Trouble with PH sensor

Hi (my first post and can I say many thanks to Kyle for setting this forum up - Im a bit of a newbie to electro chem or electro mech stuff and Ive found it hard to find info)

Ive designed a set up with one PH probe one ORP probe their EZO boards mounted on a T2 mounted on Arduino. It is designed to test swimming pool water and fire a dosing pump to add liquid chlorine when readings are within parameters. The calculation formula requires the PH to convert the ORP into Cl,ppm but Im falling over at the first step of getting the PH probe working

I desk tested and calibrated - all looked fine and realistic ( calib using test solutions, used pool water and compared to a chem test kit results)
the actual installation calls for the probes to be some 20 feet away from the Arduino
I bought properly insulated extension cables and these are routed well away from power supplies and pumps.
I know youve got to calibrate again after the extension
trouble is this did no make much sense - first the PH was reading 0 - issuing the Phext,1 command showed it was reading -2.45 - proceeded with calib mid point first - great it returns 7.00
changed solution to pH4.0 reading comes back 7.00 - wait a full 10 mins - now jumping 3.990 to 7.0
carried out calib moved on to cal,high,10 and using appropriate solution(Ph10.0). this time could not get it to return anywhere near 10.00 even after 20 mins.
the status reads ?STATUS,P,3.88
the slope reads ,?Slope 0.0,-2.00,518.81
Im not sure but that seems to tell me I’ve got a healthy 3.88 v at the probe despite the long cable
but the slope seems all over the place
I’ve tested the connections - they are good and tight bnc ones
I’m just not sure what else to test - all suggestions gratefully received - If anyone’s interested in a similar project I can post my arduino code (not pretty!) in the public domain in a separate post but right now I’ve just using a test rig where I can send individual commands to the EZO devices. Its nothing to do with my Arduino code

TIA and all the best

1 Like

Hi John. Are you not using a Raspberry Pi and Mycodo?

no, well yes and no. I have a pi4 which runs a lot of other systems around the pool turning pumps on and off, lights covers etc. the Atlas sensors PH/ORP are connected to an arduino which on a loop sends the readings (and calculated Cl ppm) to the mysql database on the raspi. as I have it at the moment if the readings go out of bounds the EZO pump is switched on for a calculated dose of liquid Cl
the raspi also runs a web server - Im not sure of my logic but I thought there was a bit to much going on with the raspi to load it with the sensors and I had a spare arduino with a ethernet shield. probably not overloaded at all but hey

Anyway the issue is/was with the T2 on the arduino. I was getting readings but they made no sense and the calibration wasnt reporting right. I dismantled everything to ensure no problems with joints even to taking the shields off the arduino. In doing so I found that the BNC connectors on the T2 were loose. I think I got a dodgy T2 because another connector simply dropped off with what looked like a dry joint. to cut long story … .a few drops of superglue and a dab of solder on the central connector - reconnected and, abracadabra, the calibration worked and everything is fine.

the one issue I have now is that the ORP sensor takes an age to wind up to sensible readings after the rig has been switched off for any length of time

I did promise to post my code but its such a mess I am completely rewiting it

Many thanks for your attention Kyle

1 Like

You probably will not be overloading it. I’ve had dozens of sensors and outputs running at the same time and it handled it fine. Especially a Pi 4, you could probably connect a hundred sensors and be okay.

Using microcontroller are often more reliable than Pis because you don’t have an entire Linux operating system or an SD card as points of failure. However, you can use an SSD with a Pi (recommended) and have a small battery backup system to allow the Pi to shutdown properly during a power outage, which helps to reduce data corruption that can cripple a Pi.

Glad to hear it’s working.

How long are you talking? Several hours?

Yes several hours.
If it’s been off for a day or more it can take 4 hours to wind up from200mv to realistic readings 600+

That doesn’t sound like it’s operating properly, though I wouldn’t know what would cause that effect.

HI @Kyle and fellow forumites. I’ve got over the problem with the ORP - it just seems to take an age to settle down. I got fed up manually takking readings and hooked it up to write to SQL on predetermined intervals averaging in between. As you do I walked away for a few days- when I came back it was reading sensibly and consistently. I think I have other issues though. The sensors (ph/orp) are plugged into the pipework and though not close to the pump they do need pump to be circulating to take an upto date reading. The second problem is that the dosing pump injects into another bit of the pipework and I suspect is not delivering the calculated amount because of back pressure if pump is on. I can get round these thiings programmatically by dosing whilst pump is switched off then pumps on to mix the solution. Anyway thanks for helo

1 Like