Options For Compensation Of The Reference Junction Temperature - Siemens SIMATIC S7-300 Reference Manual

Compensation of the reference junction temperature
You can compensate for the effects of temperature fluctuations at the reference
junction by means of compensating leads.
There are several options for you to choose from for acquiring the reference
junction temperature in order to obtain an absolute temperature value from the
difference in temperature between the reference junction and measuring point.
You can use internal or external compensation, depending on where you want the
reference junction to be.
Table 4-44 Options for Compensation of the Reference Junction Temperature
Option
No compensation
Internal compensation
(refer to Figure 4-23 for connection)
External compensation with compensating
box in the leads of an individual
thermocouple (refer to Figures 4-24 and
4-25 for its connection)
Only for SM 331; AI 8 x TC:
External compensation with resistance
thermometer for acquisition of the reference
junction temperature
(refer to Figure 4-26 and 4-27for
connection)
Theory of operation of internal compensation
With internal compensation, you can establish the reference point across the
terminals of the analog input modules. In this case, you must run the
compensating lines right up to the analog module. The internal temperature sensor
acquires the temperature of the module and supplies a compensation voltage.
Note that internal compensation is not as accurate as external compensation.
Theory of operation of external compensation with compensating box
If you employ external compensation, the temperature of the reference junction of
the thermocouples is taken into account via a compensating box, for example.
The compensating box contains a bridge circuit calibrated for a definite reference
junction temperature. The reference junction is formed by the connections for the
ends of the thermocouple's compensating leads.
If the actual temperature deviates from the compensating temperature, the
temperature-sensitive bridge resistance changes. This results in a positive or
negative compensating voltage, which is added to the thermo-e.m.f.
Programmable Logic Controllers S7-300 Module Data
A5E00105505-03
Explanation
If you want to acquire only the difference in temperature
between the measuring point and the reference junction.
If you employ internal compensation, the internal
temperature (thermocouple internal comparison) of the
module is used for comparison purposes.
You have already acquired and compensated the
reference junction temperature (thermocouple external
comparison) using a compensating box, which you have
looped into an individual thermocouple.
No further processing is necessary owing to the module.
You can acquire the reference temperature by means of a
resistance thermometer (platinum or nickel) and have it
calculated by the module for any thermocouple.
Analog Modules
4-59