Two pH probes, both newly calibrated, 0.2 apart

[AlanM]

I just started using two pH probes.  Just as a hedge against one of them slowly going bad without me knowing it.  I figured I'd have two probes in with about 6 months in age between them and just watch the two trend together.  I calibrated both of them in the same solutions yesterday and they are now reading 8.06 and 8.26, on the new BRS-brand lab grade probe and the standard grade one that Neptune Systems sends with the Apex, respectively.

 

Any idea what gives from those of you who have used them for a long time?  Should I calibrate them again?  I calibrated the BRS one, and then pulled the old Apex one out of the tank wiped it off and put it in the same calibration solution.  Could the liittle bit of contamination from the tank water have blown the calibration? 

 

Should I have swished in RO/DI when going from the 7.00 to the 10.00 solution?

 

Since calibration yesterday they are both following the same line as pH increases during the day, just 0.2 apart.

[wade]

I typically do use RO when switching from tank water.  Tanks are heavily buffered so it could have an impact, although the amount transferred is unlikely to be a problem (as the solutions are also well buffered).  Does one f them have any growth of critters on it or corrosion?

 

I've noticed in the past that probes are often just not accurate and they drift like crazy.  

[AlanM]

I typically do use RO when switching from tank water.  Tanks are heavily buffered so it could have an impact, although the amount transferred is unlikely to be a problem (as the solutions are also well buffered).  Does one f them have any growth of critters on it or corrosion?

 

I've noticed in the past that probes are often just not accurate and they drift like crazy.  

 

I did the "thermometer" flick to get any drips off the probe when pulling it out of the tank and I wiped the old probe off along the plastic body but didn't touch the glass bulb on the bottom.  Incidentally, the old probe calibrated at a much higher number than it previously was at.  I kind of believe the new one more as it is around what the old probe was showing me before calibration.  Maybe I just blew the calibration of the old one.

[Origami]

It can take a little while for the new probe to settle. Try calibrating again in a couple of days.

[AlanM]

It can take a little while for the new probe to settle. Try calibrating again in a couple of days.

 

OK.  Will try that.  I have plenty of solution.  We get it for free at the lab when gear ships with it to be used for calibrating said gear, but the science types prefer to use the stuff in jars rather than packets.

[wade]

Tom has a good point. Many new probes are fickle... still no clue why, but they are and most recommend you wait and recalibrate.

 

I calibrate my probe about every 2-3 weeks. It drifts even over that time.

 

Another reason to use RODI on an old probe is that it kills off anything living on it pretty effectively.  It won't remove it, but it will kill it.

[zygote2k]

get a third and average them or throw one out.

[Coral Hind]

Try to make sure the probes and wires are not running near electrical wires or devices such as pumps or heaters as those items can induce voltage and change the probes value some. Try swapping the plug connectors around just to rule out any resistance in a connector that is causing an issue.

[jaddc]

I use pH meters when I'm doing my (evil-mad) science in the lab.

 

For your measurements, the probes are essentially the same (8.06 ~= 8.1 and 8.26 ~= 8.3). I doubt you can get better precision than that without driving yourself mad in the process. Plus the probes are not the same -- and I would trust the lab quality more than the hobby quality. 

 

It doesn't quite matter what the exact pH of the tank is (as long as it is within a good range), rather we are looking to guard against pH drift. And to measure a change in pH, you don't need to be concerned with a difference between the probes, just the difference in readings over time. If your probes are always 0.2 off from each other, then all is good. If not, then you have an issue.

 

In the lab, I calibrate my probe frequently, because there are many many factors that influence precision and accuracy. 

 

Here is a video that demonstrates how to use a pH meter properly. While obviously you are not using a pH meter, pay attention to how he works with the probe. The key point is to rinse the probe with fresh RODI water when moving between tank water and calibration solutions. Tank - RODI- Cal1 - RODI - Cal2 - RODI - tank.

