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Best sensor for temperature reading

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lepass7

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Hello,
As a part of my project I would like to be able to know the temperature inside a water tank. This project has some limitations:
1. Distance (sensor will be 2-15 meters away from the controller)
2. Sensor wire will have to pass next to power cable (220-240Vac 50/60Hz).
3. Sensor will be immerse into media (water)
4. Temperature range 0-100 Celsius (max)
5. Cheap solution.
6. Accuracy 1-2 Celsius is acceptable.
7. Microcontroller (probably PIC or arduino)

Please have a look the attached drawing (yes I know I am not an artist).
I would love to hear your suggestions, regarding the type of sensor I should use (pt100, lm35, thermocouple ... etc).

Thank you,

 

I would use a MCP9701 or MCP9701A from Microchip. They are inexpensive, cover the temperature range you want with sufficient accuracy and best of all, produce an analog output voltage which is already scaled to suit a standard ADC. They can also drive large capacitances so you can easily filter out any noise picked up on the cables. I use them in one application with a cable about 30m long.


Brian.
 

2. Sensor wire will have to pass next to power cable (220-240Vac 50/60Hz)

Depending on the type of load and most important the current level, you may have significant induced noise to warrant the use of shielded cable. You may not need it, but have it in mind.
 

Why not DS18S20? Easy to use
 

DS18S20 may not work over 15m cable length. In practice it will if the cable is good enough but it is stretching it's limits.

An analog signal can be carried over great distances and passive/active filtering can remove noise picked up along the way.

A trick I use in extreme environments is to connect a DS18S20 directly to an SMD 10F202 processor and use it to convert the temperature data to bi-phase serial format. It can then be sent along long cable lengths (>1Km) with a high degree of noise immunity.

Brian.
 

Hello guys,
Thank you all for your time. I believe the optimum solution is to use thermocouple or RTD (eg pt100).
@Brian: MCP9701 or MCP9701A are excellent but i have to make something in order to be able to submerse them into water
@Mr.Bigt and @betwixt: DS18S20 not submersible and there not cheap.
@schmitt trigger: Maximum current 20 Amperes (ac).

I think i should use a pt100 with a wheatstone bridge and a two stage amplifier (using opamps).

Thank you,

P.S: more ideas is more than welcome.
 

Hello,
I would like to share my drawing for future reference and for improvments
pt100.jpg
 

Why R6? Do you realize it is in parallel with the sensor itself?
You don't need it.
 

Hello,
I realized that :) . But without this resistor the output regarding PT100 sensor (100-138 Ohms) will be 2.5Volts to 5 Volts for 0-100 Celsius. Using this parallel resistor the output will be 115mV to 5Volts which is better for me. Actually R6 resistor will be a variable resistor 0-1.5K and it will be used as zero point callibration. Also attached you can find a new design (Instrumentation Amplifier) which i think is better. What do you think?IA_pt100.jpg
 

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