Can I use my FU20 or FU24 pH sensor in a Tank by just dropping it in the tank?

The short answer to this question is NO.

But, there is a long answer that will give you another option.

Yokogawa has created a Submersible Sensor Kit (part number M1289XT) for the FU20 pH sensors - M1289XU for the FU24 sensor). It is also known as the “Dangler.” 

 

It consists of a short enclosed section with threads that fit the threads on the back of the FU20/24 sensor. The enclosed section is large enough to contain the strain relief on the standard FU20/24 or the connector on the Variopen version. There is a compression fitting that the cable passes through that is designed to make a watertight seal with the cable. Once installed in the Dangler you CAN simply drop the FU20/24 into your tank or open channel.