Yet they read the same when dipped in 7.0 calibration fluid
This is baffling me.
My res is at 6.0 with my new doser but my combo is coming up 6.2.
I take both probes out and rinse and dry and put them into 7.0 calibration fluid and they both say 7.0
I tried replacing the combo meter probe cause the old probe was over a year old and it has gave me wonky readings when they were old before.
The calibration for PH meters usually employ something of a straight line curve with the 7.0 being at the center (0 mV). Any voltage at PH 7.0 is an offset.
The slope and offset of the curve (a straight line) will determine the readings at locations other than 7.0. Depending on what the values are stored at the other calibration points, let say 4.0 or 10.0, the slope of the line can differ.
For instance, here is a graphic of where they take two far end points to correct calibrate the meter/probe combo. Correction is the dashed line.
You can imagine a case where one of the points acts a bit differently causing the slope of the corrected line to change. The value at the 7.0 might be accurate but as you move along the slope to different PH values farther away from the 7.0, the error increases.
There could be some different reasons perhaps, thinking out-loud, 1) old or contaminated calibration solution 2) something in the solution is interfering with the PH chemistry of the probe 3) one of the points on the calibration curve is incorrect. e.g. need to re-run full cal. 4) temperature sensor is not calibrated or temperature has not stabilized when reading 5) probe needs to be cleaned or replaced.
Note that for three point calibration, if the manufacturer is using a straight line curve for compensation, two points define a line. Now, with three point calibration, this can actually throw off one end of the PH scale if all of the point do not naturally fall on a straight line. But, they should be close.
One might try changing the order of the calibration if doing a three point cal. This entirely depends on how the meter calculates the compensation which I don’t know. If they recalculate on each new data point, it “may” make a difference at least for one-half of the curve. For instance, if the order was 7-10-4, try 7-4-10.
Here are some general thoughts on what can effect the accuracy of PH probes / meters:
Have you tried doing a two point calibration?
I have found that just doing a single point calibration just isnt good enough for me. I always calibrate at 4 and 7. That way everything between those two points should be much closer.
Also, dont know what meter you have, but my Apera needed to be re-booted a while back. There is an option in the menu to re-set to factory default. Doing that plus gentle cleaning of the bulb got mine working great again.
@Northern_Loki beet me to it - and did a better job of explaining it 
Other meters may well be different - so RTFM is important, but my Apera wont let me do it out of sequence. Its programming expects the data points to be done in a specific order. For two point, its 7, then 4. For three point, you do 7, 4, and then 10 or some other value last. It wont calibrate at all unless it senses that the calibration solution is in the range it is expecting.
My combo meter does only 2 points and my doser does two as well but let’s me choose 7.0 and either 4.0 or 10.0
Both probes are less than 2 months old.
I have calibrated my combo over 8 times in the past month because of this.
Each time I do this I also do my doser since the solution is already poured and the doser is always on the money 4.0 and 7.0. It’s like clockwork the combo always turns out 0.2 pH higher than the doser. I calibrated it back and put it back in the reservoir and even sometimes right after a calibration it’s off again. Usually a few days later it drifts off course.
New calibration solution is filled every time. Kind of pissed cause I’m going through so much of it.
In my DWC I’m not too concerned with pH since it drifts so if I set it to 5.5 and it’s really 5.7 I will drop it when it’s 6.1 when it’s really 6.3 but 5.7 to 6.3 is still covering all the nutrients.
What I have an issue with is not being accurate for single dose feedings like starter cubes or coco.
The probe is a Milwaukee. But it’s the same style probe as the Bluelab. So this shouldn’t matter.
Yeah mine are the same way. One called button. And it directs you which one to use first.
You might try cleaning it gently and then re-set to factory defaults and re-calibrate. If that doesnt work, maybe you got a dud? Wont hurt to call customer support and see what they say.
Bluelab would say “should have bought a Bluelab replacement probe” and I’m sure Milwaukee would say the reverse lol.
So I calibrated last night and was reading fine. Today it says 6.4 when the doser says 6.2 lol.
The doser probe is 2 months old and the Milwaukee is < a month
I’m not sure if the combo can be reset. I’ll take a look.
Try cleaning and soaking your probe in pH 4 over night, then recalibrate and see if that might help.
LOL Yeah, but maybe they are correct 
From that link @Northern_Loki posted above:
The electrodes are usually connected using cables. Different cables are manufactured by different manufacturers and thus, incompatibility may arise between different pH meters of different manufacturers. An O-ring can be used in the connection between the electrode and cable to prevent the entry of moisture.
They are both gel filled double junction with BNC connectors.
I’ll try the 4.0 soak.
I used my pen for the first time in a year or so a few months back and it was still within .2 and calibrated well. Response was really slow so I used the 2-part cleaning sol’ns I have and it worked liked new again. It’s kept in proper storage sol’n since new about 7 years ago.
I only calibrate with pre-mixed 4 and 7. Really no need to use 10 as it’s so far out of the range we use that accuracy isn’t important in that range.
If you now your pen is always .2 high then I’d just take that into account when using it and not bother recalibrating all the time. I was using AN nutes with pH perfect and RO water so didn’t check pH ever. Just ppm. Even in soilless I didn’t bother checking. Now using MegaCrop too and it checks out at 6.0 in RO all mixed up so no need to check that either. Comes out at 6.5 using my dugout tap water that’s pH 8+ and around 400ppm.
How much pH Up and Down do you go thru with the doser? I’ve read they can be using lots trying to keep the pH at a set point. I’ve had the same two bottles of up and down for over 10 years and they are both near full still.
:peace;
It does use a bit if u have it set on auto all the time.
How I use mine is I set it in control mode once pH hits 6.1 and that brings it down to 5.6 pH and then I set it back to monitor mode and repeat once I gets to 6.1 again.
That sounds like a much better way to use one of those things.
