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378 posts

Ultimate Geek
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Topic # 201718 31-Aug-2016 13:39
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Hi all,

 

what are your thoughts regarding the following: I have a solar panel (evacuated tubes), the controller is installed indoors on the hot water cylinder cupboard wall ( visible all the time from the hallway, so I know instantly if there is water for a shower or not...).

 

as you know, the controller requires (at least) two temperature sensors, for the HWC and for the solar panel header. My issue is that I noticed some erratic behavior which occurs quite often. Everything is fine in the morning until the controller starts the pump. After that, the panel temp shows that it either keeps RAPIDLY going up (99C is the max temp on display) or drops after the pump starts (as expected), but the temp does not drop enough for the pump to stop. It is set to start the pump on dT of 15C and to stop it at dT of 4C (dT between the HWC and the water in the solar collector). Because the temp reading for the panel does not drop, the pump keeps going continuously. The temp rises to ((C in a matter of 15-20 sec, I am absolutely sure this is an error, the panle does not heat the water in the collector that fast.

 

I bet this is because the approx 12-13m run of cable from the controller to the outdoor temp sensor is done with ordinary wire like the telephone wiring, twisted red and white, silvery looking inside (solid, not mult-strand). It appears to me there is interference from the power wiring serving the circulation pump, wiring which runs fairly close to the temp sensor wiring - not really near it, but approx 200mm.

 

I am looking at replacing the temp sensor wiring with a screened cable (which should have been used from the beginning). How good results can I expect if I use a 15m USB cable (off TradeMe) or a 15m "2RCA-to-2RCA cable"? These are definitely screened, so it should do the trick... (cut the original connectors). The RCA cable is around $9 and the USB is approx $15 (for the 15m long cables).

 

Cat5, CAT6 - these are all screened, and fairly inexpensive ($8 for 15m on TradeMe)- I guess they would be a good candidate as well?

 

The higher the temp to the sensor, the higher the sensor's resistance (I believe it is a PT100 probe?), so if the temp goes up that is also consistent with a faulty contact (which would add more resistance to the circuit which would appear as higher temp recorded). I have checked the contacts and appear to be in good condition, they are not loose or corroded.

 

Many thanks in advance for your comments.





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1597 posts

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  Reply # 1620283 31-Aug-2016 15:05
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If it's proper PT100 it should be 3 core, the 3rd core been for cable resistance compensation, the cheaper sensors on use 2 core with no length compensation

 

If you want to do it right then get some proper cable, like b5102

 

 

 

 


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  Reply # 1620305 31-Aug-2016 15:46
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Dont count on trademe RCA cables being screened either.





Richard rich.ms



378 posts

Ultimate Geek
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  Reply # 1620309 31-Aug-2016 16:00
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the temp sensor is definitely 2 cores only, so maybe it is not PT100, it is 10k probe?

 

the screened cable mentioned above looks the real deal but I could not find other reference on internet other than the catalogues from the manufacturer. I will approach an electrical distributor and inquire about price.

 

it looks like the ethernet cable remains the strongest competitor?


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  Reply # 1620311 31-Aug-2016 16:04
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Common Ethernet (UTP) is not shielded either.  FTP or SFTP are the shielded ones.


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  Reply # 1620315 31-Aug-2016 16:09
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Also, I'm no expert with this stuff but just putting it out there for others to comment...  Wouldn't it need to be something more like a ground loop or something to create an offset to a DC sensor circuit.  If the interference was from the nearby AC cable it would just make the temp signal "noisy" wouldn't it?

 

If its twisted pair already that should take care of most induction etc?


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  Reply # 1620459 31-Aug-2016 22:10
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 Sounds like you have inappropriate cable at the moment.  Take the suggestion from @gregmcc and use some decent cable. Before that, you can diagnose some of your trouble by bringing the sensor inside and temporarily connect it directly to the controller. That should eliminate cross-coupling as the trouble.

 

You can also simulate your sensor with an appropriate value resistor. Heat / freeze spray the resistor to get some temperature change out of it - cheap resistors move further. Use a 10 turn potentiometer if a fixed resistor is no good.

 

See how you go


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  Reply # 1620471 31-Aug-2016 22:34
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Are you 100% sure it's PT100 (chances are that it is), PT100 can be 2 or 3 cores, like I said before if it's 2 core there is no cable length compensation, if you goes to screened cable, one end of the screen needs to be earthed for it to work correctly.

 

 

 

As per the previous poster simulate the PT100 with resistors or a POT, there are charts on line that give you the resistance for temperature for PT100 0 deg C = 100ohms

 

 

 

 


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  Reply # 1620486 31-Aug-2016 23:36
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 Is your solar system direct circulating or indirect? As if indirect you will need a higher dT cutoff temp. As it will take awhile for the primary circuit temp to equalise with the cylinder temp. And check the panel sensor location. Often they are in the outlet pipe and not directly in the panel. This causes them to read low when the pump is off. When the pump starts, the superheated water in the panel hits the sensor causing it to read really high. A good solar panel will easily get to 160deg if you let it.

 

Have also seen an installation where the cylinder temp sensor was located too low on the cylinder - below the level heated by the solar - As soon as the solar had heated the cylinder more than the dT cutoff temp - the pump ran continuously as the warmer cylinder temp was not getting sensed.

 

I also get a temp spike when the pump starts on my own solar panels - goes approx 10deg higher than the start temp. Will be more in reality due to the response time of the sensor.






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