Offset Error Rosemount 1056 Analyzer with 3900VP pH Probe

R

Thread Starter

roundyroundy

This is my first post. I've tried the search function and read the posting guidelines, so I hope this is acceptable.

I could use some guidance because the manufacturer's user manual for the Rosemount 1056 offers no troubleshooting help, and I would like to correct this problem without simply replacing the entire 1056.

I checked the pH probe (Rosemount 3900VP-10-02) with buffer solutions (4.01, 7.00, and 10.01) and got readings of 6.80 in the 4.01, 5.60 in the 7.00, and 7.80 in the 10.01. Attempted to calibrate probe using the buffers and was met with the offset error.

Replaced the probe with a brand new one, attempted to calibrate and received similar results to above.

Took the original pH probe and connected it to a known functioning pH analyzer and got an acceptable indication in the process water.

The troubleshooting I've done so far seems to indicate there is a problem with the analyzer, not the probe. If anyone has experienced a similar problem before, I would appreciate some input about additional troubleshooting and possible permanent fixes. Additionally, if anyone has input for a more effective troubleshooting process for a problematic pH probe, I'd be interested in hearing about that as well.
 
Welcome to Control.com

Substitution with a known good probe and running a cal check is a good troubleshooting technique for checking the analyzer.

You did not mention what pH value you read when you connected the pH probe to the 2nd analyzer, only that it was "acceptable". Given that a broken, open circuit pH probe, will produce a reading of 7.0 pH, I'm not sure how valid your second 'test' was, if you read pH of 7. If you run the same cal procedure with the pH probe on the second analyzer and the readings are acceptable, then you have a known good probe.

You did not mention whether the probe had ever worked with the original analyzer or whether you'd just connected it and tried it.

If you just connected it, there are 1056 settings, like where the preamp is located that could affect the readings.

Your cal readings make no sense, they're not at all linear. A pH probe outputs 0.0mV at a pH of 7.0, so to get a serious offset of 5.60 pH at 0.0mV is very suspicious.

If the probe tests/cals good, and you've checked the 1056 setup for pH on that channel, there's no reason to toss the 1056. If the display lights up and gives you a reading, the power supply, display and processor are running OK, and the fault is very likely in the pH input card. I suspect that you can replace an input card for a fraction of the cost of a new analyzer.
 
R

roundyroundy

> You did not mention what pH value you read when you connected the
> pH probe to the 2nd analyzer, only that it was "acceptable".

The indication was about 6.8 and rising which is an expected value for the process water. Response was a bit sluggish, but I believe that's normal for this probe.

> You did not mention whether the probe had ever worked with the
> original analyzer or whether you'd just connected it and tried it.

The probe was working as expected for about 6 months before suddenly failing during normal operation. It was working fine for several hours after the start of my shift during which I know no one tampered with it because I was the only one around.

> Your cal readings make no sense, they're not at all linear.
> A pH probe outputs 0.0mV at a pH of 7.0, so to get a serious
> offset of 5.60 pH at 0.0mV is very suspicious.

I agree. I've seen this before at the last plant I worked at with a gel type pH probe (brand escapes me) connected to a Honeywell UDA2182. I don't know what causes it.

> fault is very likely in the pH input card. I suspect that you can
> replace an input card for a fraction of the cost of a new analyzer.

That is very helpful info. I've seen input cards fail in the past, but I was never able to be sure the card was the issue other than plugging/wiring a new card and hoping it worked after all other troubleshooting failed.

Alternatively, Have you seen problems with cables in the past? the VP cables I have on site were installed five years ago and the terminals in the female end of the cable look a bit worn. I'd like to try cleaning them off with contact cleaner. One of my co-workers suggested using dielectric grease in the connection. Because of their location they are mildly susceptible to water intrusion from spraying/splashing.
 
> Alternatively, Have you seen problems with cables in the past?

Yes, corrosion in pH cabling can be a problem.

Increased resistance will definitely affect a relatively low voltage high impedance pH signal. It's one reason for using a pH transmitter, but corrosion problems on the probe-to-transmitter cable can still be problematic.
 
R

roundyroundy

>Yes, corrosion in pH cabling can be a problem.

Update for anyone who sees this from now on. Replacing the cable with the VP connector solved the problem. I had tried to clean the contacts inside the connector to little avail. I wired in the new cable, connected it to the probe, and it calibrated just fine.

Lead time on the cables was a few weeks, so I'd recommend keeping at least one or two on hand for troubleshooting.
 
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