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MagicJigPipe
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[*] posted on 19-3-2010 at 21:06
pH Probe Computer Interface?


I would like a pH probe (possibly even one with a temp. probe built in) and some sort of interface so that I could connect it to my computer. I figured that since I already have a computer, why buy a small device (i.e. computer) that will do the same thing except less awesome-like. I could then just use software to collect pH and temperature data.

I have found this interface so far (I posted the wrong link prior to this edit):

http://www.phidgets.com/products.php?product_id=1058

And possibly one of these probes:

http://www.omega.com/ppt/pptsc.asp?ref=PHE2100_2700_3000_990...

I just figured I would check here for knowledge of better deals, quality and ideas. Is this feasible? Would I be able to do at least one reading per second with this setup and have it display on my computer (and of course save the readings to a file). I know this software exists but will it work with this interface (for example Vernier Logger Pro)? I do know some C (language) but I doubt I would be able to whip something up on my own. All I know how to do is program calculations--no hardware interface stuff.

Also, if possible the software should compensate for temperature (if I can find a decently priced temp/pH probe).

Thanks.

P.S. Why will pH meters not read highly concentrated, strong acids (negative pH)? What is the mechanism most responsible for this limitation?

[Edited on 3-20-2010 by MagicJigPipe]




"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think, free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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[*] posted on 20-3-2010 at 19:29


Did you ever thought about a data aquisiton systhem? This is pready much what you should get if you want to save data in your computer.

This is some of the places that you can find them:

http://labjack.com/
http://www.dataq.com/
http://sine.ni.com/nips/cds/view/p/lang/en/nid/14605

You can get a simple USB data logger or you can buy the DAQ and record more that one type of data and also interact with the results (output Voltage). The problem with the DAQ is that you need Signal Conditioning and this thing can cost a lot.

If you want a good program to capture the data and interact with them, I think Labview is the best. The price is really high but you will be able to find free torrent of the program.
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not_important
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[*] posted on 20-3-2010 at 21:16


The problem with general purpose A/D device is that they have too low input impedance to function with pH probes. The probes have resistances on th order of 50 to 1000 (thousand) megohms, so the amplifier-A/D needs to be higher than that. You're looking at input currents of fractions of a pA, impedance something like 10^13 ohms.

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MagicJigPipe
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[*] posted on 20-3-2010 at 22:02


So I could get a LabJack, a pH probe and some sort of buffer/amplifier for the probe? I suppose the advantage of this would be that the LabJack could be used for lots of other things. This is pretty cool I must say. This will be a little more than I wanted to pay but hopefully it will be worth it. I suppose I could just make my own amplifier for the probe but I am not good enough with electronics to design my own fancy amplifier. Could I not just use a single op-amp? What would be the disadvantages of something that simple?

Fractions of a pA? 10^13 ohms! ! ! ! I can see why but that just seems unimaginably tiny/large.

Thanks for the info.



[Edited on 3-21-2010 by MagicJigPipe]




"There must be no barriers to freedom of inquiry ... There is no place for dogma in science. The scientist is free, and must be free to ask any question, to doubt any assertion, to seek for any evidence, to correct any errors. ... We know that the only way to avoid error is to detect it and that the only way to detect it is to be free to inquire. And we know that as long as men are free to ask what they must, free to say what they think, free to think what they will, freedom can never be lost, and science can never regress." -J. Robert Oppenheimer
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[*] posted on 21-3-2010 at 01:59


pH DATALOGGERs are available for OK money in the UK, this is a site (Timstar) selling stuff for school labs. Go search your country`
TURTLE pH LOGGER. A computer interface that turns your PC
into a pH meter. The pH Turtle has easy to use Windows® software that controls all the functions and allows the user to log and graph results.
Calibration can be at 1, 2 or 3 points through the PC. Min-Max alarms can be set to give either a visual or audible alarm. Temperature compensation is manual. Range 0-14pH. Size: 80mm dia. Supplied with pH electrode, amplifer, Windows® Software and 1M cable with RS232 connector.
PH25020 pH Turtle £49.00/ea.
PH25022 Spare Electrode HI1333B £34.94/ea.
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[*] posted on 21-3-2010 at 02:24


