Operating The Stap - BRECHTEL STAP 9406 Instrument Manual

7

Operating the STAP

7 Operating the STAP
The STAP measures the light attenuation by absorbing particles deposited on the sample
filter and compares the attenuation to that measured simultaneously through a 'refer-
ence' filter on which no particles are deposited. From the change in measured atten-
uation with time the light absorption coefficient may be determined. The STAP is an
extremely sensitive measurement device, capable of signal-to-noise ratios of 1 when
sampling only 20-40 ng/cubic meter absorbing mass concentrations.
Practicing good electrostatic discharge (ESD) procedures and understanding how to change
and initialize filters in the unit are two key aspects of properly operating the STAP.
There are three ways to interact with the STAP, the mini front panel, serially via a RS232
port, and through the STAP UAV Reader GUI program which runs on windows computers
and connects to the mini USB port. The STAP can also store its readings to an on-board
micro SD card.
Caution: Each time the STAP is powered up the filter state MUST be de-
fined by the user. Either a new initialization process is required if new
filters have been installed or the previous initialization results can be
used if the filters have not been changed.
Caution: It is NOT recommended to operate the unit at light attenua-
tion levels lower than 0.7 (a 30% reduction of intensity from clean fil-
ter state) as the uncertainties due to filter loading corrections become
greater.
Absorption measurements are made by measuring the rate at which the sample filter
gets darker as sampled aerosol are deposited on it. The sample flow flows through the
sample filter and then flows through a reference filter. The reference filter allows the
measurement to be done as the rate of darkening of the sample filter compared to the
reference filter. This ratiometric measurement makes the readings much more robust
and tolerant to changes in LED brightness.
The STAP uses a 20 bit A/D converter to read the sample and reference photodiodes. This
means the raw sample and reference readings range from 0 to 1,048,575. The tricolor
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