March 2005 

Series 1 – 

Small Volume Provers: 

Identification, Terminology and Definitions 

By G. Diane Lee 

In 1996 changes were made to Section N.3.5. “Wholesale Devices” in the Liquid Measuring 

Code of the National Institute of Standards and Technology (NIST) Handbook (HB) 44 

“Specifications and Tolerances and other Technical Requirements for Weighing and Measuring 

Devices.” This change provided for recognition of small volume provers (SVPs) for use as 

standards in the field to test wholesale liquid measuring devices. In conjunction with the 

process of adopting changes to NIST HB 44 for recognition of these provers for field use, 

specifications and tolerances for these field standards were also documented and published in 

NIST HB 105-7 in 1997. 

Since officials have begun encountering SVPs in their jurisdictions, NIST WMD has received 

questions about the design, use, operation, and implementation of SVPs in the inspection and 

testing of commercial measuring devices. In response to these inquiries, WMD has developed a 

series of articles designed to assist jurisdictions in addressing these questions. 

A number of different types of provers, including SVPs are used to test large meters. 

Terminology used to describe these provers is often used interchangeably and can be confusing. 

Consequently, this series begins with the following article in which we define the terms most 

commonly used to refer to “SVPs”, the terms used to describe other common types of large 

meter provers, and terms used to describe the operation of these provers. 

Subsequent articles in the series will cover the history, design, operation, use, and 

implementation of these provers in the inspection and testing of commercial measuring devices. 

Types of Provers and Associated Components and Operation 

The diagram below shows the different types of pipe provers and their subcategories: 
Pipe Provers

Conventional

Pipe Provers

require equal to or

greater than

10,000 meter

pulses

Small Volume

Provers

(Also Known As

Compact, Ballistic,

Captive Displacer,

Loop, Piston)

Unidirectional

Spheroid

Prover

Bidirectional

Piston

Prover

Bidirectional

Spheroid

Prover

Conventional Pipe

Provers that require

less than 10,000

meter pulses

and use pulse

Interpolation

Compact and

Ballistic

Internal By-Pass

and External ByPass

The following definitions are associated with SVPs: 

Bidirectional prover. A prover which uses a displacer that travels in one direction to actuate 

detectors in the calibrated section of the pipe and then in the opposite direction through the same 

calibrated section and the same detectors. The measured volume corresponds to the sum of both 

passages of the displacer. (OIML R 119) 

Conventional pipe prover. See “pipe provers.” 

Captive displacer prover. Same as “small volume prover.” 

Compact prover. Same as “small volume prover.” 

Double chronometry. A pulse interpolation method used in small volume provers to provide 

for fractional meter pulse counting. (NIST HB 105-7) Generally, double chronometry uses a 

component that increments time very precisely and operates two counters. Counter one is started 

when the first detector switch is tripped. The second counter is started after the first counter is 

started. The first counter is stopped when the second detector switch is tripped. The second 

counter is stopped after the first counter has stopped. Using the ratio of time counter one to time 

counter two will allow for accurately counting a fraction of a flow meter pulse. 

Detectors. Optical sensors or electronic switches used to start or stop counters and to determine 

the calibrated section of the prover. (NIST HB 105-7) 

Displacer. A piston or sphere (ball) in a pipe prover that is displaced by a flowing liquid 

between two signaling detectors. The displacer is made of material that is appropriate for the 

prover’s operating pressure, temperature, and the desired resistance to degradation by the liquid 

that is being metered. 
Sphere displacers may be made of neoprene rubber, which is used for low pressure crude oil and 

anhydrous ammonia; nitrile, which is used for refined petroleum products such as gasoline, fuel 

oil, kerosene, and high pressure crude oils; urethane, which is used where abrasive resistance is 

critical and in low temperature applications; or viton, which is used with liquids that have a high 

aromatic content. With the exception of solid viton spheroids, the sphere is typically hollow and 

filled with a 50:50 mixture of glycol and water using a needle valve. The resulting diameter of 

the sphere is between 2 to 4 % larger than the inside prover dimension and chosen such that the 

sphere can travel freely but remain sealed to the pipe wall. 

The piston displacer is made as light as possible and of non-magnetic material, such as aluminum 

or a special stainless steel. Wear rings made of non-metallic materials are used to prevent the 

metal portion of the piston from contacting the inside of the pipe. The piston is equipped with 

seating rings and cups to achieve the required seal and to provide a running surface. The rings 

and cups may be constructed of teflon, nitrile, polyurethane, or other appropriate material. 

K-factor. A value obtained by dividing the number of meter signal pulses collected during the 

proving sample by the actual, corrected prover volume. (NIST HB 105-7) 

K-Factor = meter signal pulses

 corrected prover volume 

Loop prover. Same as “small volume prover.” 

Master meter prover. A meter that serves as the reference for the proving of another meter. In 

this proving method, a “standard” or “master” meter is compared to a meter under test. The 

master meter must be reliable, consistent, and well maintained. (See American Petroleum 

Institute (API), Manual of Petroleum Measurement Standards (MPMS), Chapter 4, Section 5). 

