Thermo Scientific 48iQ Instruction Manual page 105

Operation
Settings
is active, the range mode selection status output is at 0 volts. When the
high range is active, the range mode selection status output is at half of
full-scale.
When the high range is active, the concentration must drop to 95% of
the low CO range for the low range to become active.
In addition, each CO range and analog output has a span coefficient so
that each range can be calibrated separately. This is especially necessary
if the two ranges are not close to one another. For example, the low
CO range is set to 0–50 ppm and the high CO range is set to 0–10000
ppm.
Note When using auto range mode, the high and low ranges should
not vary by more than 1 order of magnitude. For instance, if the low
range is set to 20 ppm, then the high range should be set to no more
than 200 ppm. This is because the concentration response is non-linear
and it is possible that the low range readings could become
unpredictable above the top of the low range, resulting in a substantial
jump in concentration readings when switching between ranges.
One possible fix for this would be to perform the multipoint (3-point)
calibration on both ranges and make sure that the low range high point
matches or is greater than the high range low point. This will ensure
that the high and low range concentration curves will overlap. If both
of these methods are unacceptable due to local regulations, then the
dual range mode should be used so that the range selection may be
evaluated by the user to meet their regulations. ▲
Thermo Scientific 48iQ Instruction Manual 3-77