Electromagnetic Waves and the History of Temperature Measurement – Visaya Weekly Episode 27

Last week on Visaya weekly we talked about the mechanical principles for temperature measurement. but here it comes the era of the thermometers based on electrical principles.

The Resistance Temperature Detectors and Thermocouples

As I explained in one of our past videos, RTDs are resistance based sensors used in temperature measurement.

There is a relationship between the change of temperature and the resistance of the sensor.

If we apply a voltage to the PT100 resistor there will be a current moving through the platinum film. The change in temperature modifies how this current moves through the platinum film.  

Imagine you are in the middle of a concert and in the best part you have to go to the bathroom!

Well if you are in a chilling concert like Norah Jones people will be quite still and it will be easy to go through with little resistance.  But let’s turn the temperature up and imagine we have a smells like teen spirit from Nirvana.

The same operation of going to the bathroom will be much more difficult since you will not be able to go through the crowd so easily and there will be much more resistance.

Well RTD temperature measurement obeys the same principle. At cold temperatures, the platinum atoms move much less and so the resistance value of the sensor decreases with lower temperatures.

On the contrary, when the temperature rises the movement of the platinum atoms will hinder the current movement increasing the resistance of the sensor.

RTD sensors are really stable and accurate but they have one limitation they don’t work well above 600 degrees Celsius actually above 400 degrees Celsius the common practice is to use thermocouples.

The thermocouple is one if not the simplest of sensors used for measurement of any parameter not only temperature.

What you have is two metallic wires of different allows.

An example is the type K Thermocouple that has one wire in Cromel {90% nickel and 10% chromium}  and the other in alumel {95% nickel, 2% aluminum, 2% manganese, and 1% silicon.}

When you connect them together and expose one side to higher temperature a phenomenon called the Seebeck effect takes effect.

A small voltage is produced due to the electromagnetic force produced by the temperature gradient. The voltage value will depend on the alloys used in the thermocouples.

Platinum-rhodium thermocouples are able to withstand temperatures up to 1600 degrees Celsius.

So what about the scale? Well for RTDs and Thermocouples there are tables that relate the Resistance or voltage to a temperature value.

History of Temperature Measurement PT100 Table
Courtesy of Industrial Automation and Mechatronics

Fortunately, you don’t have to take out the calculator and do the math, Plants use this temperature transmitter that has these tables and equations stored.

Temperature transmitters are able to deliver the temperature value right in the field or send it to your control room and with the communication protocol that is used in the plant.

Ok but if we want to go even hotter or we want to measure the temperature of a medium that we cannot touch.

We use infrared sensors like this thermal imaging camera that is measuring the thermal or infrared radiation of the bodies and relates it to the actual temperature of the surface.

Max Planck described the radiation law that describes the relationship between the wavelength of an emitted electromagnetic wave and the temperature of the emitting body.

Above 525 degrees Celsius, every black body will glow this is called the Draper Point.  

The hotter the object gets the more it will glow, if you remember our video regarding platinum you may remember that the pure platinum was glowing when it was melted or when it was being hammered into a lingot.

And if I could heat a piece of platinum up to approximately 720 degrees Celsius I would see that the platinum will be emitting a wavelength of 611nm.

And I love this well this is the wavelength because is the wave length that corresponds to the Visaya orange.

So the next time you think about what wavelength we travel, I can tell you is at 611nm.

Thank you for watching, Till next time.


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