UMAXTC20CO V1.1.1 Preliminary Documentation – May be Subject to Change 7-59
1.2. Thermocouple
Measurements
Figure 1 – Thermocouple Input Block Diagram
The block diagram shown in Figure 1 capture the objects associated with each thermocouple
channel. Each channel, 1 through 4, operates in the same fashion as described below.
Objects $2000
Cold Junction Auto Correct
, $6110
Thermocouple Sensor Type
and $6112
Thermocouple Operating Mode
determine how the scanner processes the raw microvolt reading
and converts it into a temperature value in degrees Celsius, which is written to read-only object
$6100
Thermocouple Input Field Value
.
Objects $6126
Thermocouple Scaling Factor
and $6127
Thermocouple Scaling Offset
are
used to convert the field value to read-only object $7130
Thermocouple Input Process Value
,
which is mapped to TPDO1 by default.
The formula to convert the field value (FV) to process value (PV) is:
Process Value = (Field Value * Scaling Factor) + Scaling Offset
While the FV is a real number, containing the temperature in °C, the PV is a 16-bit integer value.
The default scaling has been selected such that the PV will send the temperature with a resolution
of 0.0625 °C/bit and a offset of -273°C. [Scaling Factor = 16, Offset = 4368] Since the maximum
temperature the scanner can measure for a thermocouple is 1735°C, this means the range of the
PV data will be 0 to 32123 (-273°C to 1735°C.)
Alternatively, it may be desired to send the temperature in Fahrenheit with a 0.1°F resolution per
bit. In this case, the Scaling Factor would be set to 18, and the Offset to 320. Other scaling can be
selected as desired by the user.
