The degree to which the
measured leak rate agrees with the induced leak rate on the average.
If a system is accurate, it has a very small or zero bias.
Refers to the state of a
qualitative detector's response when indicating the presence of product.
A fuel or petroleum fuel blend
containing any amount of non-petroleum component, including but not
limited to ethanol, methanol, or animal/vegetable oil, with the
exception of petrodiesel blends containing no more than 5% biodiesel.
An indication of whether
the device’s measured leak rate consistently overestimates (positive
bias) or underestimates (negative bias) the actual induced leak rate.
Bulk Modulus (of Elasticity):
The ratio of hydrostatic
pressure to the relative change it produces in volume.
Bulk Underground Storage
Generally applies to underground storage
tanks 50,000 gallons or greater.
Combines product level and temperature monitoring from the tank with
data from dispensing meters. Data from delivery records may also be
included. In addition, it may address leaks or unexplained losses of
product from the tank vessel, the pressurized lines, or a combination to
monitor the tank and line system. It allows a combination of monitoring
data from a static tank and inventory data from a dynamic tank to be
combined in monitoring the system for a leak. It is also designed to
meet the monthly monitoring performance standard of detecting a leak of
0.20 gallon per hour or 150 gallons per month with 95% probability and
5% false alarm.
Continuous Automatic Tank
automatic tank gauge probe to collect data continually and combine this
with software to identify time intervals when there is no activity in
the tank and the data are stable enough for analysis. An algorithm then combines data from a number of such periods
until there is enough evidence to make a determination about the leak
status of the tank. This
type of system functions like an automatic tank gauge except that it
does not require that the tank be taken out of service for a set period
of several hours whenever a test is to be done. Instead, it uses data from shorter stable time periods and
combines the results to estimate a leak rate and perform a test. The system may default to a standard or shut down automatic tank
gauge test (requiring the tank to be out of service for a few hours) at
the end of the month if sufficient good quality data have not been obtained
over the month. These
systems are designed to meet the monthly monitoring performance standard
of detecting a leak of 0.20 gallon per hour or 150 gallons per month
with 95% probability of detection (PD) and 5% probability of
false alarm (PFA). They test the tank vessel itself.
Detectors that operate
continuously, are always present and are never turned off.
Continuous In-Tank Leak Detection Method:
Designed to allow the tank to operate continuously or nearly
continuously without interruption for leak detection tests. They typically have some sensors permanently installed in the
tank, combined with a microprocessor in a console. In addition, they may be connected to the dispensing meters,
allowing for automatic recording and use of dispensing data. There may also be a provision for direct input of data from a
keyboard or pad, to allow for entry of delivery receipts.
Currently, there are two
types of such Continuous In-Tank Leak Detection Methods:
Continuous Automatic Tank
Gauging and Continual Reconciliation.
that are electronic and/or automated mechanisms that perform leak
detection on an uninterrupted basis and immediately communicate an alarm
condition to an individual, independent of the actions of an observer.
The sum of rise time and
Diesel or Diesel Fuel:
Hydrocarbon oil that may contain up to 5% biodiesel in accordance with the current edition of ASTM standard D975.
The elapsed time after a
detector has responded to a test hydrocarbon and is removed and has
recovered to 95% of its original baseline level or there is no
detectable signal output.
Declaring a tank to be
leaking when in fact it is tight.
A blend of finished motor gasoline containing alcohol (generally ethanol
but sometimes methanol) at a concentration between 5.7 percent and
nominal 10 percent by volume; also referred to as "E 10" when the
ethanol component is a nominal 10% of the blend.
Water table or water
within the excavation around a tank.
Independent Third-Party Evaluator:
Consulting firms, test laboratories, not-for-profit research
organizations, or educational institutions; with no organizational
conflict of interest.
Induced Leak Rate:
The actual leak rate, in
gallons per hour (gph), used during the evaluation against which the
results from a given test device will be compared.
Detectors that monitor on
a periodic basis. An
intermittent detector may be a hand held device that is portable or a
permanently installed device that is used to periodically test for the
presence of product.
The elapsed time from the
detector's first contact with test product to the first detectable
Generally, a pipeline that has a diameter
of 6 inches and above.
