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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.
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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.
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