There's a serious RTD error, I'm sure someone will help. Currently, there are many RTD sensors installed in our factory. The transmitter is not used and the RTD resistance value is read from the IO card as it is. The problem is that sometimes the values of several sensors increase gradually over several hours. Touching the driver to the terminal of the marshalling cabinet or closing the knife switch again will return normal temperature. The same symptoms continued when the whole RTD sensor was re-tighting to find the cause, so I checked the terminal and found that the corrosion of the cable pin and terminal was seriously progressed due to cabinet humidity. We are preparing for a report, but we cannot explain the problem of returning to normal if we touch some tool as driver after a gradual rise or fall, or close the circuit after opening it. If you have any relevant evidence or explanation, please reply. You can send it by e-mail, please. I wish you good luck all the time.
Thank you

 

The reason for increase in indicated temperature readings is due to the method used to get an RTD reading.

The AI card generates a constant current. The current flows through the wiring, wiring connections and the RTD itself. Currect flowing through a resistance creates a voltage drop, by Ohms Law.

The 3rd wire compensates for the voltage drop in the wiring, assuming all three wires are the same size or gauge. The voltage drop across the RTD resistance is measured and converted to a reported temperature value. The AI card does not and cannot know about other resistances in the circuit, it assumes that the resistance in the wiring circuit is measured and compensated for.

When there corrosion, the corrosion creates higher resistance that create voltage drops that are not compensated and appear as higher reported temperatures.

Screwdriver pressure on corroded connections likely reduces the corrosion resistance by forcing the contact material closer together and providing a lower resistance for the source current, momentarily correcting the false corrosion induced temperature increase.

A friend who serviced a food plant that had a similar problem because of continual washdowns (and people leaving panel doors open) replaced the terminal blocks with new terminal blocks, stripped the wire back until it was clean copper, remade the connections and then applied a liquid coating with a small paint brush that dried and protected the connections from water or humidity exposure. He used a product called Fluid Film, but I'm sure there are others that are suitable for protection from water.

 

My personal experience is that RTDs are:
- electrically sensitive with regard to cabling and connection. This influences the reading, but can be compensated by using 3 or 4 wire compensation
- mechanically sensitive and easily break
Therefore in factories I only use thermocouples, why?
- mechanically very sturdy and stable
- no compensation wire needed
but:
- you need to use thermocouple wire, or thermocouple compensation wire for connecting it to your reading instrument, and the input channel on that card must be able to read thermocouples.
You can also use transmitters to convert the thermocouple to a 4-20mA reading.
But I -prefer- reading thermocouples directly with a dedicated thermocouple input.

 

Hi,
When using 3 wire RTDs, terminals corrosion may lead to either under-estimating or over-estimating the temperature.
4 wire RTDs are not affected by corrosion as long as electrical continuity is maintained (on the 4 wires).
RTDs are more accurate but usually more fragile than thermocouples.

 

I fully agree with Fil. For industrial applications thermocouples are way more mechanically robust and RTDs are too fragile.
I only use RTDs for calibration purposes when I need to be accurate to about 0.01°C or even less, then I use 4 wire.
We also do a lot of calibration over here.
See: https://cascade.net/en/products-for-temperature-calibration/

 

RTD's are traceably accurate, but are extremely vulnerable to high vibration if not mechanically secure.

For example in thermowell installations, most RTD's have a vibration limit of 10G's (1 G is equivalent to a drop to the floor from a height of 1 meter), but vulnerable to sustained vibration.

T/C are more robust with vibration limits of 20-30G's.