Temperature

PT100 Info: []
 * Temperature **

Details of PT100 The principle of operation is to measure the resistance of a platinum element. The most common type (PT100) has a resistance of 100 ohms at 0 °C and 138.4 ohms at 100 °C.

The relationship between temperature and resistance is approximately linear over a small temperature range: for example, if you assume that it is linear over the 0 to 100 °C range, the error at 50 °C is 0.4 °C. For precision measurement, it is necessary to linearise the resistance to give an accurate temperature. The most recent definition of the relationship between resistance and temperature is International Temperature Standard 90 (ITS-90).

This linearisation is done automatically, in software, when using Pico signal conditioners. The linearisation equation is: Rt = R0 * (1 + A* t + B*t2 + C*(t-100)* t3) Where: Rt is the resistance at temperature t, R0 is the resistance at 0 °C, and A= 3.9083 E-3 B = -5.775 E-7 C = -4.183 E -12 (below C = 0 (above 0 °C) For a PT100 sensor, a 1 °C temperature change will cause a 0.384 ohm change in resistance, so even a small error in measurement of the resistance (for example, the resistance of the wires leading to the sensor) can cause a large error in the measurement of the temperature. For precision work, sensors have four wires- two to carry the sense current, and two to measure the voltage across the sensor element

Problens to watch out for with PT100

The current through the sensor will cause some heating.If the sensor element is unable to dissipate this heat, it will report an artificially high temperature. This effect can be reduced by either using a large sensor element, or by making sure that it is in good thermal contact with its environment. Also its important to keep signal cables from PT100 away from any power cables which may emit electrical noise. Using screened cable, with the screen grounded at one end, may help to reduce interference. These problems will not effect our PT100

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Lukas researched max and min temperature values on []

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 * February 14**


 * I design and checked on LTSpice circuit for PT100 sensor,and is working fine on the software so far.All i have to do now is to get all the components i need and build the same circuit on breadboard and get the measurements.See below picture of this circuit taken from the LTSpice software:



February 23**

Discussion with James:

Testing of PT100/bridge combination at a variety of temperatures to determine lineararity and scale of signal. This is required to decide on the best type of amplification to employ to obtain final output signal. This can be either be a 0 - 10V or 0 - 5V signal as the PLC analogue card can be configured to handle either signal.

Design of a successful second stage amplifier could be useful for the other sensors.

Lukas is currently running tests on a variety of circuits on LT Spice and checking outputs. We await results.

Here is the basic circuit he is looking at:



As can be seen, this is a basic non-inverting amplifier. The challenge is to correctly calculate the ratio between R1 and R2 to give the correct gain.