2 perfectly calibrated pH meters giving different readings in solution

[RaggedyMan]

Just as the heading says. I have a blue lab guardian meter and an Apera meter that both calibrate at 7.0 & 4.0 but when I’m measuring my rez, tap water or whatever, I get different readings by sometimes as much as .8 points.
I don’t know which one to trust, can someone help me understand what’s going on here?

 

[TysonOG]

Are you giving them plenty of time to "settle". I have 2 bluelab guardians and 1 takes 30 secs to get accurate, the other about 5 min. I also have a cheapo ph meter that's pretty accurate but takes 30 min to acclimate.

Also, i had to replace the pH probe on one of my bluelabs awhile back. Got a $20 off Amazon and it actually works pretty well. Check the bulb part on yours for damage/corrosion.

 

[RaggedyMan]

I wasn’t giving them much longer than I was when I calibrated them. We’ll see, I’m checking on that now.
The probe to my Apera is relatively new but the blue lab probe is a little older. Both have been kept in storage solution whenever they’re put away for a while

 

[Weaselcracker]

They both read true (7 and 4) when you put them in the calibration solutions?
My Guardian usually alway reads 0.1 low after a month- but never been off more than that except the time a probe went kaput.

 

[RaggedyMan]

They both read true (7 and 4) when you put them in the calibration solutions?
My Guardian usually alway reads 0.1 low after a month- but never been off more than that except the time a probe went kaput.

I’m more inclined to go with the blue lab cause, like you, I’ve never seen it out of whack when calibrating.

Are you giving them plenty of time to "settle". I have 2 bluelab guardians and 1 takes 30 secs to get accurate, the other about 5 min. I also have a cheapo ph meter that's pretty accurate but takes 30 min to acclimate.

Also, i had to replace the pH probe on one of my bluelabs awhile back. Got a $20 off Amazon and it actually works pretty well. Check the bulb part on yours for damage/corrosion.

After 30 minutes they were different by .4

 

[Prescription Blend]

I wasn’t giving them much longer than I was when I calibrated them. We’ll see, I’m checking on that now.
The probe to my Apera is relatively new but the blue lab probe is a little older. Both have been kept in storage solution whenever they’re put away for a while

Recalibrate and let them acclimate to the standards for longer then retest to see if they are closer...

 

[RaggedyMan]

Recalibrate and let them acclimate to the standards for longer then retest to see if they are closer…

Well damn……
Put both probes in the same sample of 7.0 solution and both pens went immediately to 7.0. I left them for 15 minutes and no change. Dropped them in my rez water and after 15 minutes I got 5.6 and 6.3, WTF?

 

[Prescription Blend]

Well damn……
Put both probes in the same sample of 7.0 solution and both pens went immediately to 7.0. I left them for 15 minutes and no change. Dropped them in my rez water and after 15 minutes I got 5.6 and 6.3, WTF?

If you wear 2x watches you'll never know the time as my grandpa used to say. This is an interesting issue, narrow band pH test stripes would show you which one is accurate.

pH test strips on walmart.com or get them at a homebrew supply (garden shops would probably stock these also)

 

[RaggedyMan]

If you wear 2x watches you'll never know the time as my grandpa used to say. This is an interesting issue, narrow band pH test stripes would show you which one is accurate.

pH test strips on walmart.com or get them at a homebrew supply (garden shops would probably stock these also)

Ain’t that the truth. I keep thinking how I should just stop with using 2 meters but I like the second one that I can take out of the grow room on occasion when I’m mixing mutes or whatever. I ordered another probe for my blue lab as it’s been through a constant couple years use

oh, and I did use strips but I’ve never been able to really tell anything other than it’s “within range”

 

[ColonelBlake]

Wow, I have the exact same problem. My differences are less tho: +/- .3.

I use a Blulab Monitor ( ph, ec, temp ), and an Apera ph meter for comparison. Both were cleaned and calibrated the other day @ 7.0 and 4.0. There was a .2 difference immediately. A couple days later it's .3 off. The Blulab ph probe is less than a month old. The Apera ph pen is over a year old.

Just to note, I store my Apera ph meter in KCL solution and change the solution every time I calibrate it ( monthly ). It's meticulously maintained. The Bluelab probe(s) are always in the res. They are also cleaned and calibrated monthly.

 

[RaggedyMan]

Wow, I have the exact same problem. My differences are less tho: +/- .3.

I use a Blulab Monitor ( ph, ec, temp ), and an Apera ph meter for comparison. Both were cleaned and calibrated the other day @ 7.0 and 4.0. There was a .2 difference immediately. A couple days later it's .3 off. The Blulab ph probe is less than a month old. The Apera ph pen is over a year old.

Just to note, I store my Apera ph meter in KCL solution and change the solution every time I calibrate it ( monthly ). It's meticulously maintained. The Bluelab probe(s) are always in the res. They are also cleaned and calibrated monthly.

Well Colonel, sounds like exactly the same thing. I don’t change out my Apera pen storage solution monthly (good idea though) but everything else is identical. Since I last posted I bought a new probe for my blue lab and as I suspected, it’s spot on with the old probe so I’m going with that.
I mostly use the Apera to bring my tap water down to reasonable levels for my outdoor grow. July 1 we are able to legally grow 4 plants so I thought I’d give dirt growing a shot, not impressed so far. Love my hydro.

 

[Dankman_420]

I have a few pens. Cant trust just one. When in doubt I use the drops with color chart.

I have better luck with my Apera. Only had it few months, but so far its working well.

Im transitioning from organic soil to DWC. Amazing how important a good pen is now

 

[ColonelBlake]

So true.. My Apera seems to be the most accurate as well. I've had it 2+ years and it's still usually spot on when I calibrate it.