Handbook 44 – 2010 3.37. Mass Flow Meters 

Table of Contents 

Section 3.37. Mass Flow Meters.......................................................................................... 3-101

A. Application ................................................................................................................................................ 3-101

A.1. Liquids. ............................................................................................................................................ 3-101

A.2. Vapor (Gases). ................................................................................................................................. 3-101

S. Specifications............................................................................................................................................. 3-101

S.1. Indicating and Recording Elements. ................................................................................................ 3-101

S.1.1. Indicating Elements. .......................................................................................................... 3-101

S.1.2. Compressed Natural Gas Dispensers. ................................................................................ 3-101

S.1.3. Units................................................................................................................................... 3-101

S.2. Operating Requirements. ................................................................................................................. 3-102

S.2.1. Return to Zero.................................................................................................................... 3-102

S.2.2. Indicator Reset Mechanism................................................................................................ 3-102

S.2.3. Nonresettable Indicator...................................................................................................... 3-102

S.2.4. Provisions for Power Loss. ................................................................................................ 3-102

S.2.5. Display of Unit Price and Product Identity. ....................................................................... 3-102

S.2.6. Money-Value Computations.............................................................................................. 3-103

S.2.7. Recorded Representations, Point-of-Sale Systems............................................................. 3-103

S.2.8. Indication of Delivery. ....................................................................................................... 3-103

S.3. Measuring Elements and Measuring Systems.................................................................................. 3-104

S.3.1. Maximum and Minimum Flow-Rates................................................................................ 3-104

S.3.2. Adjustment Means. ............................................................................................................ 3-104

S.3.3. Vapor Elimination.............................................................................................................. 3-104

S.3.4. Maintenance of Liquid State.............................................................................................. 3-104

S.3.5. Provision for Sealing.......................................................................................................... 3-104

S.3.6. Automatic Density Correction. .......................................................................................... 3-105

S.3.7. Pressurizing the Discharge Hose........................................................................................ 3-105

S.3.8. Zero-Set-Back Interlock, Retail Motor-Fuel Devices. ....................................................... 3-106

S.4. Discharge Lines and Valves............................................................................................................. 3-106

S.4.1. Diversion of Measured Product. ........................................................................................ 3-106

S.4.2. Pump-Discharge Unit......................................................................................................... 3-106

S.4.3. Directional Flow Valves. ................................................................................................... 3-106

S.4.4. Discharge Valves. .............................................................................................................. 3-106

S.4.5. Antidrain Means. ............................................................................................................... 3-106

S.4.6. Other Valves. ..................................................................................................................... 3-107

S.5. Markings. ......................................................................................................................................... 3-107

S.5.1. Location of Marking Information; Retail Motor-Fuel Dispensers..................................... 3-107

S.5.2. Marking of Gasoline Volume Equivalent Conversion Factor............................................ 3-107

S.6. Printer. ............................................................................................................................................. 3-108

S.7. Totalizers for Retail Motor-Fuel Devices. ....................................................................................... 3-108

N. Notes........................................................................................................................................................... 3-108

N.1. Minimum Measured Quantity.......................................................................................................... 3-108

N.2. Test Medium.................................................................................................................................... 3-108

N.2.1. Liquid-Measuring Devices................................................................................................. 3-108

N.2.2. Vapor-Measuring Devices. ................................................................................................ 3-108

N.3. Test Drafts. ...................................................................................................................................... 3-108

N.4. Minimum Measured Quantity.......................................................................................................... 3-108

N.5. Motor-Fuel Dispenser. ..................................................................................................................... 3-108

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3.37. Mass Flow Meters Handbook 44 – 2010 

N.6. Testing Procedures........................................................................................................................... 3-108

N.6.1. Normal Tests...................................................................................................................... 3-108

N.6.2. Special Tests. ..................................................................................................................... 3-109

T. Tolerances.................................................................................................................................................. 3-109

T.1. Tolerances, General. ........................................................................................................................ 3-109

T.2. Tolerances........................................................................................................................................ 3-109

T.3. Repeatability. ................................................................................................................................... 3-110

T.4. Type Evaluation Examinations for Liquid-Measuring Devices....................................................... 3-110

UR. User Requirements ................................................................................................................................... 3-110

UR.1. Selection Requirements. .................................................................................................................. 3-110

UR.1.1. Discharge Hose-Length. .................................................................................................... 3-110

UR.1.2. Minimum Measured Quantity............................................................................................ 3-110

UR.2. Installation Requirements. ............................................................................................................... 3-110

UR.2.1. Manufacturer’s Instructions. .............................................................................................. 3-110

UR.2.2. Discharge Rate................................................................................................................... 3-110

UR.2.3. Low-Flow Cut-Off Valve. ................................................................................................. 3-110

