It's easy enough to calibrate a thermometer and save quite a few dollars. An ice-water mix is 0oC and once corrected for atmospheric pressure boiling water is 100oC. Various organic compounds can also be used. Certain compounds are better than others. The substance should be of reasonable purity and the molal freezing point depression constant must be small. A good choice would be naphthalene (one type of moth crystals) since it melts at 80.2oC and the constant is 6.9 oC/mole. The naphthalene would have to be very impure for the Mp to be significantly in error. Make sure to buy naphthalene and not paradichlorobenzene moth balls.
Melt the naphthalene making sure that it comes up to the immersion line of the thermometer. Then record the temperature as a function of time. The temperature will first drop as the naphthalene is allowed to cool. Then the temperature will level off as it begins to solidfy. Finally the temperature will again begin to drop when all the naphthalene has solidified. Now graph your results. The temperature for the middle of the straight line portion of the graph represents the freezing point 80.2oC.
When using melting/freezing points remember that both solid and liquid must be present for the result to be accurate.