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-   -   PID PV Temp Jumping all over (http://www.homebrewtalk.com/f170/pid-pv-temp-jumping-all-over-369939/)

bmick 11-25-2012 05:38 PM

PID PV Temp Jumping all over
 
Hi All,

I recently build a eHERMS control box, and used it flawlessly once. Now, every time the probe goes into the mash OR the HLT (I put it in there to get the HLT to temp for the HEX before the mash), the PV temp jumps everywhere (145, 156, 130, 142, all within 5 seconds). Strangely, when I remove it from the mash and put it into a cup of water, the temp stabilizes, accurately, in seconds. What gives? It's rendering the panel essentially useless, and I'm pretty much out of ideas. All I can think of is maybe the HLT and mash are electrically charged somehow? Any help is appreciated.

way2fastxl1200c 11-25-2012 05:42 PM

is your line/sensor near an induction motor thats running when its in those vessels?
Motors like pumps and solenoids can cause interference in theromocouples and thermistor lines.

bmick 11-25-2012 05:55 PM

1 Attachment(s)
I have a pump hooked up to the panel, but it wasn't running at the time and isn't too close to it. Pic below of the innards of the panel (rats nest, I know), but from left to right it's the SSR, PID, Switches, and outlet. I don't know that it's anything in the panel, since it works perfectly in water.

ajdelange 11-25-2012 08:15 PM

Could be any of several things. Is this an RTD, thermo couple or thermister?

Check that all connections are sound i.e. check crimps, soldering and especially connectors if you have them. Loosen screws under any spade lugs and retighten firmly after making sure spades are bright and shiny. Make sure pins on connectors are shiny. How does the probe 'go into' the mash/hlt? Is in inserted somewhere manually or does it screw into a thermowell? If the former try grounding it to the pot i.e. holding it against the side of the pot so that its metal parts make good contact with the pot. Check the ground wires to the pot(s). Make sure all other electrical connections are sound i.e. that there is no arcing at a heater wire connection.

bmick 11-25-2012 09:26 PM

1 Attachment(s)
This is a "Premium Stainless steel waterproof PT100 RTD thermistor sensor probe", or this thing http://www.amazon.com/gp/product/B005OUJTZI/ref=oh_details_o04_s00_i00. I just put it into the MLT at the top for now, holding it so the probe is about 2" in (not submerged) where the wort returns from the HEX. I tried the wort in a few different containers just as an experiment, and it jumped around in a few other pots, but again, ran normally in water. Could the wort become charged from improper element grounding?

The element in the MLT (that the HEX is in) is coming through the bottom of a rubbermaid cooler, with an outlet box and grounded to the outlet box, below.

ajdelange 11-25-2012 10:14 PM

First off, it's a RTD. There is no such thing as a RTD thermister. I deduce that it's a RTD from the PT100 (standard description of a RTD) and that it has 3 wires (common configuration though some have 4 and some 2). A RTD is a device whose resistance changes with temperature. It is fed with a current source and that current is sensed in a circuit in the controller which converts the current to a voltage. Erratic readings can be caused by 4 things
1. The temperature is jumping around erratically
2. The resistance of parts of the loop other than the RTD is changing erratically
3. The current from the current source is changing erratically
4. 'Noise' induced from somewhere outside the circuit is getting to the voltage sensing part of the circuit

RE # 1. I assume that 'HEX' means 'heat exchanger' which incorporates a heater. If you are holding the probe in a stream of fluid coming from a heat exchanger and the flow is not uniform or the heat supplied to the heater element is erratic I suppose that you could get variations in temperature but 26 in 5 seconds seems a lot. Never the less I'd check the voltage at the heater terminals with the controller set to manual 100% (and fluid present).

RE #2. This is, IMO, the most likely explanation. The picture shows crimped spade lugs. Crimps often loosen and it doesn't take much of a resistance change to correspond to 10's of degrees of reported temperature change. I would redo the crimps and clean the lugs.


RE #3: This would be a failure in the controller. As things are normal when in water this can't be the explanation.

RE #4: Shut everything else in the room off. If noise goes away try turning things back on one at a time and moving the probe wiring around near where it is positioned in normal use. If moving nearer to or farther away from a piece of equipment increases or decreases the stability then that piece of equipment could be coupling noise into the lead wires and from them the noise could be coupled into downstream parts of the controller circuit (amplifier, A/D converter). If the controller reads steady when the probe is held over the surface of the wort but becomes erratic when the tip of the probe is inserted into the wort then I'd guess there's a leakage path from the heater circuit through the wort and RTD wires. Normally the vessels are metal and leakage current would be shunted to ground (tripping the GFCI in the process and warning the brewer that something was amiss). If this is what you mean by 'charging' the wort then it is a possibility.

bmick 11-25-2012 10:56 PM

AJ, thanks so much for the extensive answers. After looking into these, and doing some experimenting, it seems that everything will run normally until I plug a heating element into the control panel. I did the following:

- Run the system as normal with water, place the probe in, recirculate through the HEX with the pump. Temps were steady, recirculating fine, and the water temp was below the set temp, so the SSR should be switched on and allowing current to the plug.

- Plug the heatstick in (I did NOT use the element in the cooler, so I could see where the problem was first), and everything went haywire immediately.

So the conclusion I'm drawing here (based on my limited experience), is that when the element is drawing electricity through the SSR, that's what's causing the issue. Could that be, could it be that the SSR is too close to the controls and that's throwing it off?

bmick 11-25-2012 11:58 PM

EDIT: Well that can't be it, it works fine as long as the heatstick is in the HLT and the probe is in a separate glass of water, so it's no the SSR drawing too close or anything.


AHHHHHHHHHH!!!!!!!! (brewing rage)

ajdelange 11-26-2012 12:06 AM

Sounds as if you are zeroing in.

I'd suggest jumpering around the SSR (i.e. reconnect the lead that goes from the SSR to the heater to the terminal that brings the juice to the SSR). If this stops the instability that suggests the SSR itself though I can't off the top of my head see how the SSR could do this. If you had a scope available I'd suggest looking for noise spikes on the wiring to and from the SSR. Short of that you could try replacing the SSR if you have a spare (and you probably should).

Problems like this can be tough to run down. It's good practice to install EMI filters in circuits like this. Sometimes you need them - sometimes you don't.

[EDIT] We have to keep in mind here that it did work and now it doesn't. That suggests that something has changed (failed) such as the SSR or that something has come loose.
Yes, proximity can be enough to create problems like this but it is more probable that the 'noise' is being coupled through the wiring. The EMI filters take care of that but if a way to arrange the wiring can be found to isolate things they aren't necessary (and they do cost money).

mattd2 11-26-2012 08:53 AM

Quote:

Originally Posted by ajdelange (Post 4620642)
...but if a way to arrange the wiring can be found to isolate things they aren't necessary (and they do cost money).

On that note, how are the cables run from the control box to the kettle. Do the Sensor and power cables run close together for any length as this can be a cause of EMI. This would also be a reason why it goes away when the probe is in a different pot (i.e. the cables would no longer be running next to each other).


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