adccommunitymod (AutomationDirect) asked a question.
Created Date: July 09,2011
Created By: doncham
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I use a DL-06 and C-More EA7 to monitor and control my koi pond. I'm attempting to monitor air and water temp, pH and ORP. I measure air and water temp with type J termocouples connected directly to a thermocouple module. Air temp works perfectly. Water temp jumps all over the place. The pH and ORP probes are connected ph/OPR transmitters that provide a 4-20 ma signal to the analog module. I have a 24 v power supply powering these transmitters. ORP has worked perfectly for over a year. pH, just like the water temp, jumps all over the place. I'm considering changing the water temp sensor to an RTD. Since Automation Direct now has a line of Signal Conditioners, I'm wondering if the following setup would provide reliable 4-20 ma signals to the PLC. RTD1-T06-L06-07 RTD connected to 884114 Signal Conditioner My existing ORP probe connected to 884114 Signal Conditioner My existing pH probe connected to 884114 Signal Conditioner From the guides on the site it appears these sorts of conversions are envisioned. I'd like to hear from some one with experiece with these units before blowing more money and yet another approach.
Created Date: July 09,2011
Created by: Do-more PE
Your jumping around is likely due to a ground differential between the pond and the PLC. Or possibly you are generating a small current into the pond via a pump or other electrical device that is in contact with the water.
Drop one probe of a multimeter in the water and measure against the ground terminal of the PLC on both AC and DC settings and let us know what you measure.
Created Date: July 12,2011
Created by: doncham
Voltage Measurements
I took the requested measurements. On 2000 mV scale measures 002 mV when leads hang free. In water to PLC ground measures 211 mv.
On 200 VAC scale measures 0 when leads hang free. In water measures .1 V.
Created Date: July 12,2011
Created by: Do-more PE
I should have asked if there are lights or a pump or other devices that use AC in the pond or connected to to the pond by pipes and if so was it running when you took the measurements?
How long are your wires to the pond vs the air?
Are the ORP and Water temp wires ran alongside any higher voltage wiring?
It sounds like something is introducing noise. We just need to figure out what that something is.
Created Date: July 12,2011
Created by: a agnone
I had similar issues. Is the 24v common grounded? Are you using grounded or ungrounded J -type thermocouples (metal jacket) or is it just the two wire type J thermocouples not in a metal jacket?
Created Date: July 13,2011
Created by: doncham
Koi Pond
The thermocouple leads are 5 ' or 6 ' long. The pH probe is probably 3 ' long to the transmitter. Wires from the transmitter to PLC are a couple of feet.
The TC is ungrounded. I got a tip earlier from this forum that a grounded probe was bad news for this application.
There is a 1 hp pump circulating the pond water. All of the piping is PVC. The motor is grounded, in fact everything in the pumphouse, including the PLC is connected to a 5 ' ground rod. There is also an airpump. Again all of the plumbing is plastic.
It's clear there is that is about 200 mV of potential in the pond, but the ORP probe which is connected through an identical transmitter as the pH probe.
Given the potential in the pond it makes sense that it overwhelms the TC signal. That's why it seemed to me that RTD might be the way to go.
Created Date: July 13,2011
Created by: a agnone
Is there a 24vdc power supply involved? Is the Common grounded also?
Created Date: July 20,2011
Created by: Herman
Are the ORP and Water temp wires ran alongside any higher voltage wiring?
It sounds like something is introducing noise. We just need to figure out what that something is.
I think you mean "Are the pH and water temperature probes... etc. "
because there were no problems with the ORP measurement. You 're on the right track though.
The motor is grounded, in fact everything in the pumphouse, including the PLC is connected to a 5 ' ground rod.
Is that 5 ' ground rod connected to the ground rod(s) of your house ? If not you could have stray currents messing things up.
Rather than "smoothing " a jumpy/faulty signal, I'd try to get the signal straightened out.
Created Date: July 29,2011
Created by: doncham
All of the signal wires are as seperated from high voltage as much as possible. They all have pretty much the same path.
I finally got around to connecting the 24V negative terminal to LG on the PLC. Water temp then read 76.6 and was steady. Measured with a thermometer at 76. That's promising. It will be interesting to see if it stays accurate and steady.
pH is swinging from 9.4 - 7.9. That's been typical since I installed the latest transmitter setup. ORP and pH use a pair of Eutech Alpha pH 500 transmitters. ORP is steady as a rock. pH jumps all over. The 24 v supply is from Automation Direct.
I think I'll try a new probe, although I'm more suspicious of the transmitter.
I installed a new ground rod for the pump house. It's about 100 ft from the ground rod where power enters the property. Are you saying I should connect the two rods. What gauge wire would be required.
Created Date: July 29,2011
Created by: Do-more PE
I personally wouldn't connect the two ground rods. If the temp is now steady with the negative bonded, you have probably solved that portion of the issue.
The PH swing might just be due to the water movement although the swing seems excessive. In my experience it's usually the probe when the variation is that high.
Created Date: August 03,2011
Created by: Herman
Donchan, when I put up a TV antenna I read to ground the antenna with its own rod and connect that with the house's main ground rods with 6 gauge solid copper. The reason being to avoid 'stray currents ' in the signal wire. But actually I doubt if that would be the solution to your problem, especially since reading AAA's post above.
That said, how old is the pH probe ? And could you test its own voltage with a multimeter in a controlled environment (out of the pond), with two different buffers ?