Casella CEL Microdust 880 IS real time dust monitor - Manual (Page 19)

Microdust 880nm & WinDust Application Software
Handbook HB3255-03
Page 19 of 51
Time Weighted Average value - TWA
The time weighted average value represents the running average dust concentration since
switch on or a manual reset. The T.W.A value can be reset to zero by pressing the ‘Enter +
Esc’ key combination. It cannot be reset whilst the logger is active.
Offset / Auto-Zero function
At switch on the Microdust will display the prevailing dust concentration as a ‘Present reading’.
This represents the actual particulate concentration being measured by the Microdust.
The ‘Offset Reading ’ function enables automatic zeroing of the instrument at any given dust
concentration. This is ideally suited to applications where a relative dust measurement may be
necessary (for example, on either side of an air conditioning filter), it can also be used to
automatically zero the instrument without mechanical adjustment of the ‘zero’ control.
The dust concentration displayed within the ‘Present reading’ window may be used as a zero
reference level by simultaneously pressing the ‘Enter+Esc’ key combination.
As an example of the offset function:-
Assume the ‘Present reading’ and dust concentration is 9.50 mg/m
3
. Pressing the ‘Enter + Esc’
key combination changes the display to an ‘Offset reading ’ of zero. All future measurements
will now be relative to an ‘Offset value’ of 9.5mg/m
3
. If the dust concentration rises to 12.00
mg/m
3
then the displayed ‘Offset reading’ will be 2.5 mg/ m
3
.
Analogue output / RS232 Socket
An analogue output signal is available for interfacing the Microdust to a chart recorder or external
data logger. Wiring details of the analogue output / RS232 socket are shown below;-