement “1 Diesel Gallon Equivalent (DGE) means 6.059 lb of Liquefied 

Natural Gas” permanently and conspicuously marked on the face of the dispenser according to the method of sale 

used.

(Added 2016)
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S.6. Printer. – When an assembly is equipped with means for printing the measured quantity, the following 

conditions apply:

(a) the scale interval shall be the same as that of the indicator;

(b) the value of the printed quantity shall be the same value as the indicated quantity;

(c) the printed quantity shall also include the mass value if the mass is not the indicated quantity;

[Nonretroactive as of January 1, 2021]

(d) a quantity for a delivery (other than an initial reference value) cannot be recorded until the measurement and 

delivery has been completed;

(e) the printer is returned to zero when the resettable indicator is returned to zero; and

(f) the printed values shall meet the requirements applicable to the indicated values.

(Amended 2016)

S.6.1. Printed Receipt. – Any delivered, printed quantity shall include an identification number, the time and 

date, and the name of the seller. This information may be printed by the device or pre-printed on the ticket.

S.7. Totalizers for Retail Motor-Fuel Devices. – Retail motor-fuel dispensers shall be equipped with a 

nonresettable totalizer for the quantity delivered through the metering device.

[Nonretroactive as of January 1, 1998]

(Added 1997)

N. Notes

N.1. Minimum Measured Quantity. – The minimum measured quantity shall be specified by the manufacturer.

N.2. Test Medium.

N.2.1. Liquid-Measuring Devices. – The device shall be tested with the liquid that the device is intended to 

measure or another liquid with the same general physical characteristics.

N.2.2. Vapor-Measuring Devices. – The device shall be tested with air or the product to be measured.

N.3. Test Drafts. – The minimum test shall be one test draft at the maximum flow rate of the installation and one 

test draft at the minimum flow rate. More tests may be performed at these or other flow rates. (Also see 

T.3. Repeatability.)

N.4. Minimum Measured Quantity. – The device shall be tested for a delivery equal to the declared minimum 

measured quantity when the device is likely to be used to make deliveries on the order of the minimum measured 

quantity.

N.5. Motor-Fuel Dispenser. – When a device is intended for use as a liquid motor-fuel dispenser, the type 

evaluation test shall include a test for accuracy using five starts and stops during a delivery to simulate the operation 

of the automatic shut-off nozzle. This test may be conducted as part of the normal inspection and test of the meter.
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N.6. Testing Procedures.

N.6.1. Normal Tests. – The normal test of a meter shall be made at the maximum discharge rate developed by 

the installation. Any additional tests conducted at flow rates down to and including the rated minimum discharge 

flow rate shall be considered normal tests.

(Added 1999)

N.6.2. Special Tests. – “Special” tests shall be made to develop the operating characteristics of a device and 

any special elements and accessories attached to or associated with the device. Any test except as set forth in 

N.6.1. Normal Tests shall be considered a special test. Special tests of a measuring system shall be made to 

develop operating characteristics of the measuring systems during a split compartment delivery. Also see 

Table T.2. Accuracy Classes and Tolerances for Mass Flow Meters.

(Added 1999)

N.6.3. Repeatability Tests. – Tests for repeatability should include a minimum of three consecutive test drafts 

of approximately the same size and be conducted under controlled conditions where variations in factors such as 

temperature, pressure, and flow rate are reduced to the extent that they will not affect the results obtained. When 

conducting the tests, the discharge rates shall be within the minimum and maximum discharge rates as marked by 

the manufacturer.

(Renumbered and Amended 2019)

T. Tolerances

T.1. Tolerances, General.

(a) The tolerances apply equally to errors of underregistration and errors of overregistration.

(b) The tolerances apply to all products at all temperatures measured at any flow rate within the rated measuring 

range of the meter.

(Amended 1999)

T.2. Tolerances. – The tolerances for mass flow meters for specific liquids, gases, and applications are listed in 

Table T.2. Accuracy Classes and Tolerances for Mass Flow Meters.