 

Also, use tissue to carefully dap off all droplets of liquid if you want to reuse your calibration solutions. If you don't reuse the calibration solutions, then don't worry about that.

 

 

Frequent calibration and proper probe treatment is key. No need to discard calibration solutions -- they should last a year before you should swap them out if you dry your probe before you dip it in and keep the container tightly sealed. 

 

One final point, keep the probes clean of salt creep.

Edited January 7, 2014 by jaddc

[AlanM]

Jeff, thanks for the video and advice.  I'll try to do a better job of calibrating them, and then if they're still off from each other I'll not worry about it.

 

One question.  What was that about salt creep?  I dunk the entire probe, wire and all in the saltwater, Apex said that it was fine to submerge the whole thing.

 

I like Rob's idea about adding one more and averaging. haha

 

The probes aren't near any electrical sources, and are at the opposite end of the sump from my ground probe and heaters, which are next to each other. 

 

I'll swap the two probes around, follow the demo in the video to carefully calibrate them, and see where I'm at.  I agree that the actual pH number doesn't matter, but the trend over time does.  That said, there is a real number, and I'd like to measure that one, heh.

[jaddc]

One question.  What was that about salt creep?  I dunk the entire probe, wire and all in the saltwater, Apex said that it was fine to submerge the whole thing.

 

 

I'll swap the two probes around, follow the demo in the video to carefully calibrate them, and see where I'm at.  I agree that the actual pH number doesn't matter, but the trend over time does.  That said, there is a real number, and I'd like to measure that one, heh.

 

Ah -- dunking the entire probes solves that problem of salt creep.

 

True enough -- you did pay to see the number. :-) Unfortunately, that means calibrating frequently. Even the professional high quality probes are not able to hold calibration for very long. 

[Coral Hind]

 

One question.  What was that about salt creep?  I dunk the entire probe, wire and all in the saltwater, Apex said that it was fine to submerge the whole thing.

Try raising a probe so it is not fully submerged and see if the readings change. I remember years ago finding out that mine had different readings when fully submerged compared to just the bottom 1" or so. I used a styro holder for the probes so they didn't become fully submerged. Give it a try and see if it changes anything.

[AlanM]

Try raising a probe so it is not fully submerged and see if the readings change. I remember years ago finding out that mine had different readings when fully submerged compared to just the bottom 1" or so. I used a styro holder for the probes so they didn't become fully submerged. Give it a try and see if it changes anything.

 

Will do.  I have those magnet probe holders from Avast, but I'm using them to hold dosing tubes at the moment and decided that I liked just dropping the probe in the sump because it stayed in the water when I lowered the water level during water changes.

[jaddc]

Try raising a probe so it is not fully submerged and see if the readings change. I remember years ago finding out that mine had different readings when fully submerged compared to just the bottom 1" or so. I used a styro holder for the probes so they didn't become fully submerged. Give it a try and see if it changes anything.

 

+1

 

I'm not sure why they tell you to submerge, perhaps it is to insure that the probe is never exposed to air. But a WC is a great time to calibrate the probes anyway.

 

In the lab, I only submerge the bottom 0.5 - 1.0" of the probe. The business end is the bulb at the bottom and that needs to be completely submerged and moist at all times. 

Edited January 7, 2014 by jaddc

[wade]

I'd also recommend an acid bath/cleaning of the older probe if you haven't done so.  Due to the nature of the probe, deposition of 'hard stuff' is normal - an acid wash should remove a great deal of that any make it more responsive as well.  Vinegar or muriatic acid is fine.

[AlanM]

OK. Will disconnect, bathe them, pat them gently dry, rub lotion on their back while playing sensual music, and calibrate them and see if they will give me "The" number. 8)

[wade]

Video or it didn't happen!  

[jaddc]

OK. Will disconnect, bathe them, pat them gently dry, rub lotion on their back while playing sensual music, and calibrate them and see if they will give me "The" number. 8)

 

My probe likes Al Green.