Ph sensor

jyoung

I am building an automated home brewery for beer and I need to monitor the Ph. Has anybody ever tried to interface a Ph sensor from maybe a desktop, or handheld Ph Meter? Any ideas for a sensor would be appreaciated.

rjo_

how much do you want to spend?

jyoung

I would lilke to stay around $100. It is not a necessity but kind of a luxury for a homebrew setup.

rjo_

and what level of accuracy?

jyoung

I am not really sure the terms for ph but 1 decimal point would be fine. ex. 7.0 to 7.1

Ale

(buy yourself one). pHmeters are not _that_ simple. They need calibration. Basically is a very high impedance ADC, between -1 and 1 V. sort of 200 mV per pH unit. Use jfet OPAmps to get very high impedance, you will need three, 2 as voltage-followers to the inputs of the third in differential mode. And a Slow ADC, maybe a sigma-delta ADC made with the prop can work. Do not forget the calibration, for pH around 7 you will need a pH 7 buffer, made with monohydrogenphosphat and dihydrogenphosphat sodium salts. You can buy it already done, from Fluka or Sigma-Aldrich, Calibration is a correction curve. It changes with temperature as you will expect, and teep the electrode wet in KCl solution 4M I think, if I do remember correctly. Electrodes are expensive, and should be treated with care. You need a glass electrode, also known as pH electrode, or AgCl/KCl electrode,

Have fun

tperkins

Ale said...
you will need three, 2 as voltage-followers to the inputs of the third in differential mode.

Instrumentation amp, right?

Ale

tperkins said...
Instrumentation amp, right?

, sure, but do not forget the impedance !, go the the metrohm site and have a look at a datasheet of a glass electrode, that will give you an idea. (A TL084 could be enough, due to the low speed). An electrode can take some 30 seconds (or more) to reach equilibrium.

rjo_

If you wanted to do something different, really cheap, with many brewing applications, you could build a resonant circuit... the core development would be similar to an NMR magnetometer, which sounds fairly complicated, but all of the software techniques you would need are available either here in the forum or in the lab sections. The pH would be correlated with a shift in one of the primary resonance frequencies.

Why just measure pH... when you could measure pH and just about everything else that makes for a good beer?

Tracy Allen

A good source for process oriented pH probes is SENSOREX. Particularly the flat surface electrodes. They offer preamps/transmitters and other accessories too.

The output is single-ended from a BNC connector, and it only takes a single CMOS input op-amp to inferface to the high impedance pH probe. It must be able to swing both (+) and (-) to cover the full pH range, but the amplifier can provide an offset for an ADC attached to the Prop. The gain and offset of the amplifier (or the software) have to be variable for calibration and standardization.

Moreover, the trickiest thing about pH instrumentation is interactions. It is one thing to have a pH probe on a hand-held instrument, and quite another to have the instrument hooked up to other sensors and to the rest of the world. In troublesome situations, pH instrumentation uses isolation amplifiers to break the ground loops. Or it uses a sampling technique that draws samples of the medium (beer) into an isolated container for the measurement. In relatively small batches that might not be a troublesome issue at all.

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Tracy Allen
www.emesystems.com