Hi
I need to do some wireless measurement at long distance, over 100 meters.
the area is quite big, about 50x50 meters, and I don't need great precision, so a FOW of 30° or less should do the trick, right?
I've found some thermopile and some PIR, but I have to fight with FOW, and I don't know if they work at big distance..
any idea or suggestion?
edit: the sensor have to be wireless
FOW?
The DS1820's are very good. A little care would be needed for such long cable runs, but project should be do-able.
Do you need absolute temperatures, or realtive? (Precision or accuracy)
Readings per minute?
Number of sensor?
LM35s might also be worth a look, especially if you really don't mind somewhat inaccurate answers. (Nothing wrong with LM35 accuracy over shorter distances... though not as good as DS1820s... but, if you can accept the accuracy degradation, they might be better at 100m.)
Tell us more about what you are measuring, where the sensors will be going.
tkbyd:
FOW?
Field of view, sorry I forgot to tell you the sensor need to be wireless
tkbyd:
The DS1820's are very good. A little care would be needed for such long cable runs, but project should be do-able.Do you need absolute temperatures, or realtive? (Precision or accuracy)
absolute is better
tkbyd:
Readings per minute?
60Hz
tkbyd:
Number of sensor?
probably one
60 readings per second???
What object's temperature changes significantly in one second, let alone 1/60th second?
No scenario- limited help. We're not being "nasty"... just limited by the question.
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"Absolute better"... of course absolute is "better", but do you NEED it?? Again. No scenario...
sorry, 1Hz should be enough, but if sensor are slower i can use an array of them synchronized to NOT take measure at same time (I'm quite good at programming) 
here the scenario: an RC plane will cover a great field looking for flames.
because of the great difference from flame (at least 200C) from ambient (max 50C), a relative sensor should be ok.
the plane can move at low speed (less than 20km/h = 5m/s)
maybe this is an interesting sensor - MLX90614 - Error - Melexis
also - http://www.cheap-thermocam.tk/ -
Rob
that was one of the thermopile i've seen, other alternatives?
this kind of sensor are difficult to find!
Seen this posting?:
yes, but the sensor has a wide FOW, (like 65°).. from TPA81 Infra Red Thermal Sensor
In a warmer room at 18°C, the candle measures 27°C at 2 meters. This is because the candle only occupies a small part of the sensors field of view and the candles point heat source is
added to the back ground ambient - not swamped by it. A human at 2 meters will show up as around 29°C with a background 20°C ambient.
but it say:
They (thermopile) have a very wide detection angle or field of view (FOV) of around 100° and need either shrouding or a lens or commonly both to get a more useful FOV of around 12°.
maybe it's time to study some photography 
Use a reflecting telescope to narrow the field of view and increase sensitivity.
Building a thermopile using lots of thermocouples isn't difficult, just time consuming.
my problem is that this is first time with this kind of sensor. I don't know where buy good but cheap one, and where buy the lens, a reflecting telescope maybe it's too fragile outside and too big.
Look at "solar cooker" on Google images.
Build your own - aluminum or copper hammered into shape and turn it to face the ground. You don't need optical quality, just something that concentrates the energy to an area smaller than the aperture. A $5 laser diode mounted at the far end of the room will tell you if the focus is roughly right.
i understand what you say, and it should work, I've just to select the right material that reflect that frequency of IR (from 5.5 to 15 micrometers).
Maybe just a mirror should do the trick.
a good idea.