I have owned my MW-101 pH meter for just over 1.5 yrs now and fully have enjoyed using it.
In the past few months (4-5) I have been noticing that it has been taking a considerably longer time to calibrate on the first 7.0 buffer. Almost up to 10 minutes. Then when I go to calibrate the 4.0 solution it takes a normal amount of time (about 1 minute give or take).
I typically brew about 20-25 batches (sometimes more) per year and have taken great care with the probe always rinsing with distilled water, only using it at room temps, and storing it in proper storage solution.
Do these probes really only last about 1 solid year of brewing? Has anyone else experienced these fluctuations?
I just yesterday ordered another probe and am hoping things will go better with it. Just kind of sucks that that is a yearly (or 1.5 yr) expense when using these.
I have that same meter. After awhile, it started becoming very difficult to calibrate so I replaced the probe (I can’t remember exactly how long ago that was, maybe 18 months?).
That solved the issue, but now it is starting to happen again.
I know a lot of people like these meters, but the manual calibration drives me nuts. I’m about to replace it with an Extech 110.
My new Extech 110 was acting wonky the other day. It was giving me questionable readings and seemed to drift over the course of a couple of hours. As a test I checked some distilled vinegar and it was at 2.9 after calibration. I dumped and refilled the probe and it’s dead on 2.4. Very happy with that feature.
so it really seems like you (at best) get 1.5 yrs out of these probes depending on how often one uses them.
Kind of crappy as those replacement probes are about 40 bucks a shot.
Good to know that solved the issue. Looking forward to “nailing” my pH’s again. Or at least verifying that I am.
So when do you guys take your ph readings? If its at the 15 min mark in the mash do you draw a sample and let it cool before reading with your meter? I noticed the thermoworks model im looking at says it has temperature compensation built in.
I’m sure there is a wide range of qualities when it comes to these probes. They are a standard used by many units and manufactured by multiple companies.
I know a few people here have that meter and like it. I almost bought this meter, but switched to the Extech because it measures to the hundredths and also because I had an Amazon gift card that was “to be used on something fun that I normally wouldn’t buy on my own”
On average we state that a pH electrode will last 1-2 years. It is very possible to have a probe work longer. As long a pH electrode has a good offset (mV value in pH 7.01) and slope (mV difference from pH 4- pH 7) then the probe can be used.
A good offset value is +/- 30 mV while slope should be greater than 160 mV (90%).
Since the meter is calibrated manually and it does not have a GLP feature it can be difficult to determine the pH electrode characteristic. I would recommend the following:
Short the BNC connector on the meter with a paperclip. That is put one end in the center hole and touch the outside of the connector. This is 0 mV.
Dial pH 7.01 calibration trimmer until pH 7.01 is displayed
Reconnect the pH electrode to the meter and place in pH 7.01 fresh buffer
Record the pH value in the buffer.
It is now possible to determine the offset of the pH electrode. Ideally the meter would display pH 7.01 since the theoretical mV at pH 7.01 is 0 mV at 25 oC. An acceptable offset of +/-30 mV would be displayed as a pH from 6.5 to 7.5. Meters will typically allow a calibration of +/- 60 mV which would show as a pH reading around 6 (+60 mV) and 8 (-60 mV).
If you see reading outside pH 6.5 to 7.5 then the electrode needs to be cleaned. Any coating on the glass will shift the voltage read by the probe.
Instability in readings that occurs with aging are due to various factors including:
Dirty electrode – as there is a build up on the glass the measurement circuit is impeded.
Clogged junction – on the side of the probe should be a white plug. That is the junction. When it gets clogged the measurement circuit gets impeded.
Storage - It is very important to store the pH electrode in storage solution. The storage solution maintains a hydrated layer on the glass bulb, hydration of the junction, and hydration of the gel used inside the probe.
I cannot state for Thermoworks but as far as temperature goes:
Most pH electrodes are made of General Purpose (GP) glass. The rule of thumb for GP glass is
At 25 oC expect 1-2 years
At 50 oC expect 6 month to 1 year
At 75 oC expect 3-6 months
At 100 oC expect less than 1 month.
The guidelines are for probes in continuous use. I would expect a longer time for probes that are used intermittently.
