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realpars.com/pt100
⌚Timestamps:
00:00 - Intro
00:50 - What is a Pt100?
01:37 - Physical properties of Pt
02:30 - Alpha coefficient
04:29 - Industrial Pt100
05:38 - 2-wire Pt100
06:17 - 3-wire Pt100
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Platinum 100, or Pt100 RTDs are an important part of many process control installations.
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.
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.
1) 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.
2) 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.
3) 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.
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.
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.
The Pt100 RTD is often referred to as the Pt100 (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 Pt100 (391) sensor.
The equation is only approximate, so to know the true temperature at any measured resistance, we will need to consult a published standard table of resistance for a Pt100 (385) sensor. You can find the link to this table here: bit.ly/Pt100RTD385
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 thermowell. 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 4-20 mA signal.
In order to determine the resistance of the RTD, a special bridge circuit is used, called a Wheatstone 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.
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