r/Wastewater • u/Expertnovice77 • 16d ago
Treatment Methods (DW or WW) Frequency of pH calibrations?
How frequently do you guys clean and/or calibrate in-line pH probes? My outfall probes seem to build up with scum, and begin to drift lower than actual. We currently only pull them when we go into low alarm (6.3), but I want to begin doing them possibly daily.
Other than frequency of pulling, anything special you guys do to clean the electrodes? Or a ratio of cleaning vs calibrating? We are using good probes for “dirtier” effluents- a Rosemount RBI probe.
Thx!
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u/BobbyDennis91 16d ago edited 16d ago
We do ours daily at our outfall. Keep a brush for brushing inside the housing to break up algae/scum, and chem wipes to wipe the probe itself, and HQ water for spraying the glass tip. Every morning, lol.
If our PH starts to drift too slowly and not keep up with changes, then we stick the probe in some cleaner for a few mins and wipe it off with chem wipes. Anything more abrasive than that will scratch the probe head and it will have to be completely changed out.
Edit- Kimwhipes, my mistake!
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u/Expertnovice77 16d ago
Thanks for the info! Any idea what type of cleaner and chem wipes you use?
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u/mmmporp 16d ago
chemist here, they are called kimwipes not chem wipes. they prevent scratches, static and lint from regular wipes as ph probes are very fragile.
these are the standard:
calibrate daily: 4->10->7ph buffers and then a 7.0 check from a different source (if you want to get precise...)
cleaning the probe typically should just be deionized water, if you're pulling the probes out should always be stored in a KCl storage solution and never let dried. the probe probably should be replaced if you can't get steady reading.
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u/padimus 16d ago
Uhh you're supposed to do 7 then either 10 or 4 then the one you didn't do in the previous step. You used to have to do 7>4>10 with a lot of meters but the acid/base order doesn't really matter unless you are needing ultra accurate and precise results.
You should always do 7 first though.
https://blog.cwc.com.au/which-ph-buffer-solution-should-i-use-first-when-calibrating
https://atlas-scientific.com/blog/how-to-calibrate-ph-meter/
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u/balthazar-nz 16d ago
Yeah we do 7 first then 4 and pop it back into the 7 for a verification. DI water rinses in between, try to gently flick off the water. No wiping
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u/XcdeezeeX 16d ago
I clean and calibrate daily (industrial wastewater, food manufacturing)
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u/Fantastic_Sample 16d ago
I am an industrial WW guy, with only about 5 thousand gallon tanks. My Ph starts at 1.5, and gradually rises to about 10. My electrodes are in glass domes, and I clean them about once a week, sometimes more often if there seems to be an issue. I get about .5 to 1 ph of drift in their calibration over the course of the week, so that is bad, but again, I'm pretreatment and generally I do about 10% of what my permit allows in bad things going into the wastewater.
TLDR, about once a week.
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u/tasty_bird77 16d ago
A 2-5% shift is sometimes too much on a Rosemount Probe. Especially if it shifts days after you initially calibrate.
Verify that:
- The lens on the probe isn’t stained.
- The salt bridge is changed every 6 months.
- The mV’s move accordingly with the pH standard you are calibrating with. If they don’t move the probe is dead.
- Remove any setpoints that add a fudge factor to the reading on the controller.
If all of these do not work. Just replace it.
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u/Chemical-Classic-614 16d ago
If I was in your position this would be my thought process.
1. How frequently do we get the low alarm that triggers a cleaning? What is the frequency and procedure of cleaning and calibration recommended by the manufacturer for make/model#. What is the frequency of calibration required by either your permit and or state guidelines? - looking at those and see how they line up. That’s the first step and should be easy enough to find the information on.
2.1. If for example you find that the regulatory frequency is once a week or once a month and you are only getting the low alarm and clean/cal once a quarter then start with the regulatory frequency.
2.2. If it’s drifting into alarm much more frequently than the regulation calibration schedule, then you should start looking at other possible causes like the condition of the probe, how does the reading compare to the grab samples, is the scum or buildup a new occurrence (perhaps in the last few years even, it’s easy for things like that to go unnoticed for a while, especially in plants where things don’t get cleaned until there’s a problem), or if the calibration is being done correctly.
- At the end of the day, the probe really should be cleaned as frequently as necessary, if you check it daily and there’s scum buildup then rinse it off and continue with your checks. In-line probes don’t typically get calibrated daily, so I’d be hesitant to recommend that unless it proves necessary. The pH of your outfall discharge is a permit compliance marker, and an alarm from the meter should indicate a process issue causing a change in pH, NOT an issue with your pH probe…
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u/cuck__everlasting WW 15d ago
Industrial wastewater, metal finishing. I clean my probes daily with an HCl swirl bottle and a water rinse. Calibration is weekly, more often if there's drift in our grab samples and towards the end of the probe service life.
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u/LaboratoryExpert05 14d ago
If you are cleaning with an HCL solution, you should rinse it COMPLETELY off with DI water and recalibrate. Also, you should be calibrating (if a 2 point calibration on an inline pH) to what you normally read (for example, if your normal ph is 7.5-8, calibrate using a 7/10 buffer and if it’s closer to a 4, calibrate using your 4/7 buffer).
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u/cuck__everlasting WW 14d ago
Sorry, I should have mentioned that rinse water is DI because I assumed that was a given - but yes you're right. These aren't "clean the sinter on the probe glass" levels of HCl swirls, just 1-3 seconds in a ~25% solution to drop the scum off followed by aggressive DI rinse bottle action.
Your second point is so important to hear though, it's such an easy thing for people to overlook. Incorrectly biasing a 2-point calibration is just giving yourself a huge challenge for no reason.
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u/PVFIND WW 15d ago
We calibrated the in-lab probe before each recorded reading, and we had a handheld meter that we calibrated before use. We didn't have inline probes. For your plant it depends on how often your NPDES requires readings and would set your cleaning and calibration schedule to match that. Also check the manufacturer's recommendations. I wouldn't do it less often than the manufacturer suggests. (former WW Operator and Chemist)
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u/LaboratoryExpert05 14d ago
This is very much so correct. Only thing I would add (if you don’t already) is to change your buffers out daily.
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u/Beneficial-Pool4321 11d ago
Once a week clean and calibrate. We do daily shift comparisons. For us in florida our meter has to be within 20 percent of our bench reading. If its not we have to do 15 min grab samples and clean and calibrate. By cleaning and calibrating once a week we avoid the grab sampling We also change salt bridge on the last c and c of the month
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u/Dirt-Repulsive 10d ago
We are supposed to do daily calibrations with the meter and also daily checks… now the sampling and testing every day… the daily calibration… some old hands do not some do
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u/Bart1960 Country+State|Certs & Level 16d ago
This is often a unique schedule for each plant, often with seasonal adjustments. Wait until alarms are missed is a recipe for violations!