Glossary of Terms

Accuracy
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.

Activated
Refers to the state of a qualitative detector's response when indicating the presence of product.

Alternative Fuel
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 percent biodiesel.

Bias
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 Tank
Generally applies to underground storage tanks 50,000 gallons or greater.

Continual Reconciliation
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 percent probability and 5 percent false alarm.

Continuous Automatic Tank Gauging Method (Continuous ATGS)
These systems use an 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.

Continuous Detector
Detectors that operate continuously, are always present and are never turned off.

Continuous In-Tank Leak Detection Method (CITLDS)
These systems are 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 three types of such continuous systems that are reaching the market. These three types are referred to as “Continuous ATGS,” “Continual Reconciliation,” and “Automatic Monthly Inventory Control.”

Continuous Monitoring
Routinely performing leak detection on an uninterrupted basis.

Detection Time
The sum of rise time and lag time.

Diesel or Diesel Fuel
Middle distillate fuel that may contain up to 5 percent biodiesel in accordance with ASTM standard D975.

Fall time
The elapsed time after a detector has responded to a test hydrocarbon and is removed and has recovered to 95 percent of its original baseline level or there is no detectable signal output.

False Alarm
Declaring a tank to be leaking when in fact it is tight.

Gasohol
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 percent of the blend.

GPH
Gallons per hour

Groundwater
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.

Intermittent Detector
Detectors that monitor on a regular 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.

Lag Time
The elapsed time from the detector's first contact with test product to the first detectable signal.

Large Diameter Pipeline
Generally, a pipeline that has a diameter of 6 inches and above.

Leak threshold
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 percent, PFA < 5 percent).

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 tank.

Minimum Detectable Leak Rate
The leak rate that can be detected with a Probability of Detection (PD) of 95 percent and a probability of False Alarm (PFA) of 5 percent. The minimum threshold is calculated setting the PFA at 5 percent. For a PD of 95 percent, the leak rate is then equal to twice the threshold that gives a PFA of 5 percent 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.

MER
The Maximum Effective Range, the longest length of sensor cables and/or jumper cables that can be connected to form a leak detection network.

N/A
Not Applicable

N/D
Not Determined

N/R
No Response

Net Pressure
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).

Precision
The degree of agreement of repeated measurements of the same parameter. Precision estimates reflect random error and are not affected by bias.

Pressure
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 as negative.

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 percentage.

Probe
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 percent, PFA < 5 percent criteria).

Qualitative Responses
The type of detector response that indicates only the presence or absence of product without determining the specific product concentration or thickness.

Quantitative Response
A type of detector response that quantifies the concentration or thickness of product present.

Relative Accuracy
A function of systematic error, or bias, and random error, or precision. Smaller values indicate better accuracy. See entry for “Accuracy.”

Resolution
The smallest change in the quantity being measured which the measurement system is capable of detecting.

Response Time
A general term that refers to the more specific terms of lag time, rise time, and fall time.

Rise Time
The elapsed time from a detector's first detectable signal in response to the presence of product to an output that is 95 percent 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
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.

Throughput
The volume of product dispensed from a tank system in a month.

Total Pressure
In this document this term equals the sum of the pressure in ullage space and the pressure due to product head.

Ullage
The un-wetted portion of the tank, i.e. that portion of the tank not in contact with product.

Vacuum
In this document this term refers to any pressure that is less than atmospheric.