My DS18B20 is reading high. How can I get it to return the correct temperature?
I've put a lot of DS18B20 sensors in place now on various sensor nodes - maybe 50 or so. I've learnt a bit about the readings returned from them. I know you have isolated some of these as not being the cause, but it is good to confirm that they are issues.

I don't know if you are using Celsius or Fahrenheit, so the degree of the problem isn't clear.

Localised heating

I don't think of Arduino as a "hot" chip, but putting a DS18B20 in an enclosure with an Arduino and LCD caused me to see readings 2-3°C higher than expected.

Moving it outside the enclosure fixed this.

Heating from high Vcc/frequent conversions

If you supply the chip with a high Vcc and perform frequent conversions (i.e. as frequently as you can), the chip does warm up. I have seen this increase readings by 1-2°C.

I haven't seen the problem with high Vcc alone, but I operate most DS18B20 in parasitic mode and convert at most once every 10s. The ones operated in active mode generally have the powered turned off when not being used.

OneWire and DallasTemperature do support parasitic mode.

Not all temperature sensors are created equal

I had a number of temperature sensors before I started playing around with DS18B20. Almost without exception, these under-read. Most of them are based on thermistors and calibration is poor. I thought the DS18B20 were over reading, but when I used a good thermocouple and meter, I could validate that they were correct.

This was about learning to trust the DS18B20.

Heat conducted along wire

If you have the sensors on a wire, the wire conducts a surprising amount of heat. This can be an issue when running wiring along pipes.

Fake DS18B20

These sensors are very popular, and this seems to have caused fakes to appear. I have a batch of stainless steel waterproof sensors that over-read and the timings were out. They worked with the OneWire library, but not a DS2482 OneWire master. I have heard of the normal TO-92 cased ones being fake as well.