we’re going to walk you through one of the most commonly used temperature sensing devices… the thermocouple. We’ll discuss the basics of how they work.
A thermocouple is an extremely simple device used to measure temperature. Thermocouples tend to be inexpensive, durable, and can be fabricated into a variety of shapes and sizes.
A thermocouple is made up of two dissimilar metal wires. The metal wires are connected together in only one place, typically the tip of the thermocouple.
Lots of manufacturers call that junction by different names. Hot Junction, Measurement Junction, Sensing Point, or Sensing Junction. Those terms all refer to the same thing… the place where the dissimilar metals are joined that will measure the temperature.
The wires at the opposite end from the sensing junction are then left available to connect to some kind of measuring instrument like a temperature transmitter, a simple electronic display unit, or even directly to a PLC thermocouple input card.
The wiring terminals on the measuring instrument are most often called the Cold Junction.
While the Hot Junction refers to the tip of the thermocouple that will be exposed to the heat source of interest, the cold junction refers to the thermocouple wire connections that happen right at the measuring instrument, which typically is not exposed to the same thermal energy.
All thermocouples work the same way. They generate a small voltage when they are exposed to heat.
If you’re interested in the detailed physics of the way this works, you can research topics like The Thermoelectric Effect, or The Seebeck Effect, but to put things in simpler terms, when you heat up a piece of metal, the electrons in the metal want to move around more and will tend to move through the metal away from the heat.
Because electrons are negatively charged, the colder end of the piece of metal will have a negative charge compared to the hotter end. A thermocouple works based on the movement of the electrons in its metal wires due to the heat difference between the hot and cold junctions.
If the two wires of the thermocouple were made up of the same type of metal, electrons in both wires would move away from the heat at roughly the same rate, so you couldn’t really measure the difference in the charge of the two wires.
The different metals in those wires, or more accurately the electrons in those different metal wires, react differently to heat.
When exposed to heat, the electrons from one of the wires will want to move around at a certain rate. The electrons from the other wire will want to move around at a different rate.
– The wire that has the electrons that move more ends up being more negatively charged at the cold junction… and will therefore be called the negative wire lead.
– The wire with the slower electrons won’t build up as much of a charge, so it’s called the positive wire lead.
That difference in charge between the positive and negative wire leads can be measured and used to calculate the heat at the hot junction.
This difference in charge, also called a voltage, can be measured.
The more heat you apply to the metal wires, the more the electrons want to move around, and the more they move away from the heat.
With the two different types of metal wire, the difference in the voltage will increase and decrease with changes in heat at the sensing point.
A Type-K thermocouple is probably the most commonly used thermocouple in industrial applications because it responds predictably across a very wide range of temperatures. Type-K thermocouples are made from the metal alloys Chromel and Alumel.
we’re going to walk you through one of the most commonly used temperature sensing devices… the thermocouple. We’ll discuss the basics of how they work.
A thermocouple is an extremely simple device used to measure temperature. Thermocouples tend to be inexpensive, durable, and can be fabricated into a variety of shapes and sizes.
A thermocouple is made up of two dissimilar metal wires. The metal wires are connected together in only one place, typically the tip of the thermocouple.
Lots of manufacturers call that junction by different names. Hot Junction, Measurement Junction, Sensing Point, or Sensing Junction. Those terms all refer to the same thing… the place where the dissimilar metals are joined that will measure the temperature.
The wires at the opposite end from the sensing junction are then left available to connect to some kind of measuring instrument like a temperature transmitter, a simple electronic display unit, or even directly to a PLC thermocouple input card.
The wiring terminals on the measuring instrument are most often called the Cold Junction.
While the Hot Junction refers to the tip of the thermocouple that will be exposed to the heat source of interest, the cold junction refers to the thermocouple wire connections that happen right at the measuring instrument, which typically is not exposed to the same thermal energy.
All thermocouples work the same way. They generate a small voltage when they are exposed to heat.
If you’re interested in the detailed physics of the way this works, you can research topics like The Thermoelectric Effect, or The Seebeck Effect, but to put things in simpler terms, when you heat up a piece of metal, the electrons in the metal want to move around more and will tend to move through the metal away from the heat.
Because electrons are negatively charged, the colder end of the piece of metal will have a negative charge compared to the hotter end. A thermocouple works based on the movement of the electrons in its metal wires due to the heat difference between the hot and cold junctions.
If the two wires of the thermocouple were made up of the same type of metal, electrons in both wires would move away from the heat at roughly the same rate, so you couldn’t really measure the difference in the charge of the two wires.
The different metals in those wires, or more accurately the electrons in those different metal wires, react differently to heat.
When exposed to heat, the electrons from one of the wires will want to move around at a certain rate. The electrons from the other wire will want to move around at a different rate.
– The wire that has the electrons that move more ends up being more negatively charged at the cold junction… and will therefore be called the negative wire lead.
– The wire with the slower electrons won’t build up as much of a charge, so it’s called the positive wire lead.
That difference in charge between the positive and negative wire leads can be measured and used to calculate the heat at the hot junction.
This difference in charge, also called a voltage, can be measured.
The more heat you apply to the metal wires, the more the electrons want to move around, and the more they move away from the heat.
With the two different types of metal wire, the difference in the voltage will increase and decrease with changes in heat at the sensing point.
A Type-K thermocouple is probably the most commonly used thermocouple in industrial applications because it responds predictably across a very wide range of temperatures. Type-K thermocouples are made from the metal alloys Chromel and Alumel.
No comments:
Post a Comment