Calibration Experiments conducted at ETT observatory, 1980-2000

Data standards at Hyderabad (HYB) and Ettaiyapuram (ETT) magnetic observatories in the era of suspension variometers and secondary calibrated absolute magnetometers could be maintained only bycontinuous evaluation and self-consistent data reduction. Experiments were devised at these observatories using minimal equipment, without calibration facilities, to compare and calibrate absolute and variation magnetometers in situ. Of these experiments three are described here: i) Determination of the thermal coefficients of the La Cour suspended magnets and assessment of the impact of diurnal temperature variation, within the wooden, thermally insulated variometer room at ET. ii) Evaluation of the effect of H-variation (diurnal range similar to 100nT) on the La Cour D-variometer at ETT, that was dynamically balanced in the 'astable' position, with torque equal to the force exerted by ambient H-component. The H-field, around the D-variometer was increased in steps, using the Helmholtz coils of the assembly, to estimate the coefficient of change in D for every 10nT increase in H. This was used to re-calibrate declination values at ET. iii) Improved constant current source, bias coil assembly and in-house proton precession magnetometer(PPM) were deployed in the experiment. Baselines obtained from two different methods of vectorised measurement, Nelson's method and Serson's method, were compared for equatorial latitudes, against those obtained from DIFlux. Serson's method of vectorised measurements was found to be optimal for low values of vertical component.