Computer is way cooler than logger, since it permits to also act upon the readings, eg. put in more acid or base ...
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[*] posted on 21-3-2010 at 04:12


The Turtle logger is a PC interface module with software, The OP wanted and interface from pH probe to computer. Now if you have to rework the software etc to get automation for processes.....
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[*] posted on 21-3-2010 at 04:20


I would go for some standard linux-setup ; eg. what I have working is:
==> Data in via good Voltmeter (RS 232), any sensor can be attached ..., pH-Probe probably somehow too ...
==> Relay out via parallel port, 8 relays can be switched on and off (eg. also connecting/disconnecting different sensors to the voltmeter, though of the same type ...)

Used it for temp-control in my furnaces ..., also for stirring: Just switching on/off one of the relays once in a while (stirrer was drilling-engine + concrete-stirrer, 1 buck in the hardware-shop) ...
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[*] posted on 21-3-2010 at 04:46


The high impedance is because you want the input not to load the device being read to a appreciable degree. If the resistance of the probe was 10^9 ohms then 10^11 ohm input would be 1%, giving a noticeable even if not large error. And we're talking about the resistance through a piece of glass.

Ordinary op amps have nowhere near that Z-in. Take a look at something like the Analog Devices AD549, data sheet should be on-line. Remember that this is no ordinary circuit, keeping surface leakage down low enough is a bit tricky.

Quote:
The circuit in Figure 46 illustrates the use of the AD549 as a pH probe amplifier. As with other electrometer applications, the use of guarding, shielding, and Teflon standoffs is necessary to capitalize on the AD549 low input current. If an AD549L (60 fA maximum input current) is used, the error contributed by the input current is held below 60 μV for pH electrode source impedances up to 10^9 Ω.



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[*] posted on 21-3-2010 at 05:21


The impedance ist the problem; but before the digitalization-step in the probe there must be a point in the circuit, where the signal is amplified enough ..., just before the a/d-converter-chip ...
==> ... that would be the point where to solder 2 wires for connecting the digital multimeter ...

As I recall the name of the sortware: cdmm
http://linux.softpedia.com/get/Utilities/cdmm-23916.shtml

It can be made to give the read-out-values on the command-line; those are then fed into the selfmade software ...

Just use "cdmm", not the "QTdmm" (which is only useful for collecting date and plotting ...)
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[*] posted on 21-3-2010 at 07:16
pH Meter Schematics


Tiny pH-Meter EXCELLENT !!
http://damien.douxchamps.net/elec/ph_meter/

pH Meter Schematics (Copy and Past the Link into Your Browser):
http://images.google.nl/images?hl=nl&client=firefox-a&am...:official&channel=s&resnum=0&q=ph+meter+schematic&um=1&ie=UTF-8&ei=Yi-mS5XyOsmG4Qaxld2gCg&sa=X&oi=image_result_group&a mp;ct=title&resnum=4&ved=0CCQQsAQwAw

pH Meter Construction:
http://www.ph-meter.info/pH-meter-construction

The Simplest Possible pH Meter: :o
http://www.66pacific.com/ph/simplest_ph.aspx

Simple pH Meter:
http://freecircuitdiagram.com/2009/05/21/simple-ph-meter/

pH Controller, uS-, Redox- and Temperature Meter Schematics (Dutch Link Page to Excellent English and Russian Schematic WebSites):
http://members.tele2.nl/rsetteur/aquarium/karel/ph/index_ph....

