In your daily life and different branches of industries,
there are many cases in which it’s required to know the temperature of the environment,
inside a reactor, winding of an electrical machine and so on.
man has invented different technologies to do this.
But why different technologies?
This is because of different applications
which make one technique better than the others for each specific case
and there’s no unique way recommended for all applications.
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Most likely you’ve heard about “RTDs”,
“Thermocouples”, “Thermistors”, “Semiconductor” type elements and so on,
which will be addressed here.
Before I go into details of this subject,
let’s see what a “Temperature Sensor” or Temperature Transducer is
and what does a “Temperature Transmitter” mean.
Generally, a sensor or transducer is a physical device
which is capable of transforming one type of process variable
to my favorite signal type.
To elaborate on this generalized sentence, let me give you an example.
Temperature, pressure, flow, etc, are some process variables
and actually, they are physical characteristics of our real world.
With modern technology
and because of tremendous advances in Electrical Engineering in the past century,
we like to transform every measurable process value into an electrical signal
and a temperature sensor
is a device which will transform the temperature into an electrical signal,
no matter how tiny the amount of this signal might be!
So far I took a big “First Step”
which was the transformation of “Temperature” into “Electrical Signal”.
Based on different sensor technologies,
this signal may have different ranges
and for industrial applications,
I need to have my signals limited
to some universally accepted electrical “signal ranges”.
Today some of these globally accepted electrical signal ranges
are 4-20 mA , 1-5 V , 0-10 V , etc.
A “Temperature Transmitter” is a device which transforms the tiny output
of a “Temperature Transducer” to one of these standard signal ranges.
Now let’s get back to different “Temperature Transducer” technologies.
RTD or “Resistance Temperature Detector”
is a device the resistance of which varies with the temperature.
Since it is a passive device,
an external electrical current should be applied to it
and then the voltage drop across it can be measured.
This voltage is a good indication of the temperature.
When referring to such a device as “passive”,
it means that the device needs external current or voltage source.
To state the obvious,
a big amount of external current
can cause power dissipation in the resistor of RTD and lead to excess heat,
so to avoid this type of error,
the current should be kept at a minimum level.
This wiring diagram shows the simplest application of an RTD,
called “two-wire” configuration.
More accurate reading calls for 3-wire or 4-wire configurations.
In reality, the distance between the temperature sensing point
and measuring system calls for wiring
and since the real wiring has its own resistance,
some measurement error sneaks in hereby!
3-wire and 4-wire solutions are developed to remove this error.
One of the most common RTDs is “PT100”
which consists of a thin film of Platinum
on a plastic film and shows a resistance of 100Ω at 32°F.
Its resistance varies with temperature
and it can typically measure temperatures from -330 to 1560°F.
The relationship between resistance and temperature of PT100 is relatively linear.
PT100 is just an example of platinum RTDs
and in the industry you may find different RTD types suitable for various applications,
For example: Copper, Nickel, Nickel-Iron, etc.
Thermistors are temperature-dependent resistors
and are widely used in industrial purposes,
such as overcurrent protection, self-regulating heating elements,
inrush current limiters and so on.
Thermistors can be NTC or PTC.
In NTC or "Negative Temperature Coefficient" thermistors,
resistance decreases as temperature rises.
NTC’s are commonly used as “inrush” current limiters.
And with PTC or "Positive Temperature Coefficient" thermistors,
resistance increases as temperature increases.
PTC thermistors are commonly used as “overcurrent protection”
and in resettable fuses.
A thermocouple or simply “TC”
is comprised of a couple of specific dissimilar wires joined together,
forming the “sensing point” or “junction”.
Based on physical characteristics called “Thermoelectric Effect”,
when this junction is placed at different temperatures,
different millivolt signals are generated
which can be interpreted as an indication of the temperature.
In comparison with RTDs,
Thermocouples are self-powered
and require no external excitation current source.
Thermocouples are commonly used for furnaces,
Gas Turbine combustion chambers, high temperature exhaust ducts, etc.
The main restriction of Thermocouples is the “accuracy”
which doesn’t make it the best solution for precise applications.
Also, Thermocouples need a reference measurement point called “Cold Junction”.
The thermocouple junction is often exposed to extreme environments,
while the cold junction is often mounted near the instrument location.
Based on “range” of temperature measurement,
“sensitivity” and some other factors in each application,
different types of Thermocouples are available,
for example E, J, K, M, N, T and so on.
For instance, Type “J” is made up of “Iron-Constantan” combination
with a range of −40°F to +1380°F
and a sensitivity of about 27.8 µV/°F
while Type “K” (Chromel-Alumel)
is one of the most common general-purpose thermocouples
with a sensitivity of approximately 22.8 µV/°F.
Type K is inexpensive and a wide variety of probes are available
in its −330°F to +2460°F operating range.
Since the functionality of thermocouples is based on Thermoelectric Effect
in different types of conductors,
when the location of a thermocouple is far from the “measuring instrument”
for example the electronic transmitter,
the proper type of conductors should be used for extension purpose.
Otherwise, the tiny signal generated by the thermocouple
will be added with some error at the point where the thermocouple wires
are connected to the extension wire!
“Semiconductor Temperature Sensor”is based on the fact
that the junction voltage across a p-n combination of semiconductors,
like a diode junction or “base-emitter” junction
of regular transistors, is a function of temperature.
This technology is vastly used in electronic devices and IC technologies.
Linear characteristic, small size, and low cost are advantages of this technology,
but it should be noted that the limited range of around -40°F to 248°F
makes it suitable for specific applications.
To wrap up this video,
the comparison between different types of temperature sensor technologies
is a multifaceted task.
For example, if “accuracy” is considered as the key performance indicator,
usually RTD’s are better than Thermocouples;
approximately 10 times more accurate.
From the “sensitivity” point of view,
while both RTDs and Thermocouples respond quickly to temperature changes,
at similar costs, thermocouples are often faster.
If I have to measure electronic PCB and/or IC temperature,
silicon-based types are the best choices.
Also alongside the technical bonus-malus comparison,
sometimes cost is the key factor in selecting the proper device,
as they say: “Money Talks!”
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