(Amended 1994 and 1999)
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Table T.2. 

Accuracy Classes and Tolerances for Mass Flow Meters

Accuracy 

Class

Application or Commodity

Being Measured

Acceptance 

Tolerance

Maintenance 

Tolerance

Special 

Tolerance

0.3 - Large capacity motor-fuel dispensers (maximum 

discharge flow rates greater than 100 L/min or 

25 gal/min)

- Heated products (other than asphalt) at 

temperatures greater than 50 °C (122 °F)

- Asphalt at temperatures equal to or below 50 °C

(122 °F)

- Loading rack meters

- Vehicle-tank meters

- Home heating oil

- Milk and other food products

- All other liquid applications not shown in the table 

where the minimum delivery is at least 700 kg 

(1500 lb)

0.2 % 0.3 % 0.5 %

 0.3A - Asphalt at temperatures greater than 50 °C

(122 °F)

0.3 % 0.3 % 0.5 %

0.5 - Small capacity (retail) motor-fuel dispensers

- Agri-chemical liquids

- All other liquid applications not shown in the table 

where the minimum delivery is less than 700 kg or 

1500 lb

0.3 % 0.5 % 0.5 %

1.0 - Anhydrous ammonia

- LP Gas (including vehicle-tank meters)

0.6 % 1.0 % 1.0 %

2.0 - Compressed natural gas as a motor-fuel 1.5 % 2.0 % 2.0 %

2.5 - Cryogenic liquid meters

- Liquefied compressed gases other than LP Gas

1.5 % 2.5 % 2.5 %

(Added 1994) (Amended 1999, 2001, and 2013)

T.3. Repeatability. – When multiple tests are conducted at approximately the same flow rate and draft size, the 

range of the test results for the flow rate shall not exceed 40 % of the absolute value of the maintenance tolerance and 

the results of each test shall be within the applicable tolerance. Also see N.6.3 Repeatability Tests.

(Amended 1992, 1994, 2001, and 2019)

T.4. Type Evaluation Examinations for Liquid-Measuring Devices. – For type evaluation examinations, the 

tolerance values shall apply under the following conditions:

(a) with any one liquid within the range of liquids;

(b) at any one liquid temperature and pressure within the operating range of the meter; and

(c) at all flow rates within the range of flow rates.

(Added 1993) (Amended 1994)
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UR. User Requirements

UR.1. Selection Requirements.

UR.1.1. Discharge Hose-Length. – The length of the discharge hose on a retail motor-fuel device shall not 

exceed 4.6 m (15 ft) unless it can be demonstrated that a longer hose is essential to permit deliveries to be made 

to receiving vehicles or vessels.

[Nonretroactive as of January 1, 1998]

(Added 1997)

UR.1.2. Minimum Measured Quantity.

(a) The minimum measured quantity shall be specified by the manufacturer.

(b) The minimum measured quantity appropriate for a transaction may be specified by the weights and 

measures authority. A device may have a minimum measured quantity smaller than that specified by 

the weights and measures authority; however, the device must perform within the performance 

requirements for the declared minimum measured quantity.

UR.2. Installation Requirements.

UR.2.1. Manufacturer’s Instructions. – A device shall be installed in accordance with the manufacturer’s 

instructions, and the installation shall be sufficiently secure and rigid to maintain this condition.

(Added 1997)

UR.2.2. Discharge Rate. – A device shall be installed so that the actual maximum discharge rate will not 

exceed the rated maximum discharge rate. Automatic means of flow regulation shall be incorporated in the 

installation if necessary.

(Added 1997)

UR.2.3. Low-Flow Cut-Off Valve. – If a metering system is equipped with a programmable or adjustable 

“low-flow cut-off” feature:

(a) the low-flow cut-off value shall not be set at flow rates lower than the minimum operating flow rate 

specified by the manufacturer on the meter; and

(b) the system shall be equipped with flow control valves which prevent the flow of product and stop the 

indicator from registering product flow whenever the product flow rate is less than the low-flow cut-off 

value.

