Hi
could anyone try to explain to me this curious phenomenon?
im using two temperature sensors:
one lm35 (analog: 10mv/1C)
second i2c bus se95
and both are giving me different readings .. (se95 around 3-4 C less then lm35)
im sure about arduino side (script and hardware)
iv checked lm35 with multimeter and calculate temp p&p style and it seems to be ok
any ideas ?
Lemme guess, you're running the Arduino from USB power ?
yess ...??
USB power is frequently some way from 5v which upsets the AREF voltage and thus the reading from the LM35. The SE95 is completely digital and won't be too worried by 5v not being quite 5v.
i fear its not as simple
"iv checked lm35 with multimeter and calculate temp p&p style and it seems to be ok"
connecting lm35 to stable power source (workbench regulated power supply)im getting 240mv(multimeter) on output (with 10mv for each C degree it gives 24 C) and SE95 still gives lower values .. to witch one of them should i trust :-[
Are the LM35 and SE95 right next to each other ?, temperatures at different points in a room can easily be 3 or 4 degrees different. What does a common or garden household thermometer read in the same piece of air ?
The LM35 will accept a wide range of input voltages and still give the 10mV per degree, its the AREF voltage on the arduino that matters. Its frequently not 5v if powered from USB.
we can rule out aref because lm35 connected ONLY to multimeter gives 24 C , yes they are lying next to each other (no more then 10cm apart),
im afraid im having chicken&egg paradox here .. im not able to check temperature with household thermometer .. because i don't own one so that's the reason im building one 
because i don't own one
Maybe your neighbour has one?
I take it both sensors read higher if you warm them with your fingers ? (and drop back to 'normal' again afterwards - it can take several minutes for them to stabilize )
24 is on the warm side if you're heating the house, 21 is more normal, but if you're cooling its better the other way round. Kind of difficult which to believe if you don't know what it is to start with.
both of them react correctly to humane body waste energy (touched them with a finger :P) well il have to wait till tomorrow to knock on my neighbours doors