# RTD fails open?



## RePhase277 (Feb 5, 2008)

The obvious question would be is the RTD being used within its temperature rating?

Besides environmental, the only reason an RTD would open would be either too high of a voltage on the power leads causing it to over heat, or maybe the third wire getting grounded and sending too much current through that path?

Are you 100% sure of the wiring's integrity?


----------



## oliquir (Jan 13, 2011)

is there any vibration on the rtd, if it always fail, i would try using another brand


----------



## glen1971 (Oct 10, 2012)

RePhase277 said:


> The obvious question would be is the RTD being used within its temperature rating?
> 
> Besides environmental, the only reason an RTD would open would be either too high of a voltage on the power leads causing it to over heat, or maybe the third wire getting grounded and sending too much current through that path?
> 
> Are you 100% sure of the wiring's integrity?


 
It's within the temp range for sure. 
The power comes from a 24 vdc UPS and it is as stable as ever..
I'll triple check the wiring this afternoon, but I'm 99% sure it is good from the transmitter to the separation kit for the RTD..


----------



## mitch65 (Mar 26, 2015)

RTD bottoming out in thermowell +vibration?


----------



## Paulusgnome (Mar 28, 2009)

Controllers that use RTDs normally limit the current to the RTD to 1mA or less. This is to stop self-heating which occurs at higher current levels.
This sounds a bit like the controller is not limiting current to the sensor to a low enough level, and that is what is burning out your PT100s (I am assuming that your RTDs are PT100).
I would load up the temperature transmitter with a 100 ohm resistor and measure the current and voltage. This should shed some light on the problem.


----------



## glen1971 (Oct 10, 2012)

mitch65 said:


> RTD bottoming out in thermowell +vibration?


I can check if there is an noticeable damage to the RTD in the morning.. There isn't much there for vibration. It's on a line quite a distance from any equipment.


----------



## glen1971 (Oct 10, 2012)

Paulusgnome said:


> Controllers that use RTDs normally limit the current to the RTD to 1mA or less. This is to stop self-heating which occurs at higher current levels.
> This sounds a bit like the controller is not limiting current to the sensor to a low enough level, and that is what is burning out your PT100s (I am assuming that your RTDs are PT100).
> I would load up the temperature transmitter with a 100 ohm resistor and measure the current and voltage. This should shed some light on the problem.


I'll see if I can scrounge up a resistor in the coming days and do some checking. The transmitter is about 7 years old..


----------



## Jlarson (Jun 28, 2009)

glen1971 said:


> I can check if there is an noticeable damage to the RTD in the morning.. There isn't much there for vibration. It's on a line quite a distance from any equipment.


RTD for a flow meter? I've had issues with cable breakage on some of my compensated flow meters when straight rtds were used, they'd break we're it was bent to strap down to the pipe.


----------



## RePhase277 (Feb 5, 2008)

glen1971 said:


> I'll see if I can scrounge up a resistor in the coming days and do some checking. The transmitter is about 7 years old..


Brown-black-brown... just sayin'.


----------



## glen1971 (Oct 10, 2012)

Jlarson said:


> RTD for a flow meter? I've had issues with cable breakage on some of my compensated flow meters when straight rtds were used, they'd break we're it was bent to strap down to the pipe.


It is on a fuel gas meter run. The sensor entry is parallel with the piping and the cable (1tr16 teck) is strapped with the tubing support back to the transmitter that is mounted securely to the handrail.


----------



## glen1971 (Oct 10, 2012)

RePhase277 said:


> Brown-black-brown... just sayin'.


Yup.. But the odds of a 24 hour store that sells them popping up over night in town is pretty slim.. lol.. There might be a small selection of resistors on site or maybe at the office..


----------



## DesignerMan (Jun 13, 2008)

I've had issues on some past jobs where the signal cable was used for 4-20mA control. I ran a cable outside the conduit, point to point for testing purposes. Might be worth trying to help locate the issue.


----------



## RePhase277 (Feb 5, 2008)

glen1971 said:


> Yup.. But the odds of a 24 hour store that sells them popping up over night in town is pretty slim.. lol..


Never stop hoping.



> There might be a small selection of resistors on site or maybe at the office..


You can always combine them. Don't hesitate to use ten-thousand 1 meg resistors in parallel:laughing:


----------



## glen1971 (Oct 10, 2012)

DesignerMan said:


> I've had issues on some past jobs where the signal cable was used for 4-20mA control. I ran a cable outside the conduit, point to point for testing purposes. Might be worth trying to help locate the issue.


It's teck cable in a Zone 2 (Class 1 Div 2) area.


----------



## glen1971 (Oct 10, 2012)

RePhase277 said:


> Are you 100% sure of the wiring's integrity?


Checked the wiring yesterday.. No shorts to ground between any of the conductors. All read continuity (0.3 ohms) when tested between each other and open when apart.


----------



## mitch65 (Mar 26, 2015)

The RTD itself is failing open though? Something between the xmitter and the RTD is damaging the element. If it isn't physically damaged, (crushed by water freezing in thermowell or abrasion or vibration) I would lean towards transmitter malfunction. Being a fuel line it shouldn't be isolated for cathodic protection that could be grounding through your sensor.


----------



## glen1971 (Oct 10, 2012)

mitch65 said:


> The RTD itself is failing open though? Something between the xmitter and the RTD is damaging the element. If it isn't physically damaged, (crushed by water freezing in thermowell or abrasion or vibration) I would lean towards transmitter malfunction. Being a fuel line it shouldn't be isolated for cathodic protection that could be grounding through your sensor.


That's kinda what I was leaning towards (failing transmitter).. I'll double check the cathodic on Monday, but I think there is an insul kit upstream of the thermowell..


----------



## cmdr_suds (Jul 29, 2016)

I am really starting to miss Radio Shack

Sent from my LG-AS330 using Tapatalk


----------



## cmdr_suds (Jul 29, 2016)

glen1971 said:


> Yup.. But the odds of a 24 hour store that sells them popping up over night in town is pretty slim.. lol.. There might be a small selection of resistors on site or maybe at the office..


I am really starting to miss Radio Shack [emoji26] 

Sent from my LG-AS330 using Tapatalk


----------



## JRaef (Mar 23, 2009)

RePhase277 said:


> Never stop hoping.
> 
> 
> 
> You can always combine them. Don't hesitate to use ten-thousand 1 meg resistors in parallel:laughing:


Been there, done that. Had to make an RC circuit to suppress a nasty coil kickback on an air compressor starter with a 480V coil that was causing other local power electronics to gate themselves on. Radio Shack was my only option in the desert town of Adelanto, CA (near Death valley) on a Friday at 5:30PM. The RS clerk had no idea what resistors and capacitors were, I had to search for them and the selection was pathetic. The end result wasn't pretty, about 3ft long if I stretched all the components out, but I got to leave the job site that night and go home. I put heat shrink tubing over everything and coiled it up to stuff it all back into the starter, then told them to re-do it properly later when the right parts were available. 

I'd bet a paycheck it's still like I left it 15+ years ago.


----------