(Added 1992)

UR.3. Use of Device.

UR.3.1. Unit Price and Product Identity for Retail Dispensers. – The following information shall be 

conspicuously displayed or posted on the face of a retail dispenser used in direct sale:

(a) except for dispensers used exclusively for fleet sales, other price contract sales, and truck refueling 

(e.g., truck stop dispensers used only to refuel trucks), all of the unit prices at which the product is offered 

for sale; and

(b) in the case of a computing type or money-operated type, the unit price at which the dispenser is set to 

compute.

(Added 1993)
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UR.3.1.1. Marking of Equivalent Conversion Factors for Compressed Natural Gas. – A device 

dispensing compressed natural gas shall have either the statement “1 Gasoline Gallon Equivalent (GGE) 

means 5.660 lb of Compressed Natural Gas” or “1 Diesel Gallon Equivalent (DGE) means 6.384 lb of 

Compressed Natural Gas” permanently and conspicuously marked on the face of the dispenser according to 

the method of sale used.

(Added 2016)

U.R.3.1.2. Marking of Equivalent Conversion Factor for Liquefied Natural Gas. – A device 

dispensing liquefied natural gas shall have the statement “1 Diesel Gallon Equivalent (DGE) means 6.059 lb 

of Liquefied Natural Gas” permanently and conspicuously marked on the face of the dispenser according to 

the method of sale used.

(Added 2016)

UR.3.2. Vapor-Return Line. – During any metered delivery of liquefied petroleum gas and other liquids from 

a supplier’s tank to a receiving container, there shall be no vapor-return line from the receiving container to the 

supplier’s tank:

(a) in the case of any receiving container to which normal deliveries can be made without the use of such 

vapor-return line; or

(b) in the case of any new receiving container when the ambient temperature is below 90 °F.

(Added 1993)

UR.3.3. Ticket Printer; Customer Ticket. – Vehicle-mounted metering systems shall be equipped with a 

ticket printer which shall be used for all sales where product is delivered through the meter. A copy of the ticket 

issued by the device shall be left with the customer at the time of delivery or as otherwise specified by the 

customer.

(Added 1994)

UR.3.4. Printed Ticket. – The total price, the total quantity of the delivery, and the price per unit shall be 

printed on any ticket issued by a device of the computing type and containing any one of these values.

(Added 1993)

UR.3.5. Ticket in Printing Device. – A ticket shall not be inserted into a device equipped with a ticket printer 

until immediately before a delivery is begun, and in no case shall a ticket be in the device when the vehicle is in 

motion while on a public street, highway, or thoroughfare.

(Added 1993)

UR.3.6. Steps After Dispensing. – After delivery to a customer from a retail motor-fuel device:

(a) the starting lever shall be returned to its shutoff position and the zero-set-back interlock engaged; and

(b) the discharge nozzle shall be returned to its designed hanging position unless the primary indicating 

elements, and recording elements, if the device is equipped and activated to record, have been returned 

to a definite zero indication.

(Added 1993)

UR.3.7. Return of Indicating and Recording Elements to Zero. – The primary indicating elements (visual), 

and the primary recording elements when these are returnable to zero, shall be returned to zero immediately before 

each delivery. Exceptions to this requirement are totalizers on key-lock-operated or other self-operated dispensers 

and the primary recording element if the device is equipped to record.

(Added 1995) (Amended 1997)
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UR.3.8. Return of Product to Storage, Retail Compressed and Liquefied Natural Gas Dispensers. –

Provisions at the site shall be made for returning product to storage or disposing of the product in a safe and timely 

manner during or following testing operations. Such provisions may include return lines, or cylinders adequate 

in size and number to permit this procedure.

(Added 1998) (Amended 2016)