I am setting up a design for a small submersible ROV and planning to try cat5 cable to power the various components. I have found the ratings for the cable, however I have a question about water vs heat dissipation. Apparently, I can get approx .5-.7 amps of power transmission safely for the motors, but would I be able to push it a little more to 1amp if the cable is submerged in cold northern lake water? The cable would be 100ft and I have factored in voltage drop due to resistance. Thanks for any advice.
Electronic – Cat5 cable for low voltage underwater application
cablesmaximum-ratings
Related Solutions
While often shielded, CAT5 can be of the UTP type, meaning unshielded twisted pair. The "true" RS-485 cable you link to has two twisted pairs and a shield. If I remember right, CAT5e (and above) has a shield, at least most cables I have seen have - the exact standard can vary. I guess those would do the job just nice. Just make sure you use the twisted pair like in the original RS-485 cable (your data sheet looks like [1 White/Orange Stripe] and [2 Orange/White Stripe] are a twisted pair, and [3 Blue/White Stripe] is a third, untwisted wire.)
There seems to be a small impedance mismatch (100 Ohms for CAT5, 120 Ohms for RS-485). This will cause reflections at the driver and at the receiver, but I am pretty sure your application will still work. While you may read that 120 Ohms is typical for RS-485, the termination network uses 120 Ohms between the differential pair and 2 * 680 Ohms to VCC and GND.
Thus, the value of the termination that the cable "looks into" is smaller than 120 Ohms anyway: (120||(680+680)) Ohms = 110 Ohms.
If you have the chance to test, you could go for it. I would probably not even hesitate and use STP CAT5(e). If your contract says you pay money for every minute your installation fails, you probably want to use cables with the proper specification. (The latter will still not make sure nothing ever fails, but you're probably in a better position if you can blame it on the "true" cable instead of having someone else blaming your "wrong" cable. But you see how this last paragraph has nothing to do with physics...)
You need to use shielded cable for a 2.4GHz signal, regardless of whether it is underwater or not. Co-axial cable will contain the signal's EM field within the cable, making it very efficient at transmitting with minimal losses. It also has a consistent impedance, and this is important because where you have impedance changes you will get reflections that will degrade signal quality (this is of particular concern at your connectors).
If you don't use a co-axial cable then significant amounts of the signal will propagate into the space around the cables. If water encloses these spaces then this will absorb some of that energy, reducing the signal that makes it to the end of the cable.
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Best Answer
Nobody will give you any sort of guarantee.
It may or may not work, you have to try this experiment.
It is hard to say that the heat will not build up in the middle of the cable. There is an isolation between water and wires itself. So it will isolate heat transfer from the wires to the water. Yes, cold water will help with cooling. But will it be enough to keep the wires at acceptable temperature? Hard to say. My guess is yes, it will work. But as I said, no guarantee.