Convert your DMM to a pH Meter - By Bill Donofrio (EDN - 17 October 2002) EXCELLENT !!
http://www.edn.com/article/CA250813.html

And here you have the Article in PDF-Format:

Convert Your DMM to a pH Meter - By Bill Donofrio (EDN - 17 October 2002).pdf
Attachment: Convert Your DMM to a pH Meter - By Bill Donofrio (EDN - 17 October 2002).pdf (232kB)
This file has been downloaded 803 times

Note: The LM351 OpAmp mentioned in this Article should have been the LF351 (JFET Input Operational Amplifier) instead !!

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Lambda

[Edited on 22-3-2010 by Lambda]
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watson.fawkes
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[*] posted on 21-3-2010 at 10:48


Quote: Originally posted by not_important  
Ordinary op amps have nowhere near that Z-in. Take a look at something like the Analog Devices AD549, data sheet should be on-line.
Data sheet for AD549. Input bias current 150 fA. Input impedance 10 TΩ.

A warning note to those unfamiliar with these devices: They're sensitive to static. Handle with proper static dissipation gear. There will be nothing directly detectable by ordinary human senses when you blow the device. In all likelihood, it will become flaky rather than completely non-functional, further complicating your life.
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MagicJigPipe
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[*] posted on 21-3-2010 at 12:10


So, can I really "request samples" (and actually receive them) from Analog? Maxim also seems to have this option. I have my 'account' already set up with Mouser though, I wonder if they have it or something similar?

Looks like I'll be going with the Dataq DI-148U or the LabJack U12.

I will probably end up doing the amplifier myself... I might need some help with all that, though, once the stuff gets here.

As for the software... Does anyone here have something from Dataq? If so, do you have the WinDaq software that you could "sell" me? (if I end up choosing that one of course)

Any more opinions/advice would be great as this is all new to me.

I'm also pretty excited about the data acquisition hardware thing. I never knew there was stuff like this that was affordable. So, pretty much any instrument whose output is in varying voltage I can use with this? How cool is that. Now I'll be looking for reasons to get different devices... great...

Thanks for the help.

EDIT
Also, chief, did you write your own code to control this? If so, maybe you could teach me how to control a parallel or serial port with C ;) Trying to get this stuff to work in Windows will already be somewhat of a 'programming challenge' for me. Linux, which I am only slightly familiar with, would be much more difficult (for me at least).




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[*] posted on 21-3-2010 at 13:10


Be careful, you can easily make an amplifier, but it will not protect your systheme of the electric noise...
If you want a good program, go get Labview. It work with all DAQ and if you can find the *pro* one, you can directly add noise reducer in your systheme on you cumputer.
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[*] posted on 21-3-2010 at 16:09


Apropos amplifiers, Burr-Brown/TI has the OPA129. Input bias current 30 fA (max 100 fA), input impedance 10 TΩ.
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[*] posted on 22-3-2010 at 00:13
Forum Discussion and Article by the Author of the "pH Amplifier for Micro"


pH Amplifier for Micro Forum Link:
http://www.electro-tech-online.com/electronic-projects/41430...

The Article:

pH Probe for the PIC Temperature Controller.pdf
Attachment: pH Probe for the PIC Temperature Controller.pdf (730kB)
This file has been downloaded 775 times

And here You have the Corrected Schematic:

pH Amplifier Schematic


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[*] posted on 22-3-2010 at 04:13


And you'll note the specs on the input amp LF356:
Low input bias current: 30pA
Low Input Offset Current: 3pA
High input impedance: 10^12 ohm

A caution on this type of JFET input amps. The input current is somewhat temperature sensitive, allow good natural air flow and don't put hot components where their radiated or convected heat will warm the op amp.

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[*] posted on 22-3-2010 at 12:25


I'm told that the really cool thing to use as a buffer amplifier is the FP54.
The bias current is about 10^-17 A and the open loop gain is about 10^6
It's relatively immune to static damage too.


