this is the first time I have played around with an ORP sensor, so I may have the wrong end of the stick.
My understanding is essentially: the higher the free chlorine, the greater the oxidation reduction potential and the higher the +ve mV output from the probe.
The probe that I bought has specs that say the output is -2 ... +2v.
The results that I have been getting in various solutions are as follows:
tap water (France): 180mV
tap water with 50% chlorine (2.1% sodium Hypochlorite): 140mV
swimming pool water: 110mV
pure bleach (12.1% sodium Hypochlorite): 10mV
These are not what I expected! I would have expected that the higher the concentration of chlorine, the higher the +ve output in mV.
In terms of measurement, I am not conditioning the signal at all: just attaching my multimeter leads directly across the output. So far as I can tell there are no electronics within the probe. There are no stray currents in the vicinity that could be affecting the readings.
I understand that ORP is a relative measurement. I am not looking to calibrate, just to understand what is going on and why the measurements I am getting are more or less the inverse of what I would expect!
the Multimeter I am using has 10MOhm input impedance (Mastech MS8268). Ultimately the idea is to automate the monitoring and dosing of a pool. So far I am blindly dosing with 2L of chlorine per day and this seems to keep the pool around 0.5ppm. I've been lucky and so far avoided algae blooms.
By the way, this just sounds wrong: "tap water with 50% chlorine (2.1% sodium Hypochlorite): 140mV" - that's a mere 1:5 dilution of pure bleach, and then your swimming pool water is even higher in chlorine concentration? That would mean anyone taking a dip in your pool comes out looking like an albino.
As for connection, literally just connected with crocodile clips to the sleeve and pin of the sensor.
re your query: i meant 1:1 tap water:bleach and the relevant bleach is household bleach (2.1% Sodium Hypochlorite + surfactants and other stuff ). So 1.05% NACIO in solution (assuming insignificant Cl- in the tap water), c.10,500ppm.
But yes - I agree that the numbers/measurements don't make sense unless somehow I am misunderstanding the nature of ORP probes. Using DRP tabs I estimate the free chlorine in the pool to be around 0.5ppm.
You may be thinking of pH meters. ORP sensors (by my reading) are not tricky to use. And you can't calibrate them (in the typical sense of the term): all you can do is "check" whether the change in reading is as anticipated between two solutions as different sensors will always read differently.
A checking solution also presupposes that you have some kind of signal conditioning. At the moment I am trying simply to understand the readings with no conditioning at all.
Input impedance of my meter is 10Mohm. I'd think that anything over 10k would be fine for ORP. Ph is different. If not what impedance do you suggest as a minimum and what is the rationale?
I had read the linked document earlier today. Thank you but it was not helpful for this issue.
My working assumption is now that the probe is badly constructed as I cannot see why else the result curve would be inverted.
I doubt if a multimeter has high enough impedance to make a sensible measurement. Get a standard calibration solution to check.
Most DMM's are 10M input resistance, you probably want a lot higher than that, since you need to be
several orders of magnitude higher resistance than the water might have. Also any current flowing could
potentially (no pun intended) lead to chemical change on the electrode surfaces. A MOSFET input instrumentation amplifier might be more suitable, with input impedance more like 10^12 ohms.
Also you measure redox potential, water contains many species that affect redox potential, for instance
O2 gas might be an issue. You also need to check for a particular electrode system if it has any incompatibilities with
dissolved species that could affect the reading. Basically you need to be a chemist to understand all
the possible reactions at the electrodes!
I get very similar results with a 100Mohm ADC. So at least one order of magnitude higher.
The ORP probe has an internal resistance in the kOhm range.
Whilst I accept that there are many factors that affect potential generation this in the end is just a battery creating a pd between a reference (probably KCL) and the solution it is submerged in. Chemically this should be consistent and repeatable. And for sure a higher Cl- concentration should result in a materially higher mV than distilled water!
Ultimately the idea is to automate the monitoring and dosing of a pool.
I ve been trying to do the same for some time now both for ORP and Ph but I could never get consistent results from probes. I have tried 3-4 different ones as well and they also come with an instrumentation amplifier for signal conditioning .
It comes with its own buffer/ amplifier with calibration pots and instructions how to calibrate with it the provided solutions but I was unable to get consistent readings.
No i don't have a schematic for it. I just connected it to a multimeter as well.
Another option which looks more on the industrial side is this.
This is much more expensive as well but I havent tried it.
If you do manage to get anything usable, do let me know as I am on the same track as well!
Another option which looks more on the industrial side is this.
Just taken a proper look at these. They appear to be an opamp connect to an tiny85. The ec probe is connected directly to the tiny85. Then they all have some upc at the end that controls the i2c. I'm not sure why that is necessary or desirable when the tiny85 can provide an i2c signal.
Outrageously priced at 60$. Component cost looks like a couple of dollars disregarding the connectors. Not entirely clear where the water temperature is being measured either.
The other issue is that the pH probe doesn't look to have any negative voltage on the opamp. So difficult to see how that's being conditioned to handle the range of values seen from commons probes. Less of an issue for orp since negative readings (technically possible) would be very unusual and in any event you're really looking for above/below a threshold (taking hysteresis into account)
I'd have thought the need for electrical isolation could be circumvented by dropping a ground probe into the water being measured. That's my plan.