pH metering

[robh]

Has anybody used or seen a pH meter that is rugged enough for industrial use that can be interfaced with a pc or plc? I would prefer one that is not in constant contact with the medium to be measured. Maybe one that the operator can remove from the storage solution, measure the pH of the waste water, return to storage solution? I have been searching with little success. Thanks, Rob

[gravitar]

You might try lithographic supply houses. A dozen years ago I worked for a printing company, and the pH of the water systems on the press was one of the parameters that had to be controlled. We had handheld digital pH meters for that, and I recall one of them having a serial output (which we didn't use). I can't recall offhand who made the meters. You don't have to keep the sensor in constant contact with the process fluid, but it might not be a bad idea because the sensing element cannot be allowed to become dry. (Unless of course the sensing technology has improved since I used them!)

[robh]

Thanks I'll give it a look see. Right now we are using Oakton handheld meters. The operator takes a measurement and then writes it down on a record sheet. I would like to have that information in digital form. Well we have tried continuous probes, but they get out of calibration way to fast and we can't keep them clean. We keep our probes "wet" in the manufactures recommended storage solution. Thanks, Rob Edited June 28, 2007 by robh

[ECSI]

ProMinent Dulcometer units are very popular in my neck of the woods. http://www.prominent.com/Desktopdefault.aspx/tabid-127/

[Sleepy Wombat]

E&H. Rosemount... There are a number of Lab based probes also with hand held pH meters... At least they generally have some form of plastic protection.... pH probes can be self cleaning if the flow is ok. Also ... yes you must keep them wet... and generally water will suffice as a medium.

[Clay B.]

I used a sensor from Omega several years back and it interfaced with a PC. www.omega.com

[paulengr]

Check out www.omega.com or Cole Parmer. I believe both have good references explaining the finer points of pH probes. Wet means "KCl" solution. pH probes are essentially electrochemical cells. You get a millivolt signal off it that is temperature and pH dependent...which is why temperature correction is necessary. The glass is chemically "active". This part is relatively easy to deal with in terms of cleaning. An operator flushing it with water once per day or shift is about all that's necessary. There are some versions of pH probes that are attached to some sort of device that opens a shutter once every few minutes to stick the probe out, then draws it back in and flushes it off. I don't have names handy and they are NOT cheap. Either way, there are a variety of tip designs. Try to find one that works well for you. Also sometimes WHERE you mount it is important. You don't want to erode the glass away with abrasion but putting it some place where the liquid is flowing and not in a stagnant spot will help keep it clean. There are "chisel point" heads and other varieties other than the lab standard glass bulb looking one. The other component is the "reference" side. This has to be electrically in contact with the liquid you are measuring. There is usually a porous ceramic plug or a very tiny hole in the pH probe. The liquid inside the reference junction is KCl. It slowly seeps out into the liquid. The heavier the flow, the less likely this junction is to get gunked up but it also limits how long the probe will keep working. There are several "extra tough" probes out there that simply provide better holes or frits and frequently have fatter bodies to hold more KCl. Your "storage" solution is almost always just KCl. This just helps keep the reference solution wet as long as possible. Using distilled water keeps the probe from drying out (which allows the KCl to crystallize out and instantly destroy the probe when the salt crystals plug up the junction). Another frequent mistake is that some place on the top of the probe (or even externally through a very small tube), there will be a hole that allows the reference junction chamber to fill with air as the KCl leaks out into the liquid. On the low end probes it will be covered by a plastic ring. If you DON'T uncover the hole, the probe will fail very quickly. Another problem is that the reference junction uses a pure silver wire which corrodes fairly easily. So do NOT keep these things as a stock item. The shelf life is only 6-12 months. So if you don't turn them over on a regular basis, don't keep one around. You can get 24-hour turn around from of all places, McMaster-Carr. The connections are fairly standardized. A lot of them have BNC connectors (which again are fairly standard) because since it's a millivolt signal, you need to do as much as you can to preserve signal integrity. Usually the only thing that will trip you up is whether or not the probe uses an amplifier, whether it has a built-in thermocouple (or RTD), and that many companies try to play the "specialized connector" game to prevent you from buying the cheapest probes you can get your hands on. There are "electronic" pH probes. They eliminate the glass electrode side but they don't eliminate the reference junction. Lifespan is about 3 times longer but so is the price. On the other hand calibration wasn't really a big deal. And you have to replace the battery often which is a "1/2 AAA" (hard to find). I have heard of some very tough all-ceramic probes. They are limited in terms of their pH range but they are all ceramic and there's no liquids to mess with and such. They are over $1K from what I know. I've never bought one or found one. Anyways...the answer is there's no easy answer.

[robh]

Well thanks for all the info. Yup, but it keeps the job interesting!

[TConnolly]

Believe it or not, try Walmart, not on the racks by the registers, but back in the electronics section. Aaaarrrrruuuuuuuuuuugh! I can't believe I just told someone to go into one of those dang borg stores.

[DanW]

What you describe is called grab sampling - grabbing a sample to take a reading. Desktop type pH meter are not designed for continuous monitoring, fro one thing, their resevoirs of KCl just aren't large enough. But. when you're grab sampling, you are probably better off with a regular bench top unit, with inexpensive, interchangeable probes. Industrial pH probes are designed for continuous monitoring, but even industrial probes are considered disposable and many models are not interchangeable between brands of electronic readouts/analyzers. The fastset respondind pH probes that give fast responses for either a process or a grab samples are the solid state devices, as opposed to glass electrodes, which is why I use them. Honeywell's industrial solid state probe is called the Durafet. I don't know if lab style pH probes come in solid state versions. Dan