INPUT ANALOG CONTROL FROM THERMOCOUPLE OPERATION

Block Operation
A Thermocouple Input Block has three isolated pairs of inputs. Transformers isolate
power and optical couplers provide signal isolation.
For each pair of inputs:
1. After filtering, each signal input is sequentially switched into a common amplifier
whose output is applied to a voltage–to–frequency converter. The output signal
frequency of the VFC is applied to a frequency counter via an optical coupler. The
output frequency is counted for a 400 millisecond gate time, which is a common
multiple of all the common line frequency periods. This provides considerable
rejection of line frequency pickups.
2. The multiplexer intersperses other inputs between the two main thermocouple
input times. The other inputs come from the cold junction sensors and from internal
references. The cold junction inputs are measured and stored for later
compensation of the normal thermocouple input measurement errors.
3. To detect and correct for any gain or offset drift in the amplifier or VFC, the block
takes new readings of factory–calibrated internal reference levels during operation.
These new measurements are compared to reference values stored by the block.
4. The processor converts the cold junction temperature value to a voltage as specified
by the NBS monograph for the thermocouple type in use. This voltage is then
added to the thermocouple measurement before converting to thermal units. Since
there may be some small differences between the cold junction temperature
measurement and the actual cold junction temperature, an offset adjustment can be
entered using a Hand–held Monitor. These offsets are due to variances in the
terminal strip assembly and the correction factors are therefore stored in the
Terminal Assembly EEPROM.

Block Measurement Accuracy

The overall accuracy of the block in a given application depends on both:
1. Accurate measurement of the thermocouple millivolt signal.
2. Accurate compensation of the cold junction connections.
The block can measure the input millivolt signal to an accuracy of " 10mV (typical) or "
20mV (maximum). Depending on the thermocouple type used and the temperatures
being measured, measurement accuracy in C or F can be determined.
The output from a thermocouple varies in a non–linear manner as the temperature
being measured changes. Furthermore, each thermocouple type has a unique
characteristic.
The following table may be used as a guide to estimate accuracy in units of degrees. It
lists optimal accuracy for each thermocouple type, without allowing for conditions that
might be encountered in the application, or for the accuracy of the specific thermocouple
being used. For a more accurate estimate, consult the NBS monograph published for the
thermocouple type used in the application, referencing the temperature range to be
measured.
Thermocouple Type Average Sensitivity: mV/C Optimal Accuracy in C

Cold junction compensation can be performed internally by the block itself, or remotely.
The block has a cold junction sensor which is set at the factory to compensate the cold
junction to within " 2C typically. If a more accurate setting is required, this
compensation can be adjusted using a Hand–held Monitor, after the block is installed