sorry, but did not get your point.Hi,
Corrected = (raw_value - offset) × gain.
Klaus
how can i do it using a POT, as it does not work well, POT value changes as connected in circuit.Hi,
You may calibrate gain and offset...both can be done with a pot or with the given formula.
Klaus
but where can i get the raw value, offset or gain.
i am NEW in interfacing PT100, i got the circuit from google, and stimulated in Proteus. where it was giving me a analog voltage at different temperatures. so i tried interfacing the circuit.Hi,
before I focussed on "calibration".
but now I see your schematic. Does it make sense? Where do you have it from?
There are many issues and questionable parts and values.
Loading an OPAMP output with almost pure 150nF usually is not a good idea.
Klaus
The circuit has not improved in the last years. So the answer are similar.You should try different approach for measuring with Pt-100
Yes, the same problem, but at that time my problem was not solved, so i dropped the project.Hi,
you are new to PT100??
--> 2019´ thread: https://www.edaboard.com/threads/pt100-interfacing-with-controller.386487/post-1661077
Klaus
I have a Requirement where i have to measure Air Temperature with range of 10 degree to 55 Degree C.What do you want to measure? Temperature. Which range?
What resolution, what precision?
How often (per second, per hour) do you want a new measurement value?
Mind: the requirments so far have nothing to to with PT100 nor with the microcontroller.
Klaus
As we are designing a Pediatric Warmer, we have chosen PT100 because of Accuracy. so PT100 is the Must.Hi,
so far so good. This is just a range of 45°C with a resolution of 0.1°C .. makes a dynamic of 1:450 or 9 bits.
Why did you choose PT100? Is it a must? (Without saying it is good or bad)
Do you have a Vendor / exact part in mind? Link to datasheet..
Did you read how a PT100 works generally?
How the temperature-to-resistance behaves. (You need to know this to be able to decide a useful measurement circuit and software calculations)
Klaus
Good, then we don´t need to look for other sensors.PT100 is the Must.
Then it´s high time to get familiar with PT100 working principle, temperature-to-resistance, formula, accuracy, precision.No i dont have any idea for PT100, as i only know, the resistance of PT100 Increases as the temperature rises.
i am confused.Hi,
now guessing:
When a current goes through a resistor it will cause power to be dissipated. Dissipated as heat. Means: the sensor gets warm.
Obviously it´s not a good idea to make a "temperature sensor warm" ... because this meanse we lose accuracy.
So we have to try to keep the sensor cold - or better say: keep it as good as possible at ambient temperature.
This could be done with:
* using a suitable sensor with good heat spreading
* using the sensor in a heat spreading medium (maybe water instead of still air)
* using lwo measurement current
* powering the sensor as short as possible
... another method could be to find out how big the "temperature rise" will be .. and subtract this value from the measured value in software.
*****
values:
(now I´m using approximated values... please use the values given in the datasheet)
Determine the sensor current:
Let´s say the self heating is 70 K/W. Then to get a maximum of 0.1°C self heating the sensor power needs to be below 1/700 W or about 1.5mW.
Since P = I x I x R --> I = sqrt(P/R) = sqrt(1.5mW / 100R) = about 4mA.
sensor voltage range:
Just to be on the safe side let´s calculate with 2mA.
at 25°C we have 100 Ohms.
you want the lowest temperature to be 10°C, thus we meed to go 15°C down: with about 0.4 Ohms/°C this means 6 ohms less than 100 Ohms = 94 Ohms.
you want the highest temperature to be 55°C, thus we meed to go 30°C up: with about 0.4 Ohms/°C this means 12 ohms more than 100 Ohms = 112 Ohms.
So the measurement range is 94 Ohms ... 112 Ohms (with 9 bit resolution)
Multiplied with 2mA this gives a sensor voltage of 188mV ... 224mV. .. a range of 36mV
0.1°C resolution means 0.04 Ohms resolution or 0.08mV or 80uV resolution.
(An LM358 has a typ offset voltage of 2mV, this is 25 times the required accuracy error !!! It is absolutely unsuitable)
(although you said: 0.1°C precision - which is repeatability - I calculated for 0.1°C accuracy. Your real requirement may be relaxed)
Now we need to know the (decodable) ADC input voltage range and it´s precision.
****
Later:
Another thing to consider: maybe you want to validate the function of the sensor: Simple methods are just detect "short circuit" and "open circuit"...
or the more advanced method to check whether the temperature changes meaningful values when you change the "measurement current".
Klaus
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