OT: ORP - oxidation-reduction potential meter

I have been tasked with putting ORP meters in our cooling tower sumps, they will be the PV in a loop that meters chlorine to the sumps. Some of the towers are in Class I. Div. II areas. I did a google search and got many hits. Just looking for some feedback, if anyone has used a particular vendor/mfg. with either good or bad results.

The ideal solution would be a sensor with a built in controller, I could put everything inside a NEMA 7 enclosure and aviod the cost of running conduit for several hundred feet.

Thanks in advance.

I don't have an app for ORP, but I've used Honeywell pH, and the probes used for ORP are, in effect, pH probes. Since I don't do ORP, I'm not sure whether the glass or non-glass (they call it a Durafet) probe would be better, but their tech guy could sort that out.

The non-glass Durafet probes last about 3 times longer than the glass probes we've used and I've rebuilt the Durafets by refilling the KCL from a kit, which helps keeps probe costs down. The probes are temperature compensated and industrial grade.

Honeywell has either a 2 wire "transmitter" they call DirectLine or an analyzer box. The transmitter head can either mount directly on the probe or remotely.

The pH/ORP analyzer/controller is a UDA 2182. I bought the UDA instead of the transmitter because I wanted the control, I had the AC power out there, and because I'm fanatical about documenting configs and setups.

The UDA has an IR port on it, from which the setup can be down/uploaded from a PC or a pocket pc with their software. The IR works well and it's fast. I wish more push-button boxes had that kind of software.

I checked the documentation that came with UDA box and it has an FM class 1, div 2 rating. The plastic housing has a NEMA 4X rating and I got the 2" pipe mount kit for it.

Maybe someone has a built-in controller right on a probe, but I haven't seen one.

My advice is to stick with the industrial grade analytical probes. Lab grade stuff just doesn't stand up over time.

Dan

Ken,

We have actually done two applications where we have replaced the ORP controller and interfaced the sensor directly to the analog of our PLC.

In one instance, it was a millivolt input signal required. In the other it was a +/- 2 VDC. Both applications are currently in testing, so we don't have final results just yet, however, everything preliminary, is looking very positive.

I believe ORP controllers run anywhere from $475 - $700 (not including the cost of the sensors)

My point is - what you are looking to do can possibly be done with a PLC also.

Hope this helps.

God Bless,

danw, the Honeywell products look interesting, I'll give them a call on Monday.

Stephen, a plc is an option, but in this case, having a vendor "package" will allow the maintenance guys to maintain it vs. me. Cost is a factor, but at times the least expensive solution is not always ideal.

Most everything that I have used is a two piece sensor/element and transmitter. The two that I like best are GLI/Hach and E+H. I have used Signet also. The GLI Model 53 transmitter is list $890 and the probes run $500-800 list. (Their web site is down so I can't get a link - http://www.hach.com search for P53). These are 5 wire sensors with temperature compensation. This unit will have relays that are programmable for high/low, on/off, deadbands etc... GLI has some economy models also.

You may need to be creative with meeting the Class I Div 1 requirements. The probes are sealed/potted with the cable. I doubt that any manufacturer has intrinsic safety barriers in their equipment to handle the hazardous area. Most industries and applications don't end up in hazardous areas from what I have seen. You will need to provide for cleaning and calibration from time to time. Yes these probes require maintenance - sometimes often depending on the application (Daily or per shift for industrial wastewater, daily for municipal disinfection, weekly/monthly for cooling tower application). You will also prefer to have the transmitter/display near the probe making meeting the classification requirements difficult.

Your installation in the sump will be a problem since you will not get a good / representative sample of the ORP and get a good mix / reaction time with the chemical. The cooling towers that I have done have used sample pumps to create a sample loop with pH, conductivity, and/or ORP in line with the sample piping pulling off the cooling tower sump and chemical injection after the probes into the sample line return. The advantage is that you can locate the pumps and instruments in a clean and safe location. The down side is you need sample pumps and more moving parts to go bad.


Good luck - contact me if you wish.

My experience with ORP probes hasn't been great. Frequent probe maintenance is a must, so reliability is poor because it seldom happens. In addition you usually need to mix controlled solutions to calibrate - a pain.

I've had lousy luck with Royce and Foxboro. Endress + Hauser seems better than most. Hach's instruments may be OK, but they are a pain to work with. I suggest looking at ATI http://www.analyticaltechnology.com/ I've never used their ORP, but I've had excellent luck with any analytical instruments I've purchased from them, and their service and support is top notch.

I have worked with Fisher Rosemount (Emerson) and Yokogawa ORP and I prefer the Fisher Rosemount 3081 model.

http://www.emersonprocess.com/raihome/Liquid/products/Model_385+.asp

Since you are in the US you are probably find it easier to get a 3081 than the Japanese ORP.

Thanks for all the information, looks like I've got a lot of research to do. Tom, the ATI probes look the best, with the sealed reference, but I may not be able to use it in the classified areas.

TimeFlux, if I read the information correctly, the rosemount has a disposable sensor, is that right?

Thats right Ken. Once the membrane bulb gets contaminated where it is too slow to respond you change it out.