For those who are interested, I found a decent source from NXP on calibrating 3 axis accelerometers. Some of the points they make are :
Three-axis accelerometers supplied for the consumer market are typically calibrated by the sensor manufacturer using a six-element linear model comprising a gain and offset in each of the three axes. This factory calibration will change slightly as a result of the thermal stresses during soldering of the accelerometer to the circuit board. Additional small errors, external to the accelerometer, including rotation of the accelerometer package relative to the circuit board and misalignment of the circuit board to the final product, will also be introduced during the soldering and final assembly process.
The original factory accelerometer calibration will still be adequate for the vast majority of consumer applications. Manufacturers of premium products looking to obtain improved accuracy from a consumer accelerometer may, however, wish to perform their own calibration either by repeating the calibration performed by the accelerometer manufacturer or by using a more sophisticated calibration mode
* The apparent gravitational acceleration on the earth's surface varies by 0.7% from minimum to maximum. The apparent gravitational acceleration at the recalibration site is irrelevant if the product is to be used to provide orientation angle estimates from ratios of accelerometer channel readings but should be known if the product is required to provide high-accuracy acceleration or gravitational measurements.
•The original six parameter (gain and offset in each channel) factory calibration can be recomputed
to correct for thermal stresses introduced in the soldering process.
* A 12 parameter linear calibration model can correct for accelerometer package rotation on the circuit board and for cross-axis interference between the accelerometer’s x, y and z channels.
• The orientation angles used for the recalibration must be carefully selected to provide the best calibration accuracy from the limited number of measurement orientations available. Optimum orientation angles for a given number of measurements are listed.
•Linear least squares optimization is an efficient mathematical technique to compute the
recalibration parameters from the available measurements using simple matrix algebra. Worked
examples are provided throughout the text.
•These techniques can be extended to include temperature dependence by performing the recalibration at two or more temperatures and interpolating the fitted calibration parameters to the actual temperature.
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Accelerometers are used in applications requiring either absolute or relative acceleration measurements.
Examples of absolute acceleration measurements are determining the earth's gravitational field or the acceleration forces experienced in an automobile in units of ms-2 An example of using relative accelerations is the calculation of orientation angles using ratios of the readings from the x, yand zaccelerometer channels.
Although what follows may seem an obscure point, it does need to be briefly discussed since the objective of this application note is high-precision calibration. Although the earth's gravitational field is often stated to be 9.81ms-2, in practice the apparent gravitational field measured by an accelerometer varies by 0.7% from minimum to maximum over the earth's surface as a consequence of the earth's rotation, the earth's equatorial bulge and the effects of altitude. The apparent gravitational field at sea level at the north pole is 9.832 ms