The measured leak rate at
which the system detects the tank to be leaking. This leak rate will always be less than or equal to the leak rate
requirement for the various release detection methods given in 40 CFR §
280 Subpart D-Release Detection. (Please
note that some states and other regulatory authorities may have
different requirements). The
minimum leak threshold for declaring a leak is experimentally determined
from the results of the evaluation of the release detection system.
Lower Detection Limit:
The smallest liquid
concentration or level that a detector can reliably detect (PD > 95%, PFA
Manifolded tank systems:
Tanks connected by piping
that allow the tank system to function as a single tank. A typical manifolded tank system usually consists of two
tanks connected by a siphon tube that permits the product in the tanks
to be at the same level while product is being pumped out of only 1
Minimum Detectable Leak Rate:
The leak rate that can be
detected with a Probability of Detection (PD) of 95% and a
Probability of False Alarm (PFA) of 5%. The minimum threshold is calculated setting the PFA at
5%. For a PD of
95%, the leak rate is then equal to twice the threshold that gives a PFA
of 5% assuming the bias is not significant.
Measured Leak Rate:
A positive number in
gallons per hour (gph), measured by test device that indicates the
amount of product leaking out of the tank system. A negative number would indicate that something was being added
to the tank. The
performance of a system is based on how well the measured leak rate
compares to the actual induced leak rate.
The Maximum Effective
Range, the longest length of sensor cables and/or jumper cables that can
be connected to form a leak
In this document this
term refers to a pressure difference between the pressure in the tank
and the pressure related to the groundwater. If the net pressure is positive, the pressure in the tank is
greater than that due to groundwater. If net pressure is negative, the pressure in the tank is less
than that due to groundwater.
Nominal Leak Rate:
The set or target leak
rate to be achieved as closely as possible during the evaluation of a
leak detection system. It
is a positive number expressed in gallons per hour (gph).
Methods that rely upon a procedure with a specified
observation period to determine a leak condition. During the
observation period, an individual observes, interprets, and reports the
status of the system.
The degree of agreement
of repeated measurements of the same parameter. Precision estimates reflect random error and are not affected by
In this document this
term refers to a pressure which is at or above atmospheric. Any pressure reading at or above atmospheric is listed as
positive; any pressure reading less than atmospheric (vacuum) is listed
Probability of Detection (PD):
The probability of
detecting a leak of a given size, usually expressed as a percentage.
Probability of False Alarm (PFA):
The probability of
declaring a tank to be leaking when it is tight, usually expressed as a
A component of a
detection system that must come into contact with product before product
can be declared or measured.
Product Activation Height:
The minimum height of
liquid required to cause sensor activation (this value does not have to
meet the PD > 95%, PFA < 5% criteria).
The type of detector
response that indicates only the presence or absence of product without
determining the specific product concentration or thickness.
A type of detector
response that quantifies the concentration or thickness of product
A function of systematic
error, or bias, and random error, or precision. Smaller values indicate better accuracy.
See entry for “Accuracy.”
The smallest change in
the quantity being measured which the measurement system is capable of
A general term that
refers to the more specific terms of lag time, rise time, and fall time.
The elapsed time from a
detector's first detectable signal in response to the presence of
product to an output that is 95% of full scale for a quantitative
detector or activated for a qualitative detector.
Statistical Inventory Reconciliation (SIR), In-House System:
Data gathered and input
by owner or operator. System does analysis. If analysis presents problems, technical support and analysis are
available from vendor or vendor representative.
Statistical Inventory Reconciliation (SIR), Stand Alone System:
No human interface
required. Data gathered and
analyzed automatically without owner/operator input.
Specificity applies to
vapor and liquid sensors and lists products or components of products
that these sensors can detect. Specificity
for quantitative sensors is the ratio of sensor output, or measured
concentration, to the actual concentration of hydrocarbon test gas
expressed as a percentage. Specificity
for qualitative sensors is reported as activated if the sensor responds
within 24 hours. Otherwise,
specificity is reported as inactivated.
The volume of product dispensed from a tank
system in a month.
In this document this
term equals the sum of the pressure in ullage space and the pressure due
to product head.
The un-wetted portion of
the tank, i.e. that portion of the tank not in contact with product.
In this document this term
refers to any pressure that is less than atmospheric.