Inaccurate PH sensor?

[zsxking]

My PH sensor seems not working probably, always gives reading around 7.6 while the actual value is 8.1~8.2 from API kit. The tank is looking good so I don't think 7.6 is anywhere close to true value.

I have used 3 different kinds of PH solutions to calibrate many times, every time give similar value (545, 830). So I don't think that's a calibration problem.

Will that be because of detected sensor? Should I buy a new one?

Also even when the actual reading is off, does the reading in different time comparable? Because I am more interested in the daily night difference of PH.

[rimai]

It sounds like stray voltage.
Try disconnecting the following and see if it helps:
1. USB-TTL cable
2. float switches
Also, what happens if you place the probe in a cup with tank water?

[zsxking]

I finally get to test around stray voltage. The meter did show a 1.7v voltage. It's that considered high? I try to test water in a cup, only 0.01 difference.

I also bought another ph probe from eBay. But it basically give the same result, reading the tank ph as 7.6 ~ 7.7 after calibration.

[rimai]

Stray voltage comes from another equipment you have connected to RA.
Usual suspects are float switch, heater, water pump, power head, your PC computer and even LED drivers.

[Pny]

What pH value did you get with tank water in a cup?

[thoughtiknew]

I am having this same issue I have calibrated the meter at 7 and 10 pH and it reads correctly at both of these values however when placed in tank water either in tank or in a cup it reads between 7.7 and 7.8 I have even tested it in a cup of freshly mixed salt water in a cup with a known ph of 8.4 and it is reading 7.8 I have all reef keeper relays off all expansions disconnected including temp sensor I have been messing with this for two weeks now since I received my RA and it is really frustrating because PH monitoring was one of the primary reasons I spent 500 dollars on this system and as of now its is working as an glorified timer

[rimai]

If both reading of pH 7.0 and 10.0 are reading correctly and the pH in the cup is reading 7.8, I would think that the pH in the cup is indeed 7.8.
Are you sure that the cup is 8.4?
What calibration numbers did you get when doing your calibration for pH7 and pH10?

[thoughtiknew]

PH 7 561
PH 10. 809

I have back checked all of these values against several other pH tests including API red sea and for sh** an giggles I tested it using my pool pH test strips all other tests say pH is between 8.2 8.4, and driven a calcium of 450 and a dKH of 11.2 and a magnesium of 1400 short of a complete co2 saturation (unlikely because I aerate with outside air) a pH of 7.7 is mathematically and chemically impossible

[ReEfnWrX]

Actually, I would venture to say that you have high CO2 levels....Your alkalinity is at the high end...

As your CO2 levels rise your Alkalinity will rise and your pH will drop.... If your CO2 levels decrease, your Alkalinity will decrease and your pH will rise....

And you do not need complete CO2 saturation for this to take affect. this is the #1 affect on pH day in and day out....

http://www.reefkeeping.com/issues/2007- ... age002.jpg
http://www.reefkeeping.com/issues/2007-05/rhf/

Now if your pH is ALWAYS 7.7 and does not swing at all then I would say there is another issue...


I strongly agree with Roberto. If your PH probe was inaccurate then your calibration readings for 7 and 10 would be off and I do recall my probes calibrating in those same ranges.

If both reading of pH 7.0 and 10.0 are reading correctly and the pH in the cup is reading 7.8, I would think that the pH in the cup is indeed 7.8.
Are you sure that the cup is 8.4?
What calibration numbers did you get when doing your calibration for pH7 and pH10?

[thoughtiknew]

the likely hood of high co2 in a batch of newly mixed water and by newly I mean two day ago is very unlikey the water is stored in my basement with continuous circulation and airation with OUTSIDE air so while I agree that this is unusual given the probe reads correctly in both 7 and 10 pH solution something is awry here considering the original poster on this thread had the same issue

[rimai]

Looking at your chart, it does look like you get the variation from day/night, which is good.
A few days ago, your chart showed numbers in the 8.4 range with a beautiful sinusoidal waveform. What did you do to bring it down or change it?

chart.png (16.52 KiB) Viewed 5466 times

I think the calibration didn't go too well though.
809 looks to be in the lower end, which could explain why your numbers are low.

[thoughtiknew]

I dosed the hell out of the tank with 2 part assuming the probe the was correct back checked after 2 days only too find my dkh around 14

[rimai]

So, the pH8.4 was after you dosed?
I'd recheck your pH10 calibration number. It seems a tad bit too low.

[lnevo]

If the ph10 calibration number were low that would cause the ph number to go up, no? If it was higher that would cause his currently registering ph number to drop even more..

[rimai]

I think you are right...
What numbers do you have for yours?
I use 540 and 860

[lnevo]

540 and 835. But i fudge mine a little to compensate for stray voltage as well...

[thoughtiknew]

Those 8.4 readings were also from before I calibrated the meter and was using values from the forums as well as dosing . if I use 540 and 809 my pH reads 8.04 which I could believe although I believe it is slightly higher

[lnevo]

The 561 is high that would lower the ph...

One thing i noticed is that the ph can jump around. You should record the number you see when you hit ok and verify if that is what got saved to memory...

[thoughtiknew]

When I place the probe in solution I leave it for like 5 mins and it still seems to be flickering when I come back how long do you usually wait

[zsxking]

I finally found out that my sensor's read was correct. I thought my tank's ph was 8.2 because I use filtered nature sea water.

I also have a brine shrimp culture that is highly aerated. When testing its ph with calibrated probe, it reads 8.0, while my tank's reading is 7.6. So it's my tank has ph problem, which is also skimless btw.

To confirm, I take a cup of tank water, put it outside, run a air pump in it overnight. The next morning its ph was also 8.0.

So my ph sensor didn't have an accuracy issue. Instead, my tank is having a CO2 issue.

[ReEfnWrX]

When ever you have a pH issue, if it is high or low, and alkalinity is at the opposite high or low end as well... 99% of time is CO2...

[ReEfnWrX]

I have a Mag9 driving my skimmer at full blast. I do not regulate air flow at all, it is wide open. I regulate my skimmer water level with a pin/needle valve on the water exit side... I also run a bubbler feeding air from outside and I cannot get my pH to go past 8.2 at the end of the day before lights out.... I have been trying for the last 2 months to get it to peak past 8.2. I do run Kalk ATO but I am not increasing the kalk additive because my alk is stable at 10 which is as high as I want to go.

All tanks are different. Even though I know I am exceeding the requirement for aeration I can't stabilize peak pH beyond 8.2... I am finally accepting that it is not a bad thing and as long as it is stable that is what is important.... Look at the Zeovit system... Don't they recommend a pH around 7.8? In the end stability is far more important...

However, if anyone has any other ideas on anything else I could try to increase peak pH I would still be interested...

[lnevo]

8.2 is fine. I would say not to chase PH