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Topic: Deciphering ORP readings (Read 2787 times) previous topic - next topic

jpadie

I'm trying to keep the design at a low price point, for probes too. 

I think the ph probes I have been using are £10 each.  or less.  the ORP probe I bought a few years ago and was much more expensive.  No conditioning op-amp provided.

Incidentally I am also designing an arduino control for a variable frequency device to power a standard three phase pool pump.  Much much cheaper than buying a variable speed pump

Watcher

I'm trying to keep the design at a low price point, for probes too. 

I think the ph probes I have been using are £10 each.  or less.  the ORP probe I bought a few years ago and was much more expensive.  No conditioning op-amp provided.

Incidentally I am also designing an arduino control for a variable frequency device to power a standard three phase pool pump.  Much much cheaper than buying a variable speed pump

Thats interesting as well!

Never thought of doing that...

jpadie

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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.   

wvmarle

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.   
May work - if your probe is not being biased, as is often done to make pH values > 7 easier to read with a single sided power supply.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

jpadie

I'm biasing with a negative rail.


wvmarle

Outrageously priced at 60$.  Component cost looks like a couple of dollars disregarding the connectors.
You forget the cost of development. You may be able to reproduce that - and make it cheaper, if your time is free.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

jpadie

true.  but once you've got the product in the market, the development value is lost.  it's reverse engineerable after the first one hits the ground.

so these days you can't have an expectation of making a big mark up over the cost of production and distributions themselves (unless you can control access to the design).

Delta_G

Are you correcting for pH?  Or anything else?  ORP is strongly affected by pH.  As the pH changes the ORP changes.  ORP isn't really a good indicator of free chlorine unless everything else is tightly controlled, which won't be the case in your pool. 

The effect you're seeing with bleach in water is probably more an effect of the strong base in the bleach rather than the oxidizing power of the chlorine. 
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Please do not PM with technical questions or comments.  Keep Arduino stuff out on the boards where it belongs.

jpadie

thanks Delta_G

the pH is corrected for so we are testing for ORP when the pH is balanced. 

the initial work I did a couple of years ago was very inconclusive.  the readings for pool water and for tap water changed wildly over moments so i think there must have been stray capacitance from somewhere.


Delta_G

Quote
the pH is corrected for so we are testing for ORP when the pH is balanced.
So there's a pH probe also involved in this measurement?

So what was the pH in the bleach solution compared to the tap water?
|| | ||| | || | ||  ~Woodstock

Please do not PM with technical questions or comments.  Keep Arduino stuff out on the boards where it belongs.

jpadie

hi Delta_G

this thread spans two years.  yes - originally when I did the measurements I took the water from my pool which was pH controlled.  I did not check the pH of tap water, but it would have be within normal ranges.  Evidently adding NACIO will change the pH and that may have skewed those results, I agree.  But they should still have been repeatable.  I did not check it however.

thanks
justin





jpadie

i should add that my comment above, that you captioned, referred to the general use case for this design which is to check for pool water quality.  there is a pH probe, temperature probe and ORP probe all within the design.  In my case, the attached uProcessor logs the data and decides whether to add acid, base or, if the pH is in range, chlorine (or to turn on the electrolyser).  the liquid dosing is done by cheap peristaltic pumps although I'm having trouble sourcing liquid ph+ so I may also develop a dosing auger for the powder.  the electrolyser is turned on and off by a mains relay. 

currently the design is to have the probes in line with the chlorine cells (before) as the system has 30mm threads ready to take the ground probe and the sensor probes.   but i have wondered whether there would be significant stray current from the electrolyser, so was thinking that instead it might be an idea to t-off and run some aquarium tube up into a separate analysis area physically distant from the higher voltage electronics.  it's all a job for October.

Delta_G

#27
Aug 28, 2019, 02:56 pm Last Edit: Aug 28, 2019, 02:57 pm by Delta_G
1.  The pH control on your pool isn't tight enough to make for stable ORP readings.  It isn't.  I don't care how tight you think your control is.  It's a big open body of water and the slightest change in pH can affect that ORP reading.

2.  Don't think your tap water has "normal pH ranges".  The "normal" range for tap water is about two and a half pH units wide.  City water goes from high 6's all the way up to the low 9's.  So there's a pretty big range there.  If you have a well then it's wider than that. Well water could be anywhere from upper 5's to low 9's.

3.  Having used ORP for years to try to dose ozone into an aquarium I can tell you it's a fickle beast.  Make sure you got plenty of backup and make sure that your system can't take any drastic actions on its own.  You'll get spurious readings all over the place.  Especially if this is an outdoor pool. 

As an analytical chemist this sounds a bit crazy.  Sounds like someone who knows pools but hasn't sat through many chemistry courses.  But I hope it works for you. 
|| | ||| | || | ||  ~Woodstock

Please do not PM with technical questions or comments.  Keep Arduino stuff out on the boards where it belongs.

jpadie

Thanks.

I think the point is that ORP  _is_ used for pool measurement all over the world so there must be a fashion to make it adequately accurate for those needs.

pH may well not be controlled perfectly but again, it seems that in general pH can be controlled adequately for the needs of pool users. 

Or am i.missing something?


jremington

#29
Aug 28, 2019, 05:04 pm Last Edit: Aug 28, 2019, 05:15 pm by jremington
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Or am i.missing something?
You are evidently missing whatever it is that people "all over the world" know about using ORP sensors for "pool measurement".

Quote
ORP sensors (by my reading) are not tricky to use.
But after two years, you still have no idea how to use one.

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