UR.3. Use of Device................................................................................................................................... 3-111

UR.3.1. Unit Price and Product Identity for Retail Dispensers. ...................................................... 3-111

UR.3.2. Vapor-Return Line............................................................................................................. 3-111

UR.3.3. Ticket Printer; Customer Ticket......................................................................................... 3-111

UR.3.4. Printed Ticket..................................................................................................................... 3-111

UR.3.5. Ticket in Printing Device................................................................................................... 3-111

UR.3.6. Steps After Dispensing. ..................................................................................................... 3-111

UR.3.7. Return of Indicating and Recording Elements to Zero. ..................................................... 3-111

UR.3.8. Return of Product to Storage, Retail Compressed Natural Gas Dispensers. ...................... 3-112

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Handbook 44 – 2010 3.37. Mass Flow Meters 

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. 

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 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) 

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 liter equivalent 

(GLE) units” or “gasoline gallon equivalent (GGE) units” (see definitions). 

(Added 1994) 

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.

(a) The maximum value of the quantity-value division for liquids shall be not greater than 0.2 % of 

the minimum measured quantity. 

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3.37. Mass Flow Meters Handbook 44 – 2010 

(b) For dispensers of compressed natural gas used to refuel vehicles, the value of the division for the 

gasoline liter equivalent shall not exceed 0.01 GLE; the division for gasoline gallon equivalent 

(GGE) shall not exceed 0.001 GGE. The maximum value of the mass division shall not exceed 

0.001 kg or 0.001 lb. 

(Amended 1994) 

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. Nonresettable Indicator. – An instrument may also be equipped with a nonresettable 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) 

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 

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Handbook 44 – 2010 3.37. Mass Flow Meters 

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 money 

value 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) 

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) 

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3.37. Mass Flow Meters Handbook 44 – 2010 

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 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. Vapor Elimination. – A liquid-measuring instrument or measuring system shall be equipped with an 

effective vapor or air eliminator or other effective means, automatic in operation, to prevent the measurement of 

vapor and air. Vent lines from the air or vapor eliminator shall be made of metal tubing or some other suitable 

rigid material. 

(Amended 1999) 

S.3.3.1. Vapor Elimination on Loading Rack Liquid Metering Systems.

(a) A loading rack liquid metering system shall be equipped with a vapor or air eliminator or other 

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

designed or operationally controlled by a method, approved by the weights and measures 

jurisdiction having statutory authority over the device, such that neither air nor vapor can enter the 

system. 

(b) Vent lines from the air or vapor eliminator (if present) shall be made of metal tubing or other rigid 

material. 

(Added 1995) 

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. – 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 may 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. 

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Handbook 44 – 2010 3.37. Mass Flow Meters 

[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, and 2006) 

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 through the device or through another on-site 

device. The event logger shall have a capacity to retain 

records equal to ten 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, and 2006) 

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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3.37. Mass Flow Meters Handbook 44 – 2010 

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. (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. 

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Handbook 44 – 2010 3.37. Mass Flow Meters 

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. 

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). 

Note: The use of a dispenser key or tool to access internal marking information is permitted for retail liquid-measuring 

devices. 

[Nonretroactive as of January 1, 2003] 

(Added 2006) 

S.5.2. Marking of Gasoline Volume Equivalent Conversion Factor. – A device dispensing compressed 

natural gas shall have either the statement “1 Gasoline Liter Equivalent (GLE) is Equal to 0.678 kg of Natural 

Gas” or “1 Gasoline Gallon Equivalent (GGE) is Equal to 5.660 lb of Natural Gas” permanently and 

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

(Added 1994) 

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3.37. Mass Flow Meters Handbook 44 – 2010 

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

and delivery has been completed; 

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

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

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. (See T.3.) 

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. 

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.6.1.1. 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. 

(Added 2001) 

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Handbook 44 – 2010 3.37. Mass Flow Meters 

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. 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. (See Table T.2.) 

(Added 1999) 

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) 

Table T.2. 

Accuracy Classes and Tolerances for Mass Flow Meters

Accuracy 

Class 

Application or Commodity 

Being Measured 

Acceptance 

Tolerance 

Maintenanc

e 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 

- Asphalt at or below a temperature of 50 ΕC 

- Loading rack meters 

- Vehicle-tank meters 

- Home heating oil 

- Asphalt at or below 50 ΕC 

- 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 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 and 2001) 

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3.37. Mass Flow Meters Handbook 44 – 2010 

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. (See also N.6.1.1. Repeatability Tests) 

(Amended 1992, 1994, and 2001) 

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) 

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 

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(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) 

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 

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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) 

UR.3.8. Return of Product to Storage, Retail Compressed 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) 

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