PTC probe extension , considerations

[xsherlock]

Hi ALL,

I need to run a longer cable from the PTC bricklet to the the PT100 probe (the one from your shop).

I used 2 x1mm^2 sensor cable  to extend the 1m that comes with the probe. 7m long.   

Do you think that can affect the results much?

I have some concerns , as that is an important readout I have 2 sensors in exactly the same location, glued together with a thermal glue into one blob attached to the pipe.

They are about 1-1.1 degree apart, and output looks a bit noisy.  (I read them every 2 seconds)

What do you think?

[batti]

Hi,

as far as I understand your description you use a 2-wire probe, right? For a 2-wire probe there is no wire resistance compensation. One possibility would be to connect the 2-wire probe with 4-wires, such that you can compensate for the wire resistance.

Regarding noise a longer cable might induce more noise. In this case a 4-wire connection should also improve the situation.

[xsherlock]

Yes I you your probe from the shop that is 2 wire.

How do I connect  a 2 wire PT100 with 4 wires?

TIA

[batti]

A PT100 probe is a 2-connector probe. When you use a 4-wire connection you connect two wires to each connector as close as possible to the probe.

[xsherlock]

Ok so I rewired the setup and now 2 sensors (Output 1, Output2)  are 4 wire (7m extension).  3 named Noel are old setup (2 wire 7m extension).

And for comparison I added Poiol skimmer to the graph that is same PTC probe on 2 wire directly to PTC bricklet  on the same RPI HAT. 

As you can see the extension is the problem , adding 2 wires did not improve the noise. Probe directly into the bricklet is super clean.

What else could be a problem here?

TIA

 

bearbeitet January 13, 2021 at 20:19 von xsherlock

[batti]

Hi Sherlock,

is it possible that you send me a picture of your extension? The connection at the sensor and at the Bricklet is what I like to see.

I'm pretty sure that you have done that right, but sometimes it is much easier to see it on a picture that to explain it by words.

You have configured the PTC Bricklet for 4-wires, right?

[xsherlock]

Hi,

I have spent couple more days on testing the setup  and rewiring and unfortunately still experience the problems. So I will share all the details and maybe with your external eye you will see something unusual in this one.

I have changed a 4 x 0.75mm2  cable to one of the sensors  and replaced it with shielded CAT6 ETH cable using 2 twisted pairs as single cable. I have also run that cable in a separate duct pipe on the celling away from all the power cable to the electrical cabinet.

I have eliminated the WAGO style connector at the sensor end of the cable, and soldered the sensor cable to the extension cables. That did not have much of the influence on the output jitter.

Here is a main view on my Tinkerforge automation cabinet.

There are 2 RPI's with LARGE HATS that are my sensor hubs gathering all the data from bricklets with a python script service They are powered with 13.8V from the MeanWell PS  that has battery backup. There are also 2 RPI's on the top in enclosures, that have OpenHab and Influx/Grafana completeing the setup.

You can see in bottom left, a PTC5 proble connected the RPI 1 - that one is perfectly stable. Connected here for reference.

 

Here are some of the connection closeups on the sensor cables that are the original one and the replaced cable for PTC6 

All tightend well. The PTC 4 and 6 are set in the sooftware to the ptc6.set_wire_mode(4)

On the sensor side. There is just a junction box with now soldered cables, as I removed the WAGO connectors already

 

 

I focused my repair eforts on the pool output sensors PTC4 and PTC6 as those feed  data to the 0-10V cotroled 3way Valve for precise temp control.

The PTC1-3 in the NOEL heat storage tank are still extended with just 2 wire 0.75mm2 cable. 

So for the last day I see the following results.

 

As you can see the PTC5 that is on 2 wires and directly to the brick is rock SOLID stable. One of the NOEL 2 wire long cable sensor is much better then other2 (Why?)

Both PTC4 and 6 have huge problems with jitter and some additional spikes, that  seem to corelate to the power use of the whole setup (that is measured by the isolated VC 2.0 Bricklet, that very much can also have a problem of accuracy) 

Basically that is a closed underground technical cavern and such a sudden changes in temp are completely impossible.

No other electrical devices (like heatpums or pool pumps are switched on at that time) 

I'm running out of ideas what can I try to fix that.

My last resort would be tring to add isolator bricklets to the PTC (makes any sense)?

Adding RPI  in the juncion box to measure The PTCs on a short cable (major rebuild).

TIA for any sugestions 

Maciej 

[batti]

Hi Maciej,

I'm running a little bit out of ideas, too. Sorry. I have a few things more you can test:

• Can you connect the shield of the CAT6 cable with ground? Maybe that makes a difference
• Can you swap the sensors between the PTC Bricklets? I would assume that the noisy sensors will also be swapped. I want to only get sure that the problem is the sensor not the Bricklet
• How long is the cable from the PTC sensor to the point where you make it 4-wire? It seems that there is quite a long cable. I don't know if it is possible but maybe you can replace one PTC sensor with a real 4-wire PTC sensor (then the 4-wires are directly connected to the PTC probe).

[xsherlock]

I SOLVED THE PROBLEM! It Must be something with grounding as I added an isolator bricklet for the PTC and it went away in second.

I'm still not sure exactly why  but the hypothesis is that  all the noisy sensors are glued to the pipes with conductive termal paste. And that causes some grounding loop.

The PTC5 that was only one clean sensor was a test one, in a loop in the cabinet, It was actually the only one that metal tip of the sensor was not touching the grounded metal pipes.

Ordering the remaining 4 isolator now to put it to final test.

Anyway, thank you for assisting me in solving it.

 

bearbeitet January 22, 2021 at 12:58 von xsherlock

[Superp]

Useful knowledge, filed. I always thought isolators were there just to protect your system; turns out in some circumstances their effect goes beyond that.

Thanks for digging this deep, and finding a solution.