Micromod MOD: 30ML and Modcell Maintenance for 2001N, 2002N, and 1800R - Manual (Page 133)

Temperature values are not
accurate with B type T/C if CJC
is used in MOD 30ML/Modcell
MOD 30ML and Modcell have
Analog Output Module Error
Problem Description:
The configuration involves a Thermocouple input on the MOD 30ML/Modcell. The controller is configured
with a TIM module/ AIN block configured as T/C and TI block with CJC. The TIM/AIN block is configured
as a B type thermocouple. The compensation works properly at some temperatures, but not at others.
This problem is seen only with the B type thermocouple configuration. The type of CJC selected (internal,
built-in or modular) does not have any effect on this.
Applies to:
MOD 30ML/Modcell configured from Application Builder software or Visual Application Designer ViZapp
Software. This also applies to MOD 30ML configured from front face.
Explanation:
It appears that the compensation for a Type B thermocouple is not working, due to an incomplete table in
the instrument's firmware. The table that converts from millivolts to degrees and the reverse, has values
for the range of 200 to 1820 degrees C. This range may be fine for converting a process signal from
millivolts to degrees C.
The problem appears when the instrument tries to inverse calculate a millivolt value from room
temperature for compensation. There are no table values to support this low value, and an invalid value is
produced.
With a constant temperature at the terminal block, the number of degrees of compensation varies,
depending on where the process temperature is on thermocouple's curve. It is not simply a matter of
adding the reference temperature to the calculated thermocouple temperature.
It is noted that even though the range of a Type B thermocouple is 0 to 1700 degrees C, its useful range
begins above 800 degrees C.
Because its linearization curve is nearly flat below this point, there are no accuracy specifications below
that point.
Topic: MOD 30ML/Modcell Technote: TNML0501-1
MicroMod Automation, Inc. Rev. May-2001
B Type TC Problem.Doc Page 1 of 6