Pt100 is the common abbreviation for the most common type of resistance temperature sensor used in industry.
Because accurate tables of resistance v. temperature are available it is common practice to calibrate instruments using precision decade boxes from table values.
Pt100 elements are specified over a temperature range of -200°C to 850°C however the actual operating temperature is determined by the construction of the probe into which they are incorporated. Typical low cost probes are made by soldering the Pt100 to PVC or silicon insulated copper wires. Obviously these are limited by the maximum temperature of the insulation. For higher temperature work the Pt100 is silver soldered or crimped onto mineral insulated wires and embedded in an insulating medium such as aluminium oxide powder.
At higher temperatures the platinum film can slowly evaporate which permanently changes the resistance of the sensor.
The low resistance of the Pt100 means that lead resistance can introduce noticeable errors. Lead resistance introduces two errors, an offset error caused by the lead resistance itself (which can be trimmed out) and a change in lead resistance with temperature (which cannot be trimmed out). These can both be overcome by the use of 3-wire and 4-wire compensation circuits.
Most probes are made in 3-wire configuration with one wire attached to one terminal and two wires attached to the other. In a bridge circuit the two wires end up in opposite arms of the bridge and their resistances cancel.
Note that 3-wire compensation is theoretically perfect only in constant current bridges. Constant voltage bridges are only perfect when the bridge is balanced, however in most cases the error is insignificant.
For two wire operation (normally very short cables) the twin wires are usually joined together.
The resistance/temperature characteristic of a Pt100 is not linear although for many applications the error is acceptable without correction.
A typical example: 0°C = 100ohm, 50°C = 119.4ohm, 100°C
Calibrating an instrument such that 0°C = 0% and 100°C = 100% will give a reading at 50°C = 50.4%.
There are several differing techniques for correcting the non-linearity of a Pt100 sensor including break point linearizers, and look up tables, but a simple technique is to slightly vary the current through the sensor as its value changes. Careful component selection can reduce the error by a factor of 10 or better.
Most people regard the major disadvantages of the Pt100 sensor over other industrial sensors, such as thermocouples, as response time and physical strength.
Modern Pt100 sensors are now so small and light that the response time no longer depends on the sensor itself. The response time of a Pt100 in a stainless steel sheath will be almost identical to that of an insulated thermocouple in an identical sheath because the thermal characteristics of the sheath are the major contributing factor.
The physical strength of a thermocouple is still superior but a Pt100 sensor properly packed in aluminiun oxide in a stainless steel sheath should withstand everything short of a direct blow from a hammer.
COMPARISON OF SENSORS
|THERMOCOUPLE||PT100||THERMISTOR||SOLID STATE DEVICES|
|OPERATING RANGE||Very wide: Type T can go down below -200°C. Type W5 can approach 1800°C||Wide: -200°C to 600°C||Narrow. Typically -40°C to 300°C||Very narrow: Typically -40°C to 125°C|
|PRICE||Generally inexpensive although type R & S use expensive platinum wire.||Fairly inexpensive||Low accuracy types very inexpensive - high accuracy types more expensive than Pt100||Inexpensive|
|ACCURACY||Moderate||Excellent||Poor to excellent||Moderate|
|PHYSICAL STRENGTH||Excellent||Poor to very good - Depends on probe construction||Poor to very good - Depends on probe construction||Good to very good - Depends on probe construction|
|CHANGE IN CHARACTERISTIC WITH TEMPERATURE||Small||Reasonable||Very large||Large|
|LONG TERM STABILITY||Reasonable||Excellent||Poor to excellent||Reasonable|
|PREFERRED APPLICATIONS||Industrial processes where temperature range or physical requirements preclude other devices.||All industrial processes within operating range where accuracy and repeatability are required.||Preset temperature applications.
Control where narrow hysteresis is required.
|Simple control applications and ambient compensating circuits.|
The Pt100 is my preferred sensor for all industrial applications from -200°C to 600°C. It is accurate, relatively inexpensive and easy to use. Its output change with temperature is relatively large compared to thermocouples which means lower drift errors on the electonics.
For the majority of applications Pt100 probes may be replaced with no recalibration of instruments.
Because its resistance bears an absolute relationship to temperature
(unlike a thermocouple whose output depends on the difference between the
hot junction and cold junction) no special compensating circuit needs to
be provided in the electronics.
Fred Philpott B.Sc. (Hons. Lond.)
IQ Instruments CC
MORE TO BE ADDED AS TIME PERMITS.