The most commonly used device type of barometer for calibration duties is the Fortin barometer. This is a highly accurate instrument which provides measurement inaccuracy levels of between ±0.03% of full-scale reading and ±0.001% of full-scale reading depending on the measurement range. To achieve such levels of accuracy, the instrument has to be used under very carefully controlled conditions of lighting, temperature, and vertical alignment. It must also be manufactured to exact standards and is therefore very expensive to buy. Corrections have to be made to the output reading according to the ambient temperature, local value of gravity and atmospheric pressure. Because of its expense and the difficulties in using it, the barometer is not normally used for calibration other than as a primary reference standard at the top of the calibration chain.