When you perform a Variable Temperature (VT) NMR experiment, the displayed temperature is not necessarily the actual sample temperature. When you need to know the actual temperature, with a high degree of accuracy, it is important to calibrate the sample temperature, relative to the displayed temperature.
The common method to perform this calibration is to observe the chemical-shift separation between the OH resonances and CHn resonances in either Methanol or Ethylene Glycol (generally Methanol is used for low-temperature calibration, and Ethylene Glycol is used for temperatures above ambient). The temperature dependence of this shift has been well-studied, and a variety of conversion methods have been reported to enable the determination of sample temperature.
You can dowanload this Microsoft Excel(r) spreadsheet (.xls file), which allows easy determination of either (a) temperature from measured shift separation, or (b) predicition of peak shifts for a particular temperature (created by Rich Shoemaker). The use of this spreadsheet is simple. There are fields to enter temperature (calculating the chemical shift separation), and fields to enter peak-separation (calculating the temperature). A variety of solvent systems are included.