18
3.
CALIBRATION
Calibration must be carried out on a new sensor the next time the instrument is turned on;
with an already installed sensor, the calibration should be carried out at specified intervals. The A-
CAL firmware calibrates the Gas-Explorer using calibration gas available from Bascom-Turner
containing 2.5 + 0.05% methane and 100 ppm + 2 ppm CO in air. Calibration gas is provided in a
disposable tank containing either 105 liters of gas (Part No. MC-105) or 620 liters (part No. MC-
620). The thermal conductivity sensor is automatically calibrated for 100% methane during the
autozero routine preceding calibration with the One-Gas calibration algorithm. It can be also
calibrated with pure methane or system gas by using the Two-Gas calibration. The oxygen sensor
is calibrated with ambient air during power-up.
The calibration gas must be delivered at or near atmospheric pressure to ensure accurate
calibration. For the 105 liter gas tanks, Bascom-Turner recommends using a constant flow
regulator (Part No. FR-001) set to the sampling rate of the Gas-Explorer. For the 620 liter gas
tanks, Bascom-Turner recommends using an on-demand regulator (Part No. FR-401), which
automatically adjusts its flow to the pumping rate of the Gas-Explorer.
A-CAL Calibration.
The A-CAL procedure guides the user through the calibration
procedure and instructs when to supply the required gases. After the Autozero is successful, the
user can select a One-Gas calibration which only uses the 2.5% Methane with 100 ppm CO or the
Two-Gas calibration which uses both the 2.5% Methane with 100 ppm CO and Pure Methane or
System Gas. The Two-Gas procedure should be utilized when it is necessary for the System Gas
to report as 100% Gas such as when purging pipes.
When the gases have been connected, the screen displays “
Calibrating
” for about a minute
and then changes to “
Calibration OK
” with a display of readings taken while sampling calibration
gas or “
Does Not Calibrate
” if calibration was unsuccessful. Three tries are allowed before the
screen displays “
Please Service
” for 15 seconds and then the instrument turns OFF.
Following calibration, an intermediate screen displays the sensitivity in µw/ppm and
mw/1%. The first quantity (µw/ppm) refers to the Track Gas mode, the second (mw/1%) to the
Monitor mode. If the sensitivity of the Track Gas mode falls below 0.5 µw/ppm, the gas sensor
should be changed. Similarly if the sensitivity in the Monitor mode falls below 8 mw/1%, the gas
sensor should be changed. In both cases, the display reads
“Please Change Methane Sensor”
and
the instrument is not operable until a new sensor is installed.
Calibration of the H
2
S sensor.
Proceed as above for pure methane, but use the H
2
S
calibration gas (10ppm H
2
S in methane). The instrument will automatically calibrate the thermal
conductivity sensor as well as the H
2
S sensor.
CAUTION:
Automatic calibration presupposes and depends on using Bascom-Turner’s
calibration gas (Part No. MC-105 or MC-620). Do not use a gas of a different
composition for automatic calibration.
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