PH and other types of BNC style probes

Brian Miller

I have searched and searched the internet and can't seem to find data on how to read a ph probe.

Does anyone have any expreience with these type of·devices? http://www.prominent.de/en/products/dulcotst/inhalt.htm

K de Jong

Hi Brian,

As far as I can remember from the time I was in the 'water-business' a PH probe gives a signal in millivolts, depending on the actual PH value.

Maybe this snippet of information may guide you to some experimenting with a DVM and a probe, don't forget the water to test with .

Regards,

Klaus

AlWilliamsAWC

What you are looking for is the Nernst equation. You measure a voltage, but the voltage difference is VERY small and the measurement is temperature sensitive. Also, most of the ph cells I have seen are finicky. They require calibration and special steps to keep the reference half cell in line.

http://www.eutechinst.com/techtips/tech-tips24.htm

http://www.omega.com/green/gsc.html#1

Regards,

Al Williams
AWC
8 Channels of PWM
http://www.awce.com/pak5.htm
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jamesem

Hi Brian,

From looking at the link you gave in your original post, that link does not show any pH sensors. I looked at all the PDF's on that page and they are for free chlorine, chlorine, ozone detection.

You might try Omega www.omega.com, Endress+Hauser www.endress.com, Hach www.hach.com, or Rosemount www.rosemount.com.

For the last four years, until I got laid off, I worked for a company that designed and installed control systems for water/wastewater tretment facilities and we used primarily Rosemount or Hach. These come in several pieces. One is the sensor itself, usually a long tube that is inserted into the medium. Then a device that attaches to the sensor to convert to a voltage that is then read by a head unit with a display and the ability to have an output of several types. We used the 4-20mA current output because then within reason, no matter how long the leads are from the sensor to our controller (PLC's) you still got 4-20 mA at the controller. At the controller end we used a 250 Ohm resistor to give us a 1-5 Volt reding whcih was scaled in our PLC or in the software that we used to display graphically on a computer screen.

We also had to buy calibration material to calibrate the sensors.

This all depends on your application and how much moeny you want to spend. Oviously, in the applications our company was doing this for, we spent a good deal of money for good sensors. Your application may not need anything this elaboaret or expensive. Omega may be your best bet for less expensive alternatives.

Hope this helped and have a great day!!

James E. Merritt

Helen C

Hi Brian,

pH sensors do give a mV output that is proportionate to the pH of your test solution.· However, the output is extremely high impedance so you cannot read it easily with a standard DVM.· In order to do the measurement with a DVM or AD converter you require a buffer amplifier.· You could build your own or you could buy a very neat sensor that has one built in.· These are made by Hanna Instruments and are called AmpHel electrodes.· They give an output of approximately·59 mV per pH unit.· They are more expensive than other pH sensors but they are much easier to use!

Look at [noparse]:http:[/noparse]//www.hannainst.com/

Good luck,

Helen

Tracy Allen

You might find some information of interest here:

www.emesystems.com/OL2ph.htm

I like the industrial probes from Sensorex. www.sensorex.com They also have preamps and transmitters. The big issue for pH instrumentation will be whether or not you need isolation. If you are measuring pH in a beaker or sampler the answer is probably "no", but if it is directly in a lake or a big interconnected system, the answer is probably "yes".

-- Tracy

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Tracy Allen
www.emesystems.com

Brian Miller

Thanks for all the replies, that is great info. I didn't get the email notification. I guess I need to update my configuration.

Brian Miller

Ok, been reading all the information. My application will be for reef (saltwater) aquarium use. I'm going to need 2 ph probes. One to monitor the Ph of the general system and one to monitor the PH in a Ca Reactor used to dissolved crushed coral via Co2 injection to provide Ca for coral growth. Thus needing the Ph monitoring to control the Co2 injection into the reactor. Too much Co2 will drop the system Ph, too little and the Ca Reactor will loss efficience.

jamesem

I like the system you described with a 1-5 volt reading. You mentioned that there was a device you attached to the probe and used a 250 ohm resistor. Was that a custom device or an off the shelf solution from one of the manufactures you listed?


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jamesem

Hi Brian,

That system I described was an off the shelf solution. Depending on the manufacturer, if you look at the specification sheets you may have to buy three separate pieces. There is the probe that has the actual pH probe. The second is the piece that the probe plugs into and has a cable attached, sometimes these can be custom lengths or they can be ordered in predefined lengths. The third device is a transmitter that houses the electronics, sometimes there is a display, and also has various output options. We used the 4-20 mA option because that is standard in industrial applications. Less signal loss in long lead lengths and good noise immunity, since this is current instead of voltage. The 250 ohm resistor was to convert the 4-20ma current to 1-5 V dc. going into the analog card of the PLC. But you should be able to do this with a simple A/D converter

Since your application is not heavy industrial use, most of the manufacturers I suggested may be kind of expensive. So don't be too shocked when you see the price..smile. Also, some of these might even have one transmitter and allow the use of multiple probes. Depending on if your aquariums are close enough. Check out Omega, they have some inexpensive pH 4-20mA transmitters (PHTX-212, for instance) for less than $200. You still need to buy the probes (PHE-6461-3, for instance), also about $125.00.

Hope this helps.

James E. Merritt

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James E. Merritt
BSEE, AAS- Hi Tech Electronics/Robotics

Brian Miller

What about this product from omega.com? Would the output of this be fairly easy to read with a basic stamp?

http://www.omega.com/Green/pdf/PHE6300_5300_2114_1304.pdf

SperDirect

At Sper Scientific, we carry several excellent PH meters and refractometers that are extremely easy to use and very affordable. Here's a link to our PH meters:

http://www.sperdirect.com/cgi-bin/category/pH

Hope this helps.

Jaman987

For the last four years, until I got laid off, I worked for a company that designed and installed control systems for water/wastewater tretment facilities and we used primarily Rosemount or Hach. These come in several pieces. One is the sensor itself, usually a long tube that is inserted into the medium. Then a device that attaches to the sensor to convert to a voltage that is then read by a head unit with a display and the ability to have an output of several types. We used the 4-20mA current output because then within reason, no matter how long the leads are from the sensor to our controller (PLC's) you still got 4-20 mA at the controller. At the controller end we used a 250 Ohm resistor to give us a 1-5 Volt reding whcih was scaled in our PLC or in the software that we used to display graphically on a computer screen.

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