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Ask JACUSTOMER-1eysp1u4- Your Own Question
Category: Electrical
Satisfied Customers: 41
Experience:  Journeyman Electrician, Lead Man (Sub-Foreman) at Romanoff Electric
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I have a digital temperature display on a solar hot water

Customer Question

I have a digital temperature display on a solar hot water system. The display was provided with 10K ohm thermistors. I have two thermistors attached to the outflow pipe of the solar array. One sensor is for the solar controller and the other for the display. The sensor and read out are 100ft apart. I used a 4 conductor wire, 18ga with shielding, non-twisted. It is marked as "Alarm Cable." I thought with the long run, and a partial buried run in conduit with coaxial, that this shielded wire would be a good cable. The 4 conductor allowed me to run to both thermistors.When I hook up the digital readout, it indicates temperatures at the extreme high of its range, regardless of the real temperature of the thermistor. This indicates, essentially no resistance. However, my multimeter accurately shows the resistance of the thermistor and is correct for temp. Further experiments on the remaining 400ft of the alarm cable spool actually push the readout beyond it usable range on the high side, but resistance is correct on my multimeter. It is accurate and responsive as I can hold the thermistor and watch temp/resistance change. Similar experiments with 100ft of speaker cable and 200ft of thermostat wire produce similar results. Even in a shorter run, of 25ft, the alarm cable seems to raise the readout temp reading by a degree or two compared to resistance readings.The manufacturer recommends 20ga twisted pair wire for extending the probes up to "several hundred feet." The "Alarm Cable" appears non-twisted. Is this the issue, the linear non-twisted alarm cable is producing a weird reading just for the display? Or is the shielding a problem? It seems I have eliminated the shielding by trying other types of wire. The manufacturer did tell me thermostat wire does weird things for some reason. I can't understand why the resistance readings are correct, but the readout is effected somehow.What should I try next, two runs of 20ga twisted, non-shielded, or shielded. Can I get a 4 conductor cable with two twisted pairs, shielded, or unshielded.Thanks.
Submitted: 10 months ago.
Category: Electrical
Expert:  Rico Soma replied 10 months ago.

Welcome to Just Answer, my name is ***** ***** I will do my best to help you with your issue. If my initial response doesn’t answer your question then let me know and we can continue our conversation.

I don't understand your readings either since we routinely use 18 ga solid (not stranded not shielded) cable and have never had an issue. Since the manufacturer recommends 20 ga stranded pair then you should go with that. 20 gauge stranded is used for speaker wire. However it and just about any 20 ga wire is not UV rated so it will break down in sunlight. So run it so it's not in the sun

Customer: replied 10 months ago.
Rick, thanks for the response. I can hide the wire, I have it all tidy in conduit. I run underground parallel to some coax, as I mounted my mini TV dish on the solar structure and not my house. It is a ground mount solar array for heating hot water. I don't understand the readings either. This is the second display, I thought perhaps the first one was damaged, or I damaged it, but I get the same exact readings. I guess seeing the temps is not all that important, it was more out of curiosity and tracking performance, but I want to know that the pump controller is getting the proper inputs from its thermistor. It also shows the correct resistance. It seems the non-twisted wire, despite showing the proper resistance, is getting an odd signal feeding back to the display.
Expert:  Rico Soma replied 10 months ago.

I can't address the display readout anomaly. After you install the type of wire the manufacturer recommends, if you still have a problem then it sounds like a warranty issue that you need to take up with them. We use Resol controllers and haven't had any problems with them.

Customer: replied 10 months ago.
I will try some other wire. Can we forward this question to an electrical engineer somehow, seems like it is a conductance issue in the wire somehow.
Expert:  Rico Soma replied 10 months ago.

We don't have an electrical engineering category only an electrical category where you'll get an electrician not an engineer. I can ask the mods to move it to electrical if you like.

Customer: replied 10 months ago.
lets get there take on it, I'm just perplexed it doesn't work.
Expert:  Rico Soma replied 10 months ago.

Ok, please be patient it may take a while. Please don't respond until one of us gets back to you.

Expert:  JACUSTOMER-1eysp1u4- replied 10 months ago.

Hello, Welcome to JA/Pearl my name is***** and I will try to assist you today.

I have the better part of 20 years as a commercial electrician and about a year of electronics engineering classes under my belt, and I 'll try to give you some insight with what I know of low voltage installations such as the one you have.

The likely cause of poor sensor operation is in most cases EMI noise on the line either end of the circuit could be contributing to it.( possibly the sensors themselves interfering with each other) Generally, in commercial buildings, to remove any form of EMI from the equation engineers spec twisted pair cable or twisted pair shielded cable to be installed properly by the contractor ( properly grounding the shield in one location as well as continuing the shield throughout the entire installation.

So to answer which is the best for your problem I would run the recommended #AWG from the manufacturer in a twisted / shielded solid conductor for a permanent installation or stranded if you feel the solid cable could become damaged. Multiple low voltage wire manufacturers on the market like Belden brand although they can be very expensive. I would definitely shop around.

Let me know if that answers you problem, this thread will remain open for some time, Please do not forget to rate me if you are satisfied with my answer. Thank you for using JA/Pearl , and have a wonderful day.

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