It would be recommended to cool samples to less than 140 oF (60 oC). If it is planned to measure at a higher temperature then a pH electrode with high temperature (HT) glass would be recommended. The HT glass has a higher resistance at 25 oC than GP glass. As the temperature increases then resistance decreases. The HT glass resistance at high temperature approaches that of GP glass at an ambient temperature.
The temperature compensation is used for voltage response as according to the Nernst equation for membrane potentials. For a meter with manual temperature compensation it is important to adjust the calibration trimmer to match the value on the bottle at a specific temperature. The value on the bottle is the actual pH of that solution at a given temperature. For example pH 7.01 and 4.01 at 25oC.
The offset check (pH 7.01) will work with any meter with a BNC connector. Once the connector is shorted then a o mV signal is being sent to the meter.
Many meters, including testers, that have automatic calibration have the ability to clear the calibration data that has been stored. A meter with a default calibration will be set to 0 mV = pH 7.01 and +178 mV = pH 4.01. From a default calibration it is then possible to determine both offset and slope.
The original post was for a meter that was calibrated manually. Since it has a BNC connector it was at least possible to check the offset. The slope would be difficult to check since a +178 mV signal from a simulator would have to be driven to the meter to adjust the pH 4 trimmer. Since more than likely a simulator is not available that the slope check was left out.
The offset is very helpful since a high offset value is an indicator that the probe needs to be cleaned.
If you have a meter with auto calibration and it can be cleared to a default then knowing the readings in buffers will allow to calculate both offset and slope.
Feel free to message me and I can walk you through how perform the calculation. It is based on the Nernst equation in which theoretically a pH of 7.01 is 0 mV and each pH unit generate 59.16 mV at 25 oC. Acids the pH increases positive and bases the increase is negative. A pH electrode with 100% slope will generate +59 mV in pH 6 while in pH 5 will generate +118 mV. It would be recommended to make sure that the probe has better than 90% slope or 54 mV/pH.
For example, on default calibration my pH probe reads pH 7.25 in pH 7.01 buffer and pH 4.5 in in pH 4.01 buffer.
*pH 7.25 is 1/4 pH unit away on the base side so the voltage being generated is -15 mV. Well within +/-30 mV for an offset.
pH 4.5 is 2.5 pH units away from pH 7.00 which in mV that would equal 59 * 2.5 = 147.5 mV. But since the offset is -15 mV then the mV difference is 147.5 - (-15) = 162.5 mV. 162.5/178 x 100 = 91.3% slope. The slope is above 90%.
If in the same example the pH meter read pH 6.75 in pH 7 for a default cal then the voltage would be +15 mV (more H+ = increase in + voltage). The slope would be 147.5 - 15 mV = 132.5 mV. 132.5/178 x 100 = 74.4% slope. Most meters will not calibrate less than 85% slope but if it did then you definitely should not use the probe since the slope is too low.
The numbers above are approximate since Nernst dictates 59.16 mV/pH at 25 oC. I have rounded to 59 for convenience. Either way, knowing how to approximate the offset and slope will help you understand the health of the pH electrode. To achieve an accuracy >0.1 pH it is critical to know.
The accuracy statement of the meters stated by the manufacturers is the meter itself and not the system which includes the probe. I can achieve a higher accuracy from a $50 tester than from a $600-1K benchtop if the probe of the tester has a good offset/slope and the benchtop does not.
General rule:
New pH electrodes: +/- 10 mV offset and 95-105% slope
User tolerance: +/- 30 mV offset and slope greater than 90%
Meter limits: +/- 60 mV offset and slope greater than 85%
It is possible to calibrate a probe that is outside the limits of use since the window for calibration is pretty wide. At that point it is like throwing darts and were hitting the dart board. To hit a triple 20 you want to be well within the user tolerance range.
Another way to look at the great info from pfabsits is that when you’ve calibrated your meter and then disconnect the probe and perform the shorting technique mentioned above, the meter should read between 6.5 and 7.5.
I just checked my meter and my shorted reading was 6.57, so I’m getting near the end of that probe’s life. Not too bad for almost 5 yrs of use.
Mine bounced around between 0.75 and 4.75, then seemed to settle near 3.8, but it’s been almost two weeks since I last calibrated it, and it has never held calibration between uses.
Cool trick though, thanks to pfabsits for all the info.
