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How is the ordering code "cable length" defined?

The ordering code "cable length" is made up of the ordering code "probe length" and the real cable length.
e.g. your choice was ordering code "probe length" = 200mm and ordering code "cable length" = 5m -> then it results in a real cable length of 5m "cable length" - 200mm "probe length" = 4.8m real cable length

Can the EE45 be supplied with DC?

Yes, the EE45 can be supplied with both AC and DC.

Can the EE45 be operated with a cable length of 30m (100ft)?

The EE45 was tested with cable lengths up to 10m (33ft). But the proper operation with cable lengths of 50m (160ft) and more normally should be no problem.

Can the EE31 transmitters be employed for dew point measurements as well?

That depends on the required accuracy, temperature and dew point temperature. Please observe those diagrams about the accuracies of the calculated variables.