Operation with an external cold junction

The EL331x supports operation with an internal cold junction as standard. This means that the thermocouple is attached to the terminal points at the front of the terminal housing, so that the material transition and the cold junction are located at the front of the terminal housing. The terminal measures the cold junction temperature with its own internal temperature sensor and calculates the desired measuring point temperature value.

In special applications, operation with an external cold junction is required. The external cold junction is connected to the EL331x with a normal copper connection cable, the material transition then takes place in the external connection point. This is supported by the EL331x-xxxx.

Operation with an external cold junction 1: — External cold junction

For this operation the following must be set

all CJCompensation PDO of the terminal must be activated, even if the "external cold junction" function is only used on isolated channels

Operation with an external cold junction 2: — Activation of all CJ-PDO via Predefined PDO

In CoE 0x80n0:0C of the desired channel, the external cold junction calculation must be activated by the value "2" (external process data).

Operation with an external cold junction 3: — Setting the cold junction billing in the CoE

The cold junction temperature T_v must now be recorded by a separate temperature sensor at the cold junction and fed to the terminal via the fieldbus master and the fieldbus as a linked variable ("external") (see Fig. External cold junction).

The separate measurement can technically be done via another thermocouple connected to an EL331x, or an RTD element connected to an EL32xx, or any other temperature measurement whose value is known to the controller.

The EL331x then supplies the measured value Value, taking into account the temperature value supplied with CJCompensation:

Operation with an external cold junction 4: — Display of the "Value" in the TwinCAT tree

The comparison data is written to CoE 0x70n0:11.

	Alternative to cold junction measurement As an alternative to the procedure described above, the cold junction can be maintained at a defined temperature through ice water (0° C), for example. In this case, the temperature is known without measurement of the cold junction temperature (Fig. External cold junction) and can be reported to the EL33xx via the process data.

	EL3314-0002, EL3314-0010 The EL3314-0002 since FW01 and the EL3314-0010 since FW03 does also support the external reference point measurement. The user have to ensure, that an application of external cold junction measurement have no negative effect to the measurement accuracy.