Honeywell ST 3000 Smart Transmitters

[fredro]

I am currently working as an Instrument Tech in an oil refinery (haven't quite made it 3/4 of the way to my EE). Some of the maintenance technicians have been cutting "diodes" out of these transmitters to suppress (allegedly) noisy signals or "dropping out" of the signal.

Allow me to define the two terms in quotes.

"Diodes" here are actually the 1.5k resistor in circuit with the line powering the device. Although there exists a resistor and a diode, the guys have been removing the resistors and calling them diodes, and no amount of telling them otherwise is tolerated. Stubbornness at its finest.

"Dropping out" out of the signal is exactly as it sounds. One second the signal is there and then it disappears. Sometimes, as you can imagine, this freaks out the operators or may actually shut down a unit. I have yet to witness the latter, however.

I have had no success in ascertaining why the techs do this. Despite the apparent ignorance as to the method of correction this procedure employs, it does seem to be effective. It's hard to argue against it after seeing it first hand.

Thanks for the help in advance.

 

[danw2]

Interesting. If these components are under the scrwe cover of the transmitter, I'm mystified as to which component is being removed or clipped.

There is a link on the printed circuit board that determines fail-safe direction, which when not cut is for upscale output for a fault condition. But if the link is cut the fault mode is downscale. But it's a wire link, not a 1.5K resistor.

The field wiring terminal block doesn't expose any electrical components that I've seen.

I can think of only two things:
- these components might be part of a lightning protection block perchance? The lightning protection block is red, the terminal block without lightning protection is black.

- These are components in an intrinsically safe barrier (which does have diodes and resistors)?

Is there any chance you could get a photo of the components and the area they're in and post it here? I know a guy at Honeywell ..

 

[fredro]

The component being removed is the resistor. The technicians that I work with call it a "diode" because the person who started this originally misspoke or was a complete buffoon. It is under the screw cap for the transmitter, connected to the back of the field wiring terminal block where you would hook up a communicator to change the LRV/URV or similar functions. I will get you a picture later today when I get the chance.

 

[fredro]

Here's a picture of the back of the terminal block. The "diode" has already been removed on this particular transmitter. Let me know if you need anything else.

 

[fredro]

Sorry, that was the wrong picture. Here's the right one.

 

[danw2]

Reply from a guy at Honeywell:

"I received feedback from our (Honeywell) electrical engineer.

The picture below shows the 1N4003 diode right next to the 1.5KE68CA transorb which has already been cut out. The purpose of the transorb is to protect the transmitter from noise spikes that come down the loop wires.

The purpose of the diode is to allow a service person to connect an ME (integral, local) meter across the diode. Since they already cut out the transorb which is supposed to suppress noise, this may be the reason they are seeing noise.

Assuming the transorb and diode are not faultly these components will not add to noise. What they are doing does not make sense. Without the Transorb installed, the transmitter is no longer intrinsically safe. If they think there is a problem with these components they should replace them with new components of the same values."

No longer I/S. Hmmm.

 

[fredro]

I actually did some snooping around and after asking a lot of questions, I found out that Honeywell has acknowledged the transorb diode passes leakage current to the TDC when operating in analog out mode so we have been removing them.

I don't see what you mean by the instrument not being intrinsically safe, as all that was done was guarantee that the instrument will fail completely as opposed to sending a false signal to the TDC.

Thanks for the help.

 

[danw2]

We call this 'shopping for answers'. Interesting when the real story comes out.

 

[fredro]

I was concerned because none of the other techs could explain why were doing this and, honestly, it seemed a bit irrational to me at first. After seeing the email from Honeywell, I think I will be able to sleep a little easier now