pH Meter Temperature Compensation

Ever since I was a kid measuring the pH of the acid rain puddles in my yard with my tongue, I've wanted a pH meter...

The problem I see is that if you want to get an accurate measurement you need to compensate for temperature of the mash/wort/beer etc...

These people recommend the Hanna 98128 meter but the temperature range on that is 23F - 140F. Mash temps exceed this therefore you need to do some manual calcs.

A couple questions arise from this:

1.) Is the meter still accurate and usable outside of the temperature range if you correct the reading manually?

2.) How do you easily calculate the temperature compensated pH value?

3.) Is there a more appropriate meter for this purpose?

Unless a temperature is specified, standard temperature 25°C (77°F) is assumed. If you measure your mash pH hot, not only does the meter read differently, but the hot mash will be at a different pH than a standard sample. Also, AJ says high temperatures will shorten the life of the electrode.

So if you do this, you’re on your own. The only way to correlate your hot indication is to cool the sample and check it again. Why not just do it right in the first place?

I use a coffee cup sitting in a pan of water. It gets most of the way pretty quickly. I put the probe in when the cup no longer feels warm <100°F and it cools the rest of the way as the reading stabilizes. When the reading stops changing, I’m there.

Have fun with your new meter.

Wynne-R said:

Unless a temperature is specified, standard temperature 25°C (77°F) is assumed. If you measure your mash pH hot, not only does the meter read differently, but the hot mash will be at a different pH than a standard sample. Also, AJ says high temperatures will shorten the life of the electrode.

So if you do this, you&#8217;re on your own. The only way to correlate your hot indication is to cool the sample and check it again. Why not just do it right in the first place?

I use a coffee cup sitting in a pan of water. It gets most of the way pretty quickly. I put the probe in when the cup no longer feels warm <100°F and it cools the rest of the way as the reading stabilizes. When the reading stops changing, I&#8217;m there.

Have fun with your new meter.

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I don't currently own a meter.

The Hanna meter referenced has ATC (Automatic Temperature Compensation) but what about when you go outside the specified temperature range? How do you calculate for compensation? or can you?

ShootsNRoots said:

The Hanna meter referenced has ATC (Automatic Temperature Compensation) but what about when you go outside the specified temperature range?

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You don't. If the ATC range is limited it's a pretty good bet that operating the electrode outside that range will damage it. As Wynne-R has pointed out brewing mash pH measurements are all made at room temperature. Mash and wort samples are cooled before the electrode is immersed. This means you are not reading the pH at the reaction temperature but neither is anyone else and so it is possible for brewers to compare readings.

ShootsNRoots said:

How do you calculate for compensation? or can you?

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You are probably confused (as most of us were at one time) as to the two effects of temperature on pH measurment. One is that the pH itself changes with temperature. This is because the acid dissociation constants of the multiple acids in a mash are functions of temperature. The meter does not compensate for this - it can't as it doesn't have any information about what acids you are measuring, what their relative concentrations are ... Brewers have noted that the true pH of the mash seems to increase about 0.0055 pH per degree C temperature rise. You can, if you want to get an idea as to what actual pH in the mash tun is, correct your readings by subtracting 0.0055 times the difference between room and mash temperature from the room temperature reading.

The other side of the coin is that at any given pH the electrode produces a voltage E = Eo + s*(R*T/F)*(pH -7) where R and F are physical constants, Eo and s are numbers that depend on how old the electrode (which are determined by calibration with buffers at known pH) and T is the absolute temperature. The meter compensates for this change in voltage with temperature by solving this equation for pH given the calibration constants, its temperature measurement and its voltage reading. The details are at
http://www.wetnewf.org/pdfs/ph-meter-calibration.html.

You are probably confused (as most of us were at one time)

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That's what acid puddles will do to ya. Not to worry, I mostly do acid sheets now.

Bottom line, check at room temperature and try shoot for between 5.4 and 5.6 to start.

We have a lot of people doing it this way and it works. You WILL be assimilated....

You WILL be assimilated....

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That's what these strange beings in my backyard told me.... they had an awesome ride though... some kind of floating saucer type device...

