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App Notes

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14.1.1 Thermocouple (T7 Only) [T-Series Datasheet]

Overview - T7 Only

This feature is only supported on the T7.

AIN#_EF_INDEX values:

20: Thermocouple type E
21: 
Thermocouple type J
22: 
Thermocouple type K
23: 
Thermocouple type R
24: 
Thermocouple type T 
25: 
Thermocouple type S
30: 
Thermocouple type C

This Thermocouple Extended Feature automatically performs calculations for the thermocouple types listed above. 

Thermocouple AIN-EF indices read two analog inputs—one AIN connected to a thermocouple and a second AIN connected to a Cold-Junction Compensation (CJC) sensor.

For more information, see the Thermocouples Application Note and the Thermocouples with the T7 App Note.

Configuration

To configure, write to the following registers.

AIN#_EF_CONFIG_A - Options: Select temperature units for AIN#_EF_READ_A and AIN#_EF_READ_C:

  • 0 = K
  • 1 = °C
  • 2 = °F

AIN#_EF_CONFIG_B - CJC Modbus address: The Modbus address of the second AIN channel that will be read to acquire the CJC reading

The default is the on-board temperature sensor (TEMPERATURE_DEVICE_K, at address 60052).

AIN#_EF_CONFIG_D - CJC Slope: A slope to be applied to the CJC reading

This value is always in units of K/volt, regardless of AIN#_EF_CONFIG_A, so for the internal sensor it will nominally be 1.00 and for an LM34 it will be 55.56.

Default is 1.0.

AIN#_EF_CONFIG_E - CJC Offset: An offset to be applied to the CJC reading

This value is always in units of K, regardless of AIN#_EF_CONFIG_A, so for the internal sensor it will nominally be 0.0 and for an LM34 it will be 255.37.

Default is 0.0.

Remarks

The normal analog input settings are used for negative channel, resolution index, settling, and range.

Differential thermocouple readings have the advantage of mitigating bad ground loops and ground offset problems. See the Thermocouples App Note for more information.

The ±0.1 volt range is automatically used if the range of the applicable channel (AIN#_RANGE) is set to the default ±10 volts. Otherwise, the specified range will be used.

CJC calculations are always done in kelvin, regardless of whether AIN#_EF_CONFIG_A is used to change the output units. AIN#_EF_CONFIG_D and AIN#_EF_CONFIG_E should be used as needed to convert the CJC sensor reading to kelvin.

Results

Results are read from the following registers.

AIN#_EF_READ_A: Final calculated temperature of the remote end of the thermocouple
AIN#_EF_READ_B: Measured thermocouple voltage.
AIN#_EF_READ_C: CJC temperature.
AIN#_EF_READ_D: Thermocouple voltage calculated for CJC temperature.

Only reading AIN#_EF_READ_A triggers a new measurement.

Example

Assume a type K thermocouple is connected to AIN3/GND. To configure:

AIN3_EF_INDEX = 22       // feature index for type K thermocouple
AIN3_EF_CONFIG_B = 60052 // Set the CJC source (the address for device
                         // temperature sensor). 60052 is TEMPERATURE_DEVICE_K
AIN3_EF_CONFIG_D = 1.0   // slope for CJC reading
AIN3_EF_CONFIG_E = 0.0   // offset for CJC reading

Read AIN3_EF_READ_A to get the calculated temperature. If the remote end is at room temperature, it will read as approximately 298 kelvin.