ORP in a salt water pool

[randytsuch]

I have been on the fence on monitoring ORP, but bought a Seeedstudio ORP sensor kit from Mouser. Was cheap enough to take a chance and see how it goes.

My plan is connect it to an ESP8266 which will read the output. This kit generates an analog signal output, so you need to read it as a voltage with the ADC input.
I don't plan to calibrate it, will just see how the ORP readings compare to my FC levels.

No controlling based on ORP planned either. My goal is to get notifications if the ORP readings indicate an issue. The limits for the notifications will be based on what I should get based on my experiments with it.

And because I'm lazy, I'm going to try to drop the sensor in pool, hiding the ESP in a case nearby.

Package is on its way, due next week, so I'll see if my mad scientist plan works.

Randy

 

[PoolStored]

Problem is that an ORP sensor does not know how much CL is bound with CYA that can be freed as needed. So an ORP system with high CYA will think that CL is low when it is not.

An ORP system cannot work as designed with high CYA levels.

 

[randytsuch]

Problem is that an ORP sensor does not know how much CL is bound with CYA that can be freed as needed. So an ORP system with high CYA will think that CL is low when it is not.

An ORP system cannot work as designed with high CYA levels.

I understand the CYA interferes with the readings. But I figure if CYA is constant, and my FC is constant, then ORP should also be relatively constant. And ORP will only change if FC changes.

But not sure if this is a waste of time or not. Since I only needed the ORP kit, its relatively easy and cheap to try and see what happens.

My plan is to get a text with the ORP value once a day. And also will have a program to check the ORP levels against some level I come up with, and send another text if ORP level is too low or high.
I'll still take FC measurements to make sure pool is ok.

Randy

 

[PoolStored]

Sounds like fun. Interested in your results!

 

[JoyfulNoise]

I understand the CYA interferes with the readings. But I figure if CYA is constant, and my FC is constant, then ORP should also be relatively constant. And ORP will only change if FC changes.

Not trying to be a Debbie-Downer, but that last statement is not true. ORP can change even when FC is fairly constant. It’s affected by pH and the presence of any species that can cause the balance of oxidizers and reducing agents to shift. UV light affects it as does dissolved oxygen levels. The platinum electrode attracts a lot of “garbage” to it that can foul the surface and cause measurement delays as well as signal degradation.

Interested to see what you come up with but I wouldn’t expect too much out of it.

 

[randytsuch]

Not trying to be a Debbie-Downer, but that last statement is not true. ORP can change even when FC is fairly constant. It’s affected by pH and the presence of any species that can cause the balance of oxidizers and reducing agents to shift. UV light affects it as does dissolved oxygen levels. The platinum electrode attracts a lot of “garbage” to it that can foul the surface and cause measurement delays as well as signal degradation.

Interested to see what you come up with but I wouldn’t expect too much out of it.

Thanks for the info

I knew about the pH factor too, but with a fiberglass pool I don't see much pH variation, so hoping it will not be a factor.
Will have to see how much the "garbage" on the electrode affects it.

Randy

 

[randytsuch]

My Mouser package arrived, started playing with the sensor.

Sensor didn't really have any liquid in it when I opened it, I think it leaked out, there were just a few drops.
I added some water, and thinking back, I think it took a little while to really start working after adding the water.

Initially my filtered water was around 300 mv. Later it was around 500 mv. I now think the change was due to having to recover from being dry.
Pool was is around 250 mv.

All readings were made by sticking the sensor in the sample and using a volt meter.

I thought something was wrong with my sensor, but did some looking here and remembered that CYA will lower the reading from pool water.
So planning to experiment some more with the sensor, see how sensitive it is the changes in FC at my CYA level of 70.

Randy

 

[JoyfulNoise]

If the internal solution of the probe was missing then filling it with water won’t work. The internal reference electrode is usually a silver/silver-chloride electrode sitting in a saturated potassium chloride solution. That part of the sensor should have been sealed.

 

[randytsuch]

If the internal solution of the probe was missing then filling it with water won’t work. The internal reference electrode is usually a silver/silver-chloride electrode sitting in a saturated potassium chloride solution. That part of the sensor should have been sealed.

Internal solution is still there, the internal seal is intact.

There is a little cap and rubber seal to keep solution around the tip. That's what was missing, so the tip was exposed to air.
It seems like there used to be solution around the tip because there were a few drops of something in there.

EDIT: Attached pic of ORP tip as received.

Randy

 

[JoyfulNoise]

Internal solution is still there, the internal seal is intact.

There is a little cap and rubber seal to keep solution around the tip. That's what was missing, so the tip was exposed to air.
It seems like there used to be solution around the tip because there were a few drops of something in there.

EDIT: Attached pic of ORP tip as received.

Randy

Ah. Ok.

The tip is Pt metal. It should be fine. Just soak it in DI water for a bit. There are also calibrating and cleaning solutions for the ORP probes. Nothing wrong with using them and they can help to diagnose probe issues.