Watercooling Acceptable Error Range for Temperature Sensors

I have a water cooling loop with a temperature sensor placed on the bottom of the radiator. When idling / under low usage, the temperature on the sensor reads 30 degrees C. However, when I view the CPU temperature from the on board sensor, it reads 40 degrees C.

Is this an acceptable / normal difference?

Note: The water cooling loop is a single 120 mm radiator, CPU water block, and pump / reservoir.

What temperature differences have other people observed in their systems?

 

Trust the internal hardware sensor first. A program like Real Temp is really good and idling around 40 is normal when reading from the motherboard sensors. The actual temperature via Real Temp is around 10 degrees less in my case with an i5 2500K.

 

Calibrate any sensors with a known accurate one before fitting that way you can quickly add/subtract the difference to get a reasonably accurate reading.

If you've got the sensor on the outside of the rad then it's pretty pointless and will give you a rough idea of the temp around the rad. The most useful sensor I have is the one that reads the water temp as it quickly gives an indication of any problems.

CPU sensors can be reasonably accurate but it depends on the CPU. The Bloomfield sensors are fairly good but the Gulftown ones are crap at lower temps.

 

I know that the sensor placement is not great. However, when building I ran into a time constraint and did not have a chance to order fittings to setup an inline temperature sensor. Perhaps I will do that when I have more time.

Do you know what the CPU sensors on the Sandy bridge platform are like?

Thanks for the input.

 

I don't know about the Sandybridge sensors but the best person to ask is unclewebb at http://www.techpowerup.com/forums/showthread.php?t=64185 He's got a lot of experience with Intel DTS and he's the one who told me about the Bloomfield/Yorktown sensors.