Hi all, my first post!

I'm having trouble with RTD's failing apparently due to lack of robustness and was wondering if anyone can offer any insight. Are RTD's really generally so unreliable?

I've recently jumped ship from power generation industry to mining process commissioning, and finding RTD's more prevalent as opposed to power gen where generally higher temps require TC's.

On my last two jobs I've had a lot of RTD problems(from the same leading supplier), commonly with the lead wires breaking. Generally the RTD lead wires are a fine multi core wire, which seems to be very brittle. I've had multiple failures even operating sub 100C temperatures. Often the break is obvious at the crimp lug, sometimes internal to insulation, and others just OC.

Despite the objections of some of my colleges I've generally been swapping out to K type TC's which I can get in nice heavy 20G. In 15 yrs I've never had a TC go open circuit.

Has anyone had problems with RTD's? Any recommendations?

 

I have had issues with finer gauge wires of all types over the years. I do not think it is sensor specific. Just wire size.

IME, RTDs are every bit as robust as T/Cs. But they don't have the temperature range of a T/C.

--
Bob

 

rtd's are quite sensitive to flow induced vibration. At a minimum you will see irreversible de-calibration, but in serious cases complete destruction of the sensor element and/or extension wires.

believe the new thermowell standard lowers the velocity at which flow induced vibration is a concern, especially in liquid services.

t/c are more resistant to FIV, to a point and partly explains your finding, but they too can be made to fail in extreme cases.

all of the thermowell suppliers now use new sizing methods to help sort out the issues involved and usually requires reducing the thermowell length. your solution is fine, but check all new applications.

 

very common failure when flow induced vibration is involved. the new thermowell standard (asme) helps reduce likelihood of sensor fatigue failure

t/c are more resistant to such failure as you've found, but in severe cases both types can be mechanically destroyed.

 

re. Bob P. Comments

Bob, what little data is available on sensor endurance is mostly in regard to shock ratings, i.e. fall testing.

Depending on the sensor design (wire wound bobbin, thin-film)the shock ratings of Pt rtds ranges from 5-10 G's. Whereas T/C have reportedly been subjected to 50-100 G shocks without damage.

Less data is available for actual process measurements with the sensor in a thermowell. The only series of tests reported by a manufacturer found that for spring loaded sensors, the maximum allowable G-stress for all sensors with tip vibration (transverse to the thermowell axis) was roughly 10 G's. Above this level of tip acceleration the spring-loading was unable to keep the sensor in contact with the bottom of the thermowell. That company went on to design a specialty sensor, with extra-strong hold-down spring, and some means for damping the mid-span deflections of the sensor sheath. That design increased the sensor rating to 20-25G's sustained vibration.

In actual practice, where in-situ tip accelerations have been measured and the thermowell forced into resonance by vortex shedding or flow pusaltions, the G stresses have measured above 250 G's and the accelerometers battered to pieces. These acceleration levels are also predicted by the theory.

Temperature sensors having failed in service where the thermowell is exposed to severe and sustained resonance have the appearance of being used as a jack hammer, with damage to the sheath and the sensor completely destroyed.

At lessor tip accelerations, it is not un-common to experience RTD de-calibration due to the stress cycles applied to the extension wires or to Pt leads on the sensor.

T/C appear to be less sensitive to this type of failure, but as noted above, tip accelerations exceeding 10 G's or so, is capable of causing mechanical destruction of the sensor.

believe the new thermowell standard covers the issue of sense g-stresses or at least how to avoid them.

 

RTD VS TC'S

I'm still working in power generation industry. We used to handle with RTD and TC fault in maintenance process. Personally speaking I hate to deal with troubleshooting such as "RTD and TC" fault, especially high temperature circumstance. always appear oxygenation on wiring connection surface so that it's easy to cause "open circuit signal." On the other hand, How to compare RTD and TC. I think it's depend on which OEM Made it. top-money equal top-quality. My english just so so ...hope you can understand what I was trying to say here ...thanks!

 

Anecdotal:

There was enormous, earth shaking vibration at an open die forge shop in the middle of an Illinois corn field (located where the nearest neighbors couldn't feel the earth shake every time the hammers came down).

They used a specific brand RTD because the lead wire from the sensor to the head was a gauge or two thicker than anyone elses and they knew from experience that that brand lasted considerably longer than the others.

Of course, it cost more.

This is anecdotal info from 15 years ago, and the cost of platinum is up 5x or 6x over that time period, but I suspect that amongst the several dozens of manufacturers, someone makes a slightly heavier duty RTD than most, if you're willing to pay the difference.

Carl Ellis