Personal Ozone Monitor Manual Rev. E-2
15
5.10. To Set the Calibration Parameters
The instrument is calibrated at the factory where slope (S) and offset (Z) parameters
are entered into the instrument’s memory. These preset calibration parameters are
given in the instrument’s Birth Certificate and recorded on the calibration sticker on the
bottom of the instrument. However, the calibration parameters may be changed by
the user. Because of noise and/or an inherent offset, some measured values will be
below zero at very low ozone mixing ratios or while zeroing the instrument with an
external scrubber. Also, the instrument zero may drift by a few ppb over time. For
this reason, frequent zeroing of the instrument using an external ozone scrubber to
determine the offset is recommended. Any change in the slope (gain) of the
instrument is likely due to a serious problem such as contamination, an air leak,
obstruction of air flow, or loss of catalytic activity by the internal ozone scrubber, but it
also can be adjusted. Once the zero of the instrument is corrected, the slope may be
adjusted so that the instrument readout agrees with a standard ozone source (such as
the
2B Technologies Model 306 Ozone Calibration Source
) or with the readout from
another instrument whose calibration is considered to be accurate.
To change the calibration parameters, choose the
Cfg
submenu from the main
Menu
and click on
Cal
to obtain the display
Cal Menu
Z= -3 S= 1.01
Here Z is the offset applied in units of ppb (in this case
–3 ppb) and S is the slope
applied (in this case 1.01). The value of Z is added to the measured ozone value, and
the value of S is then multiplied by the measured ozone value. During calibration Z is
set to 0 and S set to 1.00, if the instrument reads an average of 3 ppb with the
external scrubber in place, the value of Z should be set to
–3. If after correction for
the zero, the instrument consistently reads 2% low, the value of S should be set to
1.02. If a calibration is done in units other than ppb, the offset (Z) value must be
converted to ppb before entering the value.
When the
Cal Menu
first appears, the
Z
will be underlined with a cursor. You may
move the cursor to choose the calibration parameter
S
or
Z
. Selecting
S
or
Z
will
select that parameter for change and activate a blinking cursor. Once
S
or
Z
is
selected, its value can be changed by pressing the Move button to increase the value.
The value will roll-over to negative values after reaching the maximum allowable offset
(
–50 to +50 for Z, 0.5 to 1.5 for S). After choosing the desired value, press the curved
arrow to exit stop editing mode. The cursor can then be moved by pressing the right
pointing arrow to the
S
to edit the slope or to the
to exit the submenu.
Once the
values of
Z
and
S
are set, selecting
will return the display to the
Cfg
menu, and
again selecting
will return to the main
Menu
. The calibration parameters reside in
non-volatile memory and are not affected by power failures.
In order to adjust the zero offset, after the instrument has warmed up for at least 20
minutes attach the external ozone scrubber and make measurements for a few
minutes. If the average of those measurements is 4.4 ppb, for example, subtract 4
