Weights and Measures Inspection of a Retail Motor Fuel Device

Weights and Measures Inspection of a Retail Motor Fuel Device


[ Music ] Every day, millions of Americans buy gas. Customers and service station owners
want to be certain what is paid for is what is dispensed and vice versa. That certainty derives from
the precision of the gas pump and the efforts of Weights and Measures
officials who inspect these dispensers to ensure they are operating at the high
levels of reliability and efficiency for which they were designed. This video will demonstrate
proper procedures developed by the National Institute of
Standards and Technology, NIST, for testing the accuracy of
retail motor fuel dispensers and for ensuring compliance
with NIST Handbook 44 which helps guarantee consistent,
clear, and understandable transactions. [ Music ] Accuracy and transparency are what the
public and gas station owners expect from the devices that dispense
the gasoline they buy and sell. In the next few minutes, this
dispenser will be inspected and tested for compliance with the
provisions of NIST Handbook 44. Before beginning an accuracy test,
the inspector examines the dispenser to verify compliance with
specifications in NIST Handbook 44. To verify that a dispenser
is measuring accurately, the inspector uses a
specialized container or standard whose capacity
has been verified in a NIST-authorized laboratory. Depending on their capacities
and other design features, these are called test
measures or provers. They must comply with
NIST Handbook 105-3 to be acceptable as a field standard. To the extent possible, an inspector
tries to duplicate the conditions under which a customer
will use the device. So, with a gasoline dispenser, if you
think about the meter that’s being used in the dispenser, meters
can vary in accuracy according to the rate of flow. They can perform differently at slower
rates than they can at fast rates. If you think about the way a delivery
occurs, the customer will start pumping at a slower rate, get up to some
steady state, and then at the end of the delivery, will start to slow
down until the delivery is finished. If the test draft is small, the portions
of the delivery at the beginning and the end become very large. So, it’s important the test draft size
be adequately large so that the bulk of the delivery as done by the inspector
will represent the normal steady state of that device. So, for most gasoline
dispensers, a 19-liter or five-gallon test draft
is considered adequate. If the inspector is testing a larger
meter such as used at truck stops, the meter will be operating
at a much larger flow rate, so the inspector will need
a larger test draft size. In those cases, it’s recommended
that the test draft size hold at least one minute’s worth of flow. Having selected the
appropriate standard, the inspector makes sure the device is
ready for use by looking inside for dirt or other foreign materials
that might affect the reading. Also, the security seals on the
gauge plates must be checked to be sure they are intact. If the seals are broken, the test
measure or prover must be returned to a NIST-approved lab for re-verification before it can
be used in an official testing. When the test measure or prover is
filled with liquid and then emptied, a small amount of residual
product remains inside. Because it is impractical to dry the
inside of test measures or provers between uses, they are
generally calibrated to deliver rather than to contain. A dry test measure or prover must be wet
down prior to the first use of the day or between extended periods of non-use to duplicate the laboratory
procedure during its calibration. The wet-down procedure consists of
filling the test measure or prover to capacity and following the
prescribed “pour and drain” protocol. The inspector zeroes the dispenser and
begins pumping gas into the test measure with the handle fully depressed
just as a consumer typically would. That is at the maximum flow
rate the dispenser can reach in this installation. While the test draft is being taken,
pay special attention to the fact that the dispenser’s nozzle is touching
the neck of the prover or test measure to ensure proper grounding. Trailer-mounted units must also
be grounded to the dispenser. The product is dispensed into
the test measure or the prover until the pump’s indicator
reads exactly five gallons. For a device indicating in liters,
a test measure or prover designed to show errors in milliliters
is used and test drafts of 19 liters or larger are used. This is a normal test, but there
is another kind of test needed to verify dispenser accuracy. A special or slow flow test is
conducted at a slower flow rate. The reason that an inspector
will conduct a slow flow test is to really look at the
condition of the meter. Meters operate at different
levels of performance depending on how fast or slow they’re operating. So, a meter that’s operating at a
slower flow rate can perform differently than a meter that’s operating
at a fast slow rate. So, the key purpose of doing
a slow flow test is to look at the condition of the meter. The slow flow test is conducted at
19 liters or five gallons a minute, with the nozzle set at its lowest
clip or the minimum flow rate marked on the meter, whichever is less. In many jurisdictions, trailer-mounted
standards called provers are used instead of hand-held test measures. For either, the process of taking a
test draft is virtually identical. After filling the test measure or
prover to capacity, it must be leveled. A test measure is placed on a level
surface to ensure an accurate reading, one that near as possible
duplicates laboratory conditions and is not thrown off by tilting. Level is determined by placing
a spirit level vertically against the measure’s neck on at least
two locations, at least 90 degrees apart around the circumference of the
neck and adjusting the orientation of the standard until the neck is
as close to vertical as possible. This is called plumbing the neck. Leveling the trailer-mounted
prover is slightly different. A trailer-mounted five-gallon prover
is generally mounted to the test unit with a flexible joint assembly. To level a trailer-mounted prover,
an inspector grasps the prover and gently moves it back
and forth or side to side until the prover is level according to the level indicator
mounted on the prover. Final leveling of the test measure
or prover should be done at the end of the test draft when the test
