Zero Gas; Span Gas; Response Factors; Warning - MSA Sirius Mode D'emploi

Zero Gas

Zero gas is a reference gas used during calibration to zero the
instrument. When a zero gas with no hydrocarbon content is introduced
to the monitor, the detector will still respond with a small signal. This
signal is a result of secondary background processes. During
calibration, zero gas is applied to quantify the background ionization
current.
For applications in which you are only interested in concentration
changes relative to a reference ambient environment, fresh air can be
used as the zero gas. When background hydrocarbon vapors are
present, MSA recommends using zero gas air or a carbon filter on the
inlet to zero the unit (See Chapter 8, TABLE 8-1, Accessory Parts
List).

Span Gas

Span gas is a reference gas used during calibration to determine the slope
(response per unit concentration) of the calibrated response curve.
Span Gas Recommendation: MSA strongly recommends use of an MSA
100 ppm isobutylene cylinder for calibration. See Chapter 5,
Calibration for calibration instructions.

Response Factors

When a compound is ionized by a photoionization detector, it yields a
current. This response is a characteristic property of the specific
compound which is influenced by its molecular structure. The slope of
the response curve (defined in picoamperes per ppm) is different for
different chemicals. To properly report the concentration for a given
sample gas, the Sirius Multigas Detector uses response factors. See
Chapter 3, Using the Sirius Multigas Detector - PID Setup, for
instructions on using the pre-programmed list of response factors.
It is very important to have an understanding of PID basics
when changing your PID settings. Failure to properly identi-
fy the VOC gas being measured and/or failure to select the
correct Response Factor alarm values (exposure, STEL,
TWA) that match your desired Response Factor and/or the
correct bulb, will result in erroneous readings that could
lead to serious injury or death.
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