It's $100 but I'm thinking of getting this:

http://www.phidgets.com/products.php?category=36&product_id=3551_0

and this:

http://www.phidgets.com/products.php?category=35&product_id=3108_1

and writing a little program to poll these devices while constantly monitoring the pH / temperature of the wort during the mash; displaying it in real time while correcting for the temperature via the equation provided in their documentation:

http://www.phidgets.com/docs/1130_User_Guide

Then, in addition to a real time display, a graph of the data can be made.

This should be interesting... Anyone have a spare $400 they can lend me? I'll pay you back, really I will...

The problem is your numbers won't match everyone else's who are doing room temperature readings so you will be starting over almost from scratch to collect data on conversion, beer quality, taste tests, etc... Will the correction be good for ALL worts? I found one equation that had so many terms to input my jaw dropped.

Yeah, we like to overkill hobby sometimes but you may want to find out how the probes life is de-rated at certain temps. Industry standards are based on cost benefit formulas. Burning up a probe every 2 hours makes sense when you are cooking up $10K worth of product at a time. Just because you can use these probes at high temps doesn't mean they will last.

ShootsNRoots said:

It's $100 but I'm thinking of getting this:

http://www.phidgets.com/products.php?category=36&product_id=3551_0

...

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The equation given at those links isn't of much use as it does not allow you to calibrate and without calibration you cannot do ATC properly. The proper equations are at http://wetnewf.org/pdfs/ph-meter-calibration.html. Beyond this you will need to figure out the mapping between the module output and sensed mV. It appears that their output may be offset binary and is probably scaled but it certainly isn't reporting mV directly as for pH < 7 mV increase as temperature goes up an pH goes down.

ShootsNRoots said:

This should be interesting... Anyone have a spare $400 they can lend me? I'll pay you back, really I will...

Click to expand...

ShootsNRoots said:

Look for meters that you can query and which can respond with millivolt and temperature readings. This makes doing the math pretty straight forward.

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ajdelange said:

The equation given at those links isn't of much use as it does not allow you to calibrate and without calibration you cannot do ATC properly. The proper equations are at http://wetnewf.org/pdfs/ph-meter-calibration.html. Beyond this you will need to figure out the mapping between the module output and sensed mV. It appears that their output may be offset binary and is probably scaled but it certainly isn't reporting mV directly as for pH < 7 mV increase as temperature goes up an pH goes down.

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They do state pretty much the same as you're saying, make sure it's calibrated and use the raw reading for maximum accuracy.

If you want maximum accuracy, you can use the RawSensorValue property from the PhidgetInterfaceKit. To adjust a formula, substitute (SensorValue) with (RawSensorValue / 4.095) If the sensor is being interfaced to your own Analog to Digital Converter and not a Phidget device, our formulas can be modified by replacing (SensorValue) with (Vin * 200). It is important to consider the voltage reference and input voltage range of your ADC for full accuracy and range.

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I don't see anything about calibration in that statement and I don't see any calibration parameters in their formula whatever you put into it.

Fortunately, there is only one requirement for success with the formulas at http://wetnewf.org/pdfs/ph-meter-calibration.html and that is that the reading, however it is scaled or offset, be a linear function of the voltage produced by the electrode. If you get actual millivolts the electrode voltage is
E = Eo - s*(R*T/F)(pH(T) - pHi)
with Eo being a few mV depending on the electrodes aging and s a number close to (a bit less than) 1 again depending on age and RT/F = 58.167 mV/pH at 293.15 K (20 °C). pHi is assumed to be 7 (though it actually isn't but should be close to it for most electrodes). The formula you are looking at assumes this but does not incorporate the other two calibration parameters :Eo and s. As such it can only produce approximate answers.

If you are going to write code I don't see much point in using an approximate formula when a much more robust one is readily available to you. Yes, you will have to also write code (doesn't have to be on the micro computer) that takes two mV readings (or 2 scaled mV readings) and two temperature readings (one temperature/mV pair for each buffer) and solves for s and Eo but that isn't rocket science. The formulas are there with the others.