measure or prover is full of liquid. This is because the weight of the
liquid can affect the level condition. Now, with the product drawn and
the test measure or prover level, it’s time to read the meniscus which
is found at the top of the liquid in the sight glass of the
test measure or prover. Most likely, it will
be slightly concave. Having taken a position at which the
bottom of the meniscus is at eye-level, the inspector reads the error at the
bottom of the concave-shaped meniscus. If the bottom of the meniscus is not
exactly at zero, the value will be read as a minus error if it is below the zero
line or as a plus error if it is above. When the reading is between
gauge graduations, it is always rounded off
to the nearest graduation. If the bottom of the meniscus is exactly
in the middle of a graduated interval, the value of the nearest even
numbered graduation is read. In this example, the reading would be
plus two cubic inches because the bottom of the meniscus is mid-way
between plus one and plus two and the lower value is an odd number. After determining the error for a
test draft, the inspector will compare that error with the allowable
tolerance that’s being applied to the piece of equipment being tested. If the result is at the tolerance
limit, if it’s near the tolerance limit, or even if it exceeds the
tolerance limit, it’s recommended that the inspector repeat that draft. The primary purpose of
repeating that draft is to verify that the initial result actually
reflects the performance of the device and isn’t due to any error or procedural
variation that might have occurred on the part of the inspector. Emptying a test measure
or prover is more than just pouring product
back into a storage tank. When filled with liquid
and then emptied, a small amount of residual
product will cling to the inside of the test measure or prover. This can affect the measurement,
so after each test, a specific draining procedure must
be followed to ensure that the amount of liquid remaining is consistent. For a hand-held measure, place
the neck of the test measure against a metal funnel to ground it. Never use a plastic funnel or
substitute a plastic safety cone. These can allow a buildup of a static
charge when the product is poured. Now, pour the contents slowly
enough that the main flow stops after 30 seconds plus
or minus five seconds. The initial draining complete, the
inspector holds the test measure at 10 to 15 degrees from vertical for 10 seconds thus draining
what liquid may remain. It is important not to shake the test
measure during this final draining. To empty a bottom drained
prover, the inspector begins by opening the drain
valve beneath the prover. This allows the liquid in the prover
to flow into the storage tank below. It is important that the inspector
closely observes the flow of liquid through the sight glass
under the drain valve. Once the main flow of liquid
ceases, the inspector begins timing. For bottom-drained five-gallon provers,
the drain time will be 30 seconds. The inspector can verify the
proper drain time by looking at the drain time marked on the prover or in its laboratory
calibration certificate. When the drain time has elapsed,
the inspector closes the drain valve and the prover is now ready
for the next test draft. Following specified drain procedures
helps to ensure that the amount of liquid remaining in a prover or
test measure is consistent from draft to draft and duplicates the conditions under which this standard was
calibrated in the laboratory. After the final accuracy
test is finished, the inspector performs
an anti-drain test. With the dispenser in the off
position, the inspector lifts the nozzle at least three feet above the level of
the nozzle and depresses the trigger. If a small amount of product comes out
of the nozzle and slows to a few drops, the anti-drain means in the
dispenser is operational. But if there is a continuous
trickle or flow, the anti-drain valve is likely
malfunctioning and the anti-drain means in the dispenser is unacceptable. A gasoline dispenser is what
we call a wet hose type device. The term “wet-hose” refers to the
fact that the dispenser is designed so that the discharge hose is
full of liquid at all times. Consider what happens during a delivery. At the beginning of the delivery,
the hose is full of liquid but the liquid that’s in the hose
was actually metered during the previous delivery. When the current delivery begins,
the product goes through the meter and displaces the liquid in the
hose into the customer’s tank. At the end of the current
delivery, the product that is — remains in the hose was actually
metered during the current delivery. But just as product that was metered
during the previous delivery goes into the current customer’s tank, the product that was metered
during the current delivery will go into the next customer’s tank. An anti-drain valve or
other means helps to ensure that the discharge hose cannot
be drained between deliveries and this helps to ensure that subsequent
customers are not getting charged for product that they didn’t receive. [Background Music] This video
has offered a short tutorial on proper procedures
for testing the accuracy and performance of motor
fuel dispensers. Handling such tests safely
and efficiently is how Weights and Measures officials serve
both the purchasing public and the commercial community. Their work assures market place
equity, discourages unfair competition, and promotes consumer confidence. [ Music ]

4 Replies to “Weights and Measures Inspection of a Retail Motor Fuel Device”

  1. Great video. I would enjoy seeing more like this. It reflects the real life applications of the standards for which the NIST is responsible, and explains how they apply to normal citizens on a daily basis.

  2. Why are multiple grades of product allowed to be dispensed through a single hose ? Based on my estimate of 13 feet of 3/4 inch hose and 8 feet of 3/4 inch tubing internal to the dispenser there is almost 1/2 gallon of lower grade fuel dispensed to a customer buying 93 octane fuel when the the previous customer bought 87 octane rated fuel and vice versa . With a difference of $0.40~$0.50 between grades that is like the meter jumping 20 to 25 cents.

  3. Sure, this kind of test is okay for regular consumer use, but what about industrial?

    Surely a kind of closed loop electronic form of calibration is used, like that developed by Cantest.

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