pH test cell reading does not agree with pool

[woodsage]

Hi -

I'm using the Sense & Dispense system with a Goldine Pro-Logic controller. The reading of the pH in the test cell of the Sense & Dispense does not agree with the pH reading I get in the pool.

The test cell reads less than the pool, usually about 0.2 ppm less.

Is this normal? Can this be corrected? Any ideas to have the same pH reading in the test cell as the pool?

Thanx for your help.

 

[duraleigh]

How are you testing?

 

[woodsage]

Taylor kit.

 

[duraleigh]

You can just about take the Taylor pH test to the bank, although sometimes interpretation is a little tough. I have no knowledge of the other system.

 

[Bama Rambler]

The real question is, which is correct? You should be able to adjust the readings to a standard so they both read the same. Do you have any 7.01 pH buffer to check them with?

 

[woodsage]

I'm wondering if I'm not getting adequate flow through the test cell; possible the water in the test cell is not as "fresh" or "real world" as taking a sample directly from the pool.

I trust the reading from the pool since that is the water I'm trying to keep in balance instead of the water in the cell.

 

[JasonLion]

The ideal test is to draw water directly from the probe cell at the same time you are noting the reading off of the PH probe. Then the numbers will be directly comparable. Drawing water from other places in the pool can sometimes be off by a noticeable amount, especially with direct sunlight on the water and most of the water for the probe cell being drawn directly from the skimmer (surface).

It is fairly common for PH probes to be off by 0.1 or 0.2 or perhaps a little more. Further, the error can drift around slowly over a period of several months. The probes are calibrated against an internal reference, which isn't perfect and will change slowly over time.

The Sense and Dispense system allows for this with a PH calibration setting on the maintenance menu.

Keep in mind that it is difficult to read the Taylor PH test precisely. The best most people can hope for is +-0.1, and novices can often be off by more than that.

 

[woodsage]

Jason -

Yes, the pH is dead on from the test cell and the pH probe.

If the water should be tested from the test cell, then should all the tests, cl, alk, cya, ch, be done from the test cell?

If I'm testing the water in the cell but swimming in the pool, shouldn't I be testing the pool water and not the cell water?

So, I'm balancing the water in the cell and not the pool?

If the pH in the test cell is 7.8 but the pool is 8.0 pH is this a concern? Or should I calibrate the pH probe to read 7.6 with hope that the pool will then read a 7.8 pH?

 

[JasonLion]

If you are calibrating the probe, you should be testing the water from the probe cell. The ORP controller depends on the PH in the probe cell being maintained at a specific value at that specific point in the plumbing, so you need the probe calibrated to the PH at that point.

For all other testing you should be testing the pool water. Of course it doesn't really matter for most of the tests, as the CH, CYA, Borate, and Salt levels will be the same anywhere you test them, and TA will be very very close to the same. Only FC and PH will be measurably different at the probe cell, and those differences are fairly easy to predict once you have some experience with your particular pool.

 

[woodsage]

So if I calibrate the probe with the water in the test cell at 7.8, then I test the water in the pool and it's 8.0, and I set the pH setpoint for the Pro-Logic to be 7.8, then the Sense & Dispense will not activate the acid pump since the pH in the test cell is equal to the setpoint. So I will have a 0.2 difference between the pool water and the test cell water. ??????

 

[JasonLion]

The PH difference isn't really constant. It will vary somewhat depending on the amount of sunlight shining on the water and a few other factors. But roughly speaking, yes. The difference should never be especially large. A difference of 0.2 is about the most you should see (unless your TA is very low).

 

[woodsage]

So, with a difference of up to 0.2 and not constant, I have to wonder what's the point of using the Sense & Dispense system. It's already been posted using ORP with an outdoor pool is not accurate, so I'm wondering why bother with the Sense & Dispense. Since I can't rely on the reading in the test cell to be the pH in the pool, then I would have to do another pH test of the pool water and adjust the pH accordingly based on the pool water - more work and basically doing the same thing if I didn't have the Sense & Dispense.

Since CAT Controller makes the Sense & Dispense for Hayward, is ORP and pH injection better suited for commercial applications rather than residential?

 

[JasonLion]

I don't normally recommend using systems like the Sense and Dispense system except in commercial pools and a few unusual situations. However, they sell rather well because they appear to fulfill a dream most people have of some magical automation that will relive you of needing to test and adjust the water. They don't actually do that, but many people buy them with the hope that they will. In practice, anything more complex than a SWG often ends up being more work than simply maintaining things manually. One of the most common, though still somewhat rare, reasons to have PH automation is when you have high levels of evaporation and your fill water TA level is quite high.

That said, having a system that can maintain PH to within +-0.1 or +-0.2 is just fine. There is no need to control PH more tightly than that (other than for ORP sensors).

 

[woodsage]

Jason -

I guess I'm getting hung up on the CSI index and a +/- 0.1 pH change affecting the CSI index; it's probably best not to get hung up too much on the numbers but rather looking at the pool water and if the water is clear and sparkling, all is well...

 

[JasonLion]

Trying to maintain CSI that narrowly is counter productive. More often than not you end up adding chemicals way too often and making things worse. CSI anywhere from -0.4 to +0.2 is just fine.

 

[Ohm_Boy]

People don't realize that while they are measuring with a micrometer, they are cutting with an axe.

When you dose your pool based upon whichever reading, do you compensate for any change in water level? One inch of water in a 16x32 pool is over 300 gallons, so you'd have to adjust your dosages accordingly if you really had to maintain the kind of precision you think you'd like.