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.
BIAS: An indication of whether the device’s measured leak rate consistently overestimates (positive bias) or underestimates (negative bias) the actual induced leak rate.
CONTINUOUS DETECTOR: Detectors that operate continuously, are always present and are never turned off.
DETECTION TIME: The sum of rise time and lag time.
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.
GROUNDWATER: Water table or water within the excavation around a tank.
LAG TIME: The elapsed time from the detector”s first contact with test product to the first detectable signal.
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.
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).
PROBE: A component of a detection system that must come into contact with product before product can be declared or measured.
RELATIVE ACCURACY: A function of systematic error, or bias, and random error, or precision. Smaller values indicate better accuracy.
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.
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.
ULLAGE: The un-wetted portion of the tank, i.e. that portion of the tank not in contact with product.
This glossary of terms was abstracted from a resource compiled by the National Work Group on Leak Detection Evaluations (NWGLDE, www.nwglde.org), an independent organization comprised of 10 members, nine of whom represent various states and one from the U.S. EPA.