RTDs vs. Thermocouples

It depends upon the manufacturer's recommended rating and the construction.

In the Endress + Hauser literature (usual disclaimer) for example, some types can span -200 to +600degC.

At high temperatures the components of an RTD assembly may start to melt or even catch fire, whereas a thermocouple doesn't have this problem so early, some being designed to go much higher than 600degC.

The question is surprising. RTD and Thermocouple are entirely different devices that have absolutely nothing in common. RTD is a temperature sensing device used in temp control instrumentation. Thermocouple is a miniature electric power generator. Upon heating, a thermocouple provides current flow, used wherever required. One example would be a pilot flame gas supply valve. The gas valve is a electro-magnetic valve, whose coil is energized by the current from a thermocouple, heated by the pilot flame.

OK. A couple of things:

RTD and Thermocouple are entirely different devices that have absolutely nothing in common.

Not true; they are both used for temperature measurement.

The gas valve is a electro-magnetic valve, whose coil is energized by the current from a thermocouple, heated by the pilot flame.

The current from a thermocouple's generated voltage is so small that it would never be able to actuate a solenoid. Maybe you're thinking of a bimetallic thermostat. Even so, the bimetallic strip is only used to close a contact. A separate power source is used to actuate the solenoid.

For basics of thermocouple theory, see here.

Sorry Mikerho, The current from a thermocouple's generated voltage is so small that it would never be able to actuate a solenoid. Maybe yo.......

The above is wrong, a thermocouple held in the pilot flame used to hold open the safety valve in most gas appliances, they can and do generate high currents but only at milliVolt levels.

I agree with your first part though... :-)

PS where is the formatting features of the comment box gone????

Do you know where I can see a diagram of the gas appliance using the thermocouple?

I can see my formatting stuff - what browser are you using?

Guest,

You got a "Bad Answer" mark, but you are right in every line that you've wrote in your posting. That will teach you, the n-th time, that no good deed goes unpunished

They are, both, used for measuring temperature.

The RTD is a "passive" devices, that changes its resistance as the temperature varies. That change is quite linear on a small scale. Though on a larger temperature range it is not very linear.

The thermocouple is an active devices. It generates electricity as it is heated. The electricity generated by a pilot flame is enough to keep the gas valve "on". A Google search over the subject would eliminate any controversy

RTDs Vs. Thermocouples ! They are BOTH used to measure temperature.

I have always wondered how to select one over the other for a given application.

That is, what is the difference to the user? Is there a difference in accurancy? I know Thermocouples are very accurate. Is there a difference in range? I know one type Thermocouple can measure very cold things, and a different type (as designated by letters J, K, and others,,,) can measure hot things, like the tubes in a furnace(!). Is there a big difference in durability? Thermocouples last a long time (yes, I know they can fail,,, but how often do you see that?). Is there a big difference in cost ? I think RTDs cost more.

Sooooo, why do we select RTDs?

Just curious.