Glossary of Terms
EVM Series User Manual
139
Term
Definition
Peak
The highest level of toxic gas or oxygen reached while the unit is on.
PEL (Permissible
Exposure Limit)
Permissible exposure limit. PEL's are a regulatory limitation to exposure used to
specify the allowable exposure to a substance in the workplace and assume that the
exposure takes place over an 8-hour shift in a 40-hour work week. Note that there
are more stringent exposure limits for higher levels of exposure that may occur over
a shorter time interval.
There are three types of PELs:
TWA
Most permissible exposure limits are based upon an 8-hour time
weighted average (TWA). The air concentration may sometimes go
above the TWA value, as long as the 8-hour average stays below.
(NOTE:
This measurement is not applicable with temperature.
relative humidity, or air velocity sensors.)
Ceiling Limit
The maximum allowable concentration of a chemical that an
employee may be exposed to. It must never be exceeded, even for
an instant.
STEL
The Short Term Exposure Limit (STEL) is the maximum
concentration above the time-weighted average that employees can
be exposed to over a specific time period (usually 15 minutes) no
more than four (4) times per day. This stands for Short Term
Exposure Limit and is the maximum average concentration of a
toxic gas to which an unprotected worker may be exposed over any
fifteen-minute interval during a work period. The EVM calculates the
STEL by compiling fifteen one-minute averages, and updates that
average each minute after the initial fifteen-minute exposure. If the
STEL is reached or exceeded, the alarm activates, and the STEL
enunciator turns on.
Different regulatory agencies have different acronyms for PELs:
NIOSH (National Institute for Occupational Safety and Health) uses
REL (Recommended Exposure Limit), and ACGIH (American
Conference of Governmental Industrial Hygienists) uses TLV
(Threshold Limit Value).
(NOTE:
This measurement is not applicable with temperature.
relative humidity, or air velocity sensors.)
Photodiode
A type of photodetector that is able to convert light source into either current or
voltage. With the EVM, it is used to measure the amount of light scattered from a
particle cloud.
ppm/ppb
Parts per million (or parts per billion) of concentration of the gas of interest in air. For
example, 1 ppm signifies one part of gas to 1 million parts of air.
