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There are many different types of thermocouples.
Most manufacturers have selection guides to help you decide what to purchase.
In addition, most reputable manufacturers
will also have technical support specialists available
that will walk you through a series of questions
to help select just the right type of thermocouple for your application.
You’ll need to consider things like the range of temperatures you are trying to measure,
and the accuracy you would like to have.
This will help narrow down the two different metal wires that you need to use.
The type of thermocouple is based on the type of metals used in the sensing wires.
As we mentioned before,
a Type-K thermocouple is made from Chromel and Alumel alloy wires.
It can be used to measure temperature from around -330 degrees Fahrenheit
to over +2460 degrees Fahrenheit.
That temperature range can cover a wide range of applications.
You need to keep in mind that the accuracy of a Type-K thermocouple
may only be about plus or minus 5 degrees Fahrenheit across the entire range.
Sometimes that’s fine,
but sometimes more accuracy is needed.
If you were looking at a cryogenic application that needed more accuracy,
but you didn’t need to worry about high temperatures,
you could consider a Type-T thermocouple.
A Type-T thermocouple is made from a copper wire and a Copper-Nickel wire.
Type-T thermocouples are usually accurate to within a degree or two
so that makes them about twice as accurate as Type-K thermocouples.
Type-T thermocouples can typically measure even lower than -330 degrees Fahrenheit
but the upper end of the range is usually just over 600 degrees Fahrenheit.
They are more accurate but have a more limited measurement range.
These are common trade-offs for thermocouples.
The metal pairs that can do more are often less accurate
a sort of “jack of all trades but master of none” scenario.
If you want greater accuracy, you either need more expensive metals,
or you need to narrow the temperature range.
In addition to the different TYPES of thermocouples,
there are different STYLES of thermocouples.
Many applications for temperature measurement
can get away with just using thermocouple wire with the ends twisted together.
In that scenario,
care must be taken to protect the wire from vibration or physical damage,
but the ability to use just a pair of flexible wires
can solve a lot of mechanical challenges
with getting the sensing point position in the area you need to measure.
In cases where a little more protection is desired,
the wires are encased in a probe, which is just some kind of protective sheath
and some insulating material to help protect the wires.
Thermocouple probes come in Ungrounded, Grounded, and Exposed Junction style.
The style of thermocouple probe you select will also be based on your application.
Exposed junction thermocouples are when the sensing wires are joined together
out beyond the end of the probe sheath.
Exposed junction thermocouples have the fastest response time to temperature changes,
but because the sensing junction is exposed,
it is more vulnerable to breakage.
These probes are usually used to measure gases.
For corrosives and liquids,
Grounded or Ungrounded thermocouples work best.
A Grounded probe is when the sensing junction
is in direct contact with the end of the sheath.
This makes the heat transfer at the end of the sheath faster,
which improves the response time of the thermocouple,
but it also makes the sensing wires
more vulnerable to electrical noise like ground loops.
This can take away from the accuracy of the grounded probe.
An ungrounded probe is when the sensing junction is joined
just inside the end of the probe.
This means there is a small layer of insulating material
between the sensing junction and the very tip of the probe.
It insulates the sensing wire from electrical noise,
but the heat transfer is slower because of the insulating material.
So an ungrounded probe tends to be more accurate, but slower to respond.
As discussed, thermocouple accuracy is very sensitive
to the types of metal used in the wiring.
Unfortunately, this makes installation in an industrial application challenging.
One problem that you need to deal with
is something called “Cold Junction Compensation”.
In the previous video,
we stated that the thermocouple signal
is based on the difference in temperature between the hot and cold junctions.
But we need to make a small signal correction
to convert that temperature “difference” into an “absolute” temperature.
For example, if the temperature in a reactor is 700 degrees Fahrenheit,
but the outside ambient temperature at the location
of the measurement instrument cold junction is 70 degrees Fahrenheit,
the thermocouple millivolt will only show a temperature of 664 degrees Fahrenheit.
Even worse, as the ambient temperature goes up and down during the day and night,
the measured reading can change,
even if the reactor temperature remains constant.
Fortunately, most measurement instruments can perform cold junction compensation
either with standard built-in equipment
or with an optional cold junction compensation add-on.
Cold junction compensation components measure the temperature of the metal
at the cold junction wiring connections
and then make a signal calculation correction.
That way, the reactor will now read the true 700 degrees Fahrenheit,
and the reading won’t change, day or night
as long as the reactor temperature doesn’t change.
So for the most part, cold junction compensation is pretty easy to deal with.
A bigger challenge for thermocouples
is if you need to make a temperature measurement in a remote location.
If you need to extend the wiring of the thermocouple,
you must use something called a thermocouple extension wire
in order to reduce the amount of error.
Trying to splice standard copper signal wiring
or even using standard terminal blocks to extend the thermocouple signal
can end up creating additional cold junctions in the circuit
and introduce more signal error.
Thermocouple extension wires are sold by Type... just like thermocouples.
A thermocouple extension wire
is constructed of the same types of metal as the thermocouple,
so when you need to extend a circuit,
a thermocouple extension wire of the same type as the thermocouple must be used.
Similarly, if you need to use terminal blocks to connect the wiring,
the terminal blocks must be constructed of the same types of metal
as the individual wires.
If you have a Type-K thermocouple,
you need to use Type-K extension wire
and special terminal blocks made of Chromel and Alumel.
Trying to extend a thermocouple using standard copper wire
will introduce errors into the measurement
because the electron movements will be disrupted by the different metals.
Additionally, it is not recommended to run thermocouple extension wire
over a long distance
due to the sensitivity of the signal to electrical noise.
If you are trying to go more than 50 to 100 feet,
you need to consider using a temperature transmitter
that converts the millivolt signal
over to another signal type such as four to twenty milliamp.
If you have multiple temperature measurements to make in a remote location,
a remote I/O rack from your PLC might end up being a good option.
Let’s look at two different examples.
First, we have a refrigerated warehouse
at one end of an industrial facility.
The control room or a more centralized control system main rack
is located a few hundred feet away.
A simple refrigerated warehouse may not require a lot of automation,
but you may need to make sure the temperature inside the warehouse remains cold.
You could try using thermocouple extension wire,
which is cheap,
but you now know the signal level of a thermocouple can’t run a long distance.
So now what do you do?
You could install a remote I/O rack from a PLC system,
and then put a thermocouple I/O card in the remote rack,
but the rack and the I/O card can begin to increase the overall cost of the solution.
Instead, you can purchase an inexpensive temperature transmitter
that converts the millivolt signal from the thermocouple
into a standard four to twenty milliamp signal
that can more easily travel across a larger distance
with greater resistance to electrical noise.
But what if you needed to take several measurements at that same warehouse.
What if there were multiple HVAC zones in the building
that needed to be optimized to keep energy costs down.
Now, with several measurements to take,
having to buy multiple transmitters for signal conversion
may cost more than a remote I/O rack.
Thermocouples are a good solution for temperature measurement,
as long as you do some homework upfront about your application,
and pay attention to some of the challenges that might come up.
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