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Platinum 100, or Pt100,
resistance temperature detectors
are an important part of many process control installations.
Accurate and repeatable measurement of temperature
is a requirement for many processes,
including heating and cooling, chemical reactions,pasteurization,
and many others
In this video,
I will Introduce you to the working principles
of a Platinum 100 resistance temperature sensor,
Describe the physical properties of a Pt100 sensor
that make it valuable for process control,
and,Describe the ways Pt100 RTD sensors
can be integrated into a measurement and control system.
Resistance temperature detectors, or RTDs,
are a class of sensors that change resistance
when the temperature of the medium they are inserted into changes.
This change of resistance is proportional to temperature
and varies in a somewhat linear fashion with temperature.
This means that as the temperature increases,
the resistance of the RTD also increases.
So, if I am able to measure the RTD’s resistance,
I can determine the temperature.
Why is this?
It is due entirely to the physical properties of the material
from which the RTD is constructed.
While RTDs can be manufactured from many metals,
including nickel and copper,
platinum exhibits physical properties
that make it ideal for use in RTD temperature sensors.
Let’s take a look at the physical properties of platinum.
First, platinum is a basic element, with the chemical symbol Pt.
That is the first part of the designation of the Pt100 RTD.
Platinum has a molecular weight of 195,
which makes it a rather heavy metal
with free electrons to make it a good conductor of electricity,
although not as good as copper or silver.
Secondly, platinum exhibits an electrical resistance
that varies in a nearly linear fashion with temperature
and has a resistance of exactly
100.00 ohms at zero degrees Celsius
This is where the second part of the designation
Pt100 comes from.
Another property of platinum
that makes it highly valuable to temperature measurement
is that it is quite inert.
It does not react with other compounds to any great extent.
So how much does the resistance of a platinum change with temperature?
The purity of the platinum used
affects the change in resistance when the temperature changes.
The most common Pt100 RTD used in industry
is one that changes resistance
at the rate of about 0.385 ohms
for every degree Celsius rise in temperature.
I know the resistance of a Pt100 sensor
at zero degrees Celsius is 100 ohms,
so the resistance I would expect at 100 degrees Celsius
would be 138.5 ohms.
The “385” factor comes from the equation
that approximates the resistance of an RTD
based on its physical properties.
The equation relates the resistance of the RTD
at the temperature being measured
to the resistance at zero degrees Celsius.
The coefficient alpha in this equation
describes the rate of change of resistance with temperature.
For the Pt100 RTD, I have been describing
if I substitute the resistance values of the Pt100 RTD
at zero and at 100 degrees Celsius,
I find that the value of alpha is 0.00385.
Knowing alpha, I can calculate the approximate resistance
the RTD will exhibit at any temperature within its range.
The Pt100 RTD is often referred to
as the Pt 385 RTD.
There are platinum RTD’s that exhibit different values of alpha,
and those would be designated with their respective alpha values,
such as with the Pt 391 sensor.
The equation is only approximate,
so to know the true temperature at any measured resistance,
I will need to consult a published standard table of resistance
for a Pt 385 sensor, like the one shown here.
You can find the link to this table in the video description.
A Pt100 RTD is typically constructed
by winding a thin platinum wire around a non-conductive core
which helps support the thin wire.
The entire assembly is encased in a sheath
to protect the sensor and to give it stability.
In industrial applications,
RTDs are commonly placed inside protective metal tubes
called thermowells.
The length of the RTD and the design of the thermowell
are design parameters determined by the instrument engineer.
PT100 RTDs can be constructed from a single platinum wire,
giving a sensor with two leads.
These leads can be connected to a special I/O card
designed to accept RTD inputs,
or the leads can be connected to a temperature transmitter,
which will output a standard four to twenty milliamp signal.
In either case, the I/O card or the transmitter
will have firmware that will determine the temperature read by the RTD
from the measured resistance.
The table I showed you before
is programmed into the transmitter
and the RTD analog input card.
In order to determine the resistance of the RTD,
a special bridge circuit is used,
called a Wheatstone bridge.
In this diagram, there are four resistors.
Resistors A, B, and C are of equal value.
The fourth resistor is the RTD itself
and its resistance can be deduced
from the voltage measured across the two legs of the bridge.
This 2-wire RTD design is not very accurate
because the platinum leads themselves
have an electrical resistance
due to the length of the wire and the connection points,
in addition to the resistance from the temperature detected
at the point of measurement.
To compensate for this added resistance,
a second platinum wire is added
to the sensor at a third lead.
This third lead is used to determine the resistance of the lead itself,
and the resistance is subtracted from the overall measure resistance
to give the true resistance
due to the change in temperature alone.
These 3-wire RTDs are the most widely used in the industry.
Although more expensive than a 2-wire RTD
the added stability and accuracy are well worth the added cost.
In this video, I introduced you to the working principles
of the Platinum 100 resistance temperature sensor,
its physical properties,
and how Pt100 sensors are used
in industrial measurement and control systems.
I demonstrated the linear relationship between
temperature and resistance for platinum.
This property of Pt100 sensors makes these sensors
makes these sensors reliable, accurate, and affordable
for most places that temperature measurements are required.
So, the next time you come across a Pt 385 RTD in the field,
you will have a much better understanding
of how the sensor works.
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