Although WMD has typically discouraged the use of master meters for use in legal-for- trade 

applications (unless certain provisions are in place), industry has used these types of provers for 

many years. 

Meter factor. A value obtained by dividing the actual volume of liquid passed through the 

meter (corrected prover volume) by the registered meter volume. (NIST HB 105-7) 

MF = corrected prover volume

 registered meter volume 

Pipe prover. This is the generic name for provers (including “conventional pipe provers” and 

“small volume provers”) in which a piston or sphere is enclosed within a calibrated length of 

pipe that is in series with the meter. The piston or sphere is displaced by a flowing liquid 

between two signaling detectors. The piston or sphere is known as the “displacer” (see 

definition). The displacer is designed to form a sliding seal against the inside of the pipe and 

travel at the same speed as the liquid. When the prover is connected in series with a meter, the 

volume swept out by the displacer as it passes between the detectors equals the volume displaced 

by the meter during the same interval. “Pulses” (see definition) are generated as the displacer 

moves between the start and stop detectors. When combined with a pulse totalizer and a flow 

meter capable of generating a pulse train, the basis for determining the liquid flow meter’s 

accuracy is established. 
API requires that a minimum of 10,000 unaltered pulses be generated during a prover pass for a 

“conventional pipe prover” to ensure an accurate test. This minimum is based upon the 

assumption that there is a potential error of 1 pulse each time a detector is passed, which equates 

to a 2-pulse error per run. The value of 2 pulses per 10,000 equates to a potential 0.02 % error. 

Note that another type of pipe prover, the “small volume prover” (see definition) can be used 

with less than 10,000 pulses; however, these provers use pulse interpolation to obtain 

comparable results. 

Piston prover. This is a type of conventional pipe prover or a type of small volume prover in 

which a piston is used as the displacer. 

Prover pass. A single pass of the displacer through the calibrated section. (NIST HB 105-7) 

Pulse counter or totalizer. Pipe provers rely on the pulse counter or totalizer to record the 

number of pulses generated by the meter while the displacer passes through the calibrated section 

of the prover. As the displacer passes the first detector, the totalizer starts recording the number 

of meter pulses received until the displacer passes the second switch signaling the count stop. 

Pulser (meter pulse generator). A component that transforms the mechanical motion of the 

meter shaft, that produces indications, into digital signals. The pulser generates a known number 

of discrete electrical pulses for each revolution of the meter shaft. There are various types of 

digital pulsers, but most commonly the pulser is a switch that opens and closes with the rotation 

of the shaft driven by the meter. 

Some pulsers use shaft encoders that include a disc containing narrow opaque regions spaced 

from each other by relatively wide transparent regions. As the meter shaft rotates, light from a 

light source is directed through the regions toward a light detector. The output from the light 

detector results in a train of pulses, which relates directly to the rotation of the meter. 

Pulses. Signals which are generated by a pulser (see above) for each revolution of the meter 

shaft. When a pipe prover is connected to a meter and a pulse counter the number of meter 

pulses are counted as the displacer passes the first detector until the displacer passes the second 

displacer. 

Pulse interpolation. A pulse counting technique used to calculate the total number of meter 

pulses between two detectors, including fractional pulses. (NIST HB 105-7) 

Small volume prover. A type of conventional pipe prover in which a displacer travels through a 

very short section of the pipe or cylinder. The volume displaced in the calibrated section is 

usually much smaller than that of a conventional pipe prover. For this reason, high precision 

detectors and pulse interpolation are necessary to achieve repeatability and accuracy (OIML R 

119). Small volume provers require less than 10,000 pulses to be generated while conventional 

pipe provers require that at least 10,000 pulses be generated for a prover run. 

Spheroid prover. This is a type of conventional pipe prover in which a spheroid (ball) is used 

as the displacer. 
Tank prover. See “Volumetric field standards.” 

Unidirectional prover. This type of prover uses a displacer that travels in one direction to 

actuate detectors in the calibrated section of the pipe. The measured volume corresponds to one 

passage of the displacer. (OIML R 119) 

Volume field standards. These are the standards most commonly use by State and local 

weights and measures officials and others for verification of commercial meters. The standards 

include both test measures (capacity < 40 L/10gal) and provers (capacity >40L/10gal). NIST 

Handbook 105-3 Specifications and Tolerances for Graduated Neck Type Volumetric Field 

Standards, specifies these as standards limited to non-pressurized, graduated neck type metal 

field standards with or without vapor recovery capabilities where the volume is established 

between a shutoff valve or bottom-neck zero graduation and an upper-neck nominal graduation. 

Some refer to these general types as “tank provers” (see also API, MPMS, Chapter 4, Section 4) 

or “volumetric neck-type provers.” 

We extend our thanks to Peter Syrnyk of Emerson Process Management for his assistance in 

preparing this article. 

Look for the next article in this series that will cover the history, use, and operation of small 

volume provers