15
PART 2. CALIBRATION AND ALARM LEVEL SELECTION
9.
AUTOMATIC CALIBRATION WITH A-CAL
The A-CAL firmware calibrates instruments automatically using calibration gas available
from Bascom-Turner
Calibration Gas
. The gas required for automatic calibration of the catalytic combustion
and CO sensors is Bascom-Turner's methane and CO calibration gas (Part No. MC-620)
containing 2.5
±
0.05% methane and 100 ppm
±
2 ppm CO in air. It is provided in a disposable
aluminum tank containing 620 liters of gas, sufficient for at least 300 calibrations.
The thermal conductivity sensor can be calibrated with pure methane.
The oxygen sensor is calibrated with ambient air whenever the instrument is autozeroed.
Calibration Gas Dispenser.
The calibration gas must be delivered at or near
atmospheric pressure to ensure accurate calibration. Bascom-Turner recommends using the
Calibration Gas Dispenser (Part No. CGD-001) which regulates and displays the delivery
pressure, approximately 6-7 inches of water. The flow or pressure regulator used upstream of the
gas dispenser must be capable of delivering at least 2 liters per minute.
A-CAL Operation.
In carrying out automatic calibrations with the A-CAL firmware,
always calibrate the catalytic combustion and CO sensors first
, that is, use the Bascom-
Turner methane and CO calibration gas first. Then, if necessary, recalibrate the thermal
conductivity sensor. This sequence of calibration gases ensures that no errors are introduced
from residual gas in the detector.
A-CAL Calibration with the Methane and CO Calibration Gas
1.
Place the selector switch in "AUTO ZERO" and zero the instrument in clean air.
The usual series of displays will appear on the monitor display, depending on the
model.
2.
After
"End"
appears on the display and the instrument beeps, insert the tip of the
standard probe partially into the outlet port of the gas dispenser. If a manual
calibration apparatus (for example, Part No. PCA-302) is used, attach the monitor's
water-block filter to the threaded hose barb (TB-512) with the regulator's valve off
and wait for
"bloc".
3.
After
"bloc"
appears on the monitor display, push the probe completely into the
dispenser port. The
"bloc"
display will be replaced by
"go"
and the instrument will
proceed to calibrate the catalytic combustion and CO sensors.
If a manual apparatus is used, open the flow regulator's valve and observe
"go"
on
the display.
4.
The monitor display will show
"GAS 2"
while the catalytic combustion sensor is
being re-calibrated and
"CO"
when the CO sensor is being re-calibrated.
