The Machmeter is only affected by the instrument and pressure error. The temperature and density do not affect the machmeter as these parameters are self-compensated by the design of the instrument. This can also be noticed by observing the construction of the machmeter, as it doesn’t have any temperature sensing element. Furthermore, there’s no compressibility error either, as the latest depends on the dynamic/static pressure and compressibility error is canceled out by calibrating the ratio-arm inside the instrument case.