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I believe you a are looking for a traditional wet-bulb and dry-bulb device. These are notorious for being inaccurate as many read to 2 degrees accuracy. Even one degree is not very good. An electronic temperature and relative humidity sensor is better that reads to at least 0.2 degree accuracy.
Note that when we have temperatures above 120 F, the need to know the temperature and humidity is gone. That is, a small error is of no consequence, so we do not need the secondary system but can use the main control system.
The main control system can be checked, if using wet-and dry-bulbs by temporarily making the WB into a DB, and then seeing if it reads the same as the main DB located near the WB.
Note that in a predryer, the air flow is too low to make accurate WB readings. Air flow of 600 rpm is required. Properly working electronic sensors of RH or EMC are needed. A DB sensor of 0.1 F accuracy is best.
This inaccuracy of some sensors is also why we use the dryng rate of the wood to make sure things are working well. Small errors of the main control system will affect the drying rate...large errors make even more difference in rate.
For a supplier of equipment, see Omega.com. It would seem that the model HH311 would be what you need as it has a remote probe and is quite accurate. They do make wireless devices that might be even more easily used, but cost more.
I have seen similar devices used in predryers, but today's control systems for kilns are so good that almost nobody double checks the readings. Even if the readings are checked, the air flow on the WB is low in many cases making the actual reading incorrect. So, an auxiliary fan is a good idea.