I read the "fine print" on the DO sensors and control board. If you add 1 more inch to the cable length, you will get false readings. They are tuned into the specifications.
Need to be careful with voltage signals 
I am reading that as a prohibition against lengthening, especially with jumper wire, the signal wire length from the probe itself to the small circuit that preforms the actual measurement. I was referring to the length of the serial connection from the small circuit board to the house which should be able to be pushed quite high as a 0-5v serial interface would tolerate at least 1.5 volts of voltage drop. I have a 100ft run that experiences a voltage drop of 5v to 4.9v and works fine. Of course the small sensor circuit could not be more than 30 inches from the probe.
Am I missing something that you saw?
That was it! Most people would like to measure various depths. The cables on my sensors are 5 meters. They make them to 15 meters, but then again, these are current. Current signals, like 4-20ma will go about 1000 ft. I installed a
Flowline LU27 ultrasonic level sensor on a farm pond north of here. We used direct bury wire and ran it 325 feet for the 4-20ma signal.
I have these going on my recirc. tanks:
Flowline DL10 I only have (1) now for playing with. For backup level, I'm gonna toss these on:
IFM-Efector KQ6005 I have a bunch of reed type level switches that I can use for backup, but like the fancier ones.
I usually don't mess around with serial stuff unless it's RS485, which will go about 4000 feet. I can't use any type of comm's or signaling that is not, shall we say, "rated for reliability". RS232 is pushing it at 30 feet. Voltage signals are pushing it at 100 feet. TTL pulse trains may go further, but no one uses that much anymore.
Next spring, I have to install a couple ultrasonic level sensors on an 8600 acre lake. I'll be testing out a couple wireless transmitters that are non-line of site and have a range of 5 miles. It will be interesting to see how well they work. I know tests on one that was inside a factory, the signal was picked up quite well 3 miles away thru an industrial district. A bit on the pricy side, but I ain't paying for it