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[*] posted on 23-3-2010 at 18:05
Using Your PC Sound Card for Measuring Signals


Sound Card Based Multimeter:
http://www.qsl.net/om3cph/sb/dcwithsb.htm

Impedance Measurement using a Sound Card:
http://www.icom.hsr.ch/uploads/media/RLC_Meter_EN_01.pdf

Measuring DC with a Sound Card (Note: A Sound Card Captures AC Voltage through the Microphone Jacks due to the Coupling Capacitors used):
http://www.radiosky.com/skypipehelp/V2/using_a_sound_card_to...

CheapChop: Measuring DC with a Sound Card INTERESTING !!
http://lea.hamradio.si/~s57uuu/scdsp/CheapChop/cheapchop.htm

There are many Programs that can be used to do measurements via a Sound Card (and You can also use the Outputs to give the Generation of Preprogrammed Output Signals).

Sound card Oscilloscope:
http://www.fileheap.com/dbquery/1/sound+card+oscilloscope

And here:

Voltage Measurement On Oscilloscope Software:
http://www.filebuzz.com/findsoftware/Voltage_Measurement_On_...

But, they can only measure AC Voltages. So You will have to Interface the DC Voltage Output of a pH Meter Interface so that the PC Oscilloscope with see this as an AC Voltage Input Signal. A Voltage Controlled Oscillator (VCO) can do the Job ...

DC to AC converter:
http://acoustimed.co.za/equipment.htm

And maybe you can find more here:

Measuring DC with the Sound Card (Copy and Past this Link into You Browser):
http://images.google.nl/images?hl=nl&client=firefox-a&am...:official&channel=s&q=measuring%20DC%20with%20the%20sound%20card&um=1&ie=UTF-8&sa=N&tab=wi

And here:

VCO:
http://images.google.nl/images?um=1&hl=nl&client=fir...

Maybe this kind of setup will give You a "Cheap Sweep" ;)

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[*] posted on 23-3-2010 at 18:53


You'll still need the high impedance input amp, and then either a chopper or a VCO plus software to translate the AC input into its DC equivalent. Cheap VCOs may not have the linearity and resolution needed. For those like me who are at ease with building `tronics, not a big deal; if you have to buy the stuff it likely will cost more than an entire small pH meter.

After that you need to calibrate the assembly, likely before every session, meaning a reasonably accurate voltage reference.

The FP54 may not be easy to buy these days, and they are somewhat power hungry compared say to a LF356 or LMC6081. True, the FP54 is tougher in regards to overloads and input zaps, but it is vulnerable to microphonics. ;-)


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[*] posted on 23-3-2010 at 21:28


Quote: Originally posted by not_important  
The FP54 may not be easy to buy these days, and they are somewhat power hungry compared say to a LF356 or LMC6081.
That was my clue to find this part number, which I had been previously unable to, because it's for a tube. On the other hand, this article describes the part as "new" in 1934, but they replaced it with the even newer, and doubtless more improved, part D96475, so now you have an alternate source.
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[*] posted on 23-3-2010 at 22:18



http://www.oddmix.com/tubes/931_beckman.html

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[*] posted on 24-3-2010 at 18:34
FP-54 Datasheet


The FP-54 has a Sensitivity of 10^17 Amperes, which corresponds to about 60 Electrons per Second !! :o

Here You have the Datasheet:

FP-54 (General Electric ETI-160 - 06-09-1944).pdf
Attachment: FP-54 (General Electric ETI-160 - 06-09-1944).pdf (1020kB)
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[*] posted on 24-3-2010 at 18:43


I have just started to do some programming with some of the Phidget stuff (although not pH meters), and my initial impression is that their stuff works well, I like it.

If I wanted to program a pH meter that is where I personally would look now.

I am programming to the low-level C interface, which is still pretty straightforward. They also have a COM object that you can use, which can be, depending:
a) Easier if your development environment knows how to automatically work with COM stuff (e.g. Visual Basic, C#), or
b) Insanely more complicated and confusing if you have to manage it my hand (native C code).
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