Tools used for drilling in the oil & gas industry, whether directional drilling, Measurement While Drilling (MWD), or ranging tools, all use magnetometers as a means to determine direction. These tools rely on Earth’s magnetic field measurements, for directional drilling and MWD, so are calibrated regularly to reduce errors. Calibration is performed at DC. This can be done by placing the sensor in a homogeneous and stable Earth field, which these days is increasingly difficult to find, or by placing the tools into a controlled field. Our range of Helmholtz coils is perfectly suited for calibration, offering generation of a precise field in any orientation, which is much more practical than rotating the tool. Our larger diameter coils enable tools to be more easily accommodated within the coil’s homogeneous volume. Use of an active compensation module does not remove the need for a clean environment in the immediate vicinity of the coils, but it does allow for DC measurements to be taken in a facility close to the storage and production facility.