V200-18-E3XB not reading temperatures correctly

[Todd B]

I have a V200-18–E3XB module that uses PT100 sensors for temperature readings. It’s connected to a display module and used to control boiler water temperature. It’s about 5 years old. Recently the PT100 temp sensors have started to have erratic readings fluctuating by 15-20 degrees or more. I never had this issue until recently. I tried new PT100 sensors but problem still exists. Could the V200 module be the problem? Is there any way to check the V200 module?

[Ausman]

What are the readings if you connect the PT100s right at the snap-in, instead of via the normal location?  If they are normal, then I would initially be carefully going over every single earth point in the installation, especially the earthing on the PT100 shields.  But you saying they are all going wonky implies a main earth issue as the first thing to look at.  Do a direct connection first and see.

Also, I routinely remove/replace snap-ins on an annual basis, to keep the internal contacts ok.  They always really seem to be not as tight as I would like, so I add a bit of gaffer tape on accessible seams to assist holding it together.

cheers, Aus

[Joe Tauser]

On 11/29/2018 at 2:56 PM, Ausman said:

so I add a bit of gaffer tape on accessible seams

OMG, Aus.  Duct tape?  I thought I was the only one who did that.

As Aus says, try connecting the sensor directly to the terminals.  If it's still jumpy then try replacing the module - in RTD mode these are nothing more than glorified ohmmeters and they are multiplexed into a common A/D converter.  If you're having an problem on multiple channels there's probably something wrong with it.

Joe T.

 

[Flex727]

13 hours ago, Joe Tauser said:

OMG, Aus.  Duct tape?  I thought I was the only one who did that.

I think we've discussed this on here before. Duct tape is an essential element of the Snap-In I/O module, particularly if there is any vibration in the environment. With the proper tools available, screwing them on is even better. I wish Unitronics offered this standard.

[Ausman]

17 hours ago, Joe Tauser said:

OMG, Aus.  Duct tape?

Yes we have discussed this before, and the problem remains.  I don't know what you funny Yanks call it, but to us upside down people duct tape isn't gaffer tape.  I think you call gaffer tape race tape.  It doesn't stretch, unlike duct tape which can be a sticky, half-pulled-itself-back PITA, a bit like trying to undo something wrapped in myriad layers of insulation tape after it's been in place a few years.  Gaffer has a sort of woven backer and can be easily torn into strips or whatever is suitable, and sticks like toffee to a blanket.  Duct tape has it's place, so does Gaffer.

I always laughed in the Alien movies because here we are in space many years into the future, and heroine Sigourney is tying things to her super advanced weaponry with good old gaffer tape!

cheers, Aus

[Joe Tauser]

I stand corrected.

Maybe I should stock some Gaffer tape and put a JTA / Unitronics part number on it.

Joe T.

[Ausman]

Yeah, you could pre-tear a few strips of just the right size for the snap-ins and put them onto a pretty bit of silicone faced paper, selling them for a fortune on the huge markup possible!

Anyway, we digress, but helpfully.

How's it going, Todd?  It did occur to me that you also need to be carefully checking the boiler earths, and it's inherent leakage anyway.  You may possibly have something electrically serious going on at the boiler itself, which might be then coming down the earth lines to the plc for some reason, or simply interfering with the signals a lot.  Don't forget that we are talking tiny amounts here, and the slightest induced variation from a variety of reasons can cause havoc with stability.  But being a boiler one would think that it has a lot of innate earthing anyway.  Give it a very good checkover.

cheers, Aus

[John_R]

my experience with true "gaffers tape" goes back about 30 years or so when I worked in pro audio (concert sound), and the "gaffers" used "gaffers tape" to tape down cables and hang scrim cloths and such on stage for the concert.
it was cloth tape that was more tacky that duct tape but it pulled off without leaving adhesive residue.

so far as the snap I/O, I've made a little drill template to drill holes in the four corners and screw it down to the nice little screw bosses that the creators started putting on the V280, V560 ,V570's a few years back (why they have not followed through with holes in the snap I/O beats me, as they redesigned the PCB to have clearance in these corners).

JohnR

[Ausman]

I"m sure it all comes down to tooling costs.  But it has always been an issue, so if it was me I would have been fixing it properly.

Everything I use them with is 230s so is the even older problem and only gaffer is possible. 

And yes, roadies still use reams of gaffer setting up concerts/stages!

cheers, Aus