How Does An RTD Work?
This article will describe how a resistive temperature detector (RTD) works. An RTD, otherwise known as a resistance thermometer, is used to sense the temperature of something, like a room, a freezer, a boiler, or a chemical reactor. These are just a few examples of their many uses.
It’s helpful to keep in mind that RTD’s are generally not useful on their own and will need to be supplied with an electrical current for practical use. The current supplies the means to get a signal to and from the RTD that can be picked up and interpreted by a device like a temperature transmitter (TT). To do this, the TT measures the resistance of the RTD and correlates it to a temperature. The TT displays the temperature in °C or °F, and/or may transmit the signal to another device like a baseboard heater, air conditioner or recording instrument. Check out my related article on how to select the right temperature transmitter.
What Exactly is an RTD?
An RTD is composed of a few components. One is a small piece of metal, typically platninum, called a temperature element. This element has an electrical current applied to it via a wire, this wire is called the lead wire. The current flows through the first wire, through the temperature element and back through a second wire to a device that captures the signal, usually a temperature transmitter.
The other main component of an RTD is the sheath that the temperature element and wires are inserted into. This is often a cylindrical piece of metal made from stainless steel, although it can be made from many other materials depending on the chemical compatibility requirements. It usually looks similar to a meat thermometer, but with a rounded or flat end instead of a pointed one.
Electrical Flow Inside An RTD
Electricity is the flow of electrons, or electrical current, from one point to another and is induced by an electrical potential, more commonly known as voltage. Voltage is the tendency of negatively charged electrons to repel each other and move towards a more positive destination. They do this by moving from atom to atom, for instance, moving along the atoms of a wire. To help think about the electrons repelling each other, it is fairly similar to how two magnets with the same polarity will push each other away.
When a material’s temperature goes up, the increased heat energy now inside the material forces the material’s atoms to vibrate more vigorously. These vibrations cause more collisions between the flowing electrons and the atoms, thus making it harder for the atoms to move freely across a temperature element, and so the resistance goes up when temperature goes up.
A conductor has a lot of electrons available to move freely and create the flow of electricity, where a non-conductive material, or insulater, has very few free electrons. So metals, which are good conductors, have lots of free electrons and when temperature is increased will have lots of electrons colliding with atoms and increasing the electrical resistance. This makes metals a good choice for temperature elements as their resistance changes a lot with changes in temperature. Platinum is especially useful as a temperature element as it’s change in resistance due to change in temperature is nearly linear over the temperature range that it’s used for in an RTD.
Correlating Resistance To Temperature
Now, how does the RTD actually know what the temperature is? As mentioned above, when conductors get warmer their electrical resistance goes up and when they get colder, their resistance goes down. In the case of an RTD, what matters is what the temperature element senses.
Along the useful temperature range a standard RTD works over, about -130°C to 482°C (-200°F to 900°F), a correlation has been studied, by experimentation, and well documented between the electrical resistance of the temperature element and the temperature the element is at. This is done by using a reference standard that is at a known temperature and simply comparing and recording the resistance of the temperature element at that known temperature over a range of temperatures.
Number of Wires In An RTD
Most RTD’s in North America have a third wire in addition to the two that supply and recieve the electrical signal that passes through the temperature element. This third wire is a way for the TT to determine how much of the signal is dependant on the resistance through the actual temperature element and how much is caused by the wires.The longer a wire is, the more resistance it has since there will be more atoms for the electrons to collide with as the electrons make their way over more distance. By using a second lead wire, the resistance of the wires can be cancelled out. The resulting measured resistance is caused by only the temperature element, although this method assumes that the two lead wires have identical resistance. This is a good assumption, but since nothing is perfect, includes a slight error.
Additionally, there are 4 wire RTD’s that are even more accurate than 3 wire ones. See the figure below for an explanation of 2 wire, 3 wire, and 4 wire RTD’s.
RTD’s are made of a temperature element that is embedded into a probe and often wired to a temperature transmitter in a 2, 3 or 4 wire configuration. Like any piece of metal, the element in an RTD has a natural resistance to the flow of electricity. This resistance is increased as the temperature of the element is increased, and decreased as the temperature of the element goes down.
If a material is a good conducter, like the platinum temperature elements used in most RTD’s, its resistance is greatly affected by changes in temperature. This is easily measurable and has been studied and correlated to specific temperatures at specific resistance values.
RTD’s have a wide range of applications and are one of the most useful devices I can think of.
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