XTC601 User’s Manual
Michell Instruments
23
OPERATION
2.7.6 External Compensation Page (standard version only)
COMP 20%
COMP 40%
COMP 60%
COMP 80%
COMP 100%
0.50-2.00
0.50-2.00
0.50-2.00
0.50-2.00
0.50-2.00
Figure 25
External Compensation page
A 4–20mA sensor may be used to compensate the % Target Gas reading for the effects
of process variables such as line pressure, flow, etc. The table of compensation factors
may be edited for 5 points along the compensation sensor range. The values would be
determined by applying the process variable at each point and noting the effect on the
% of Target Gas.
For example: a compensation is needed for line pressure. A 4–20mA line pressure
sensor would be ranged over the compensation range. While the instrument reads a
fixed % Target Gas
value, a table is created (see example below) while varying the
pressure at 20% of range intervals.
At each point the compensation factor can be set between 0.5 and 2.
Pressure
% of
Pressure
span
TG
reading
Effect =
(affected value
/ non affected
value)
Compensation
factor = 1 /
effect
0
0%
20.91
20.91/20.91=1.00
1.00
1
20%
21.65
21.65/20.91=1.04
0.96
2
40%
23.56
1.13
0.88
3
60%
25.99
1.24
0.81
4
80%
29.66
1.42
0.70
5
100%
38.85
1.86
0.54
Table 10
Compensation
The compensation factor values are then entered into the External Compensation table
(excluding the 0% point as this will always be assumed to be 1 = no effect).
Below 0% (< 4mA), the compensation factor is fixed to 1. Above 100% the compensation
factor is extrapolated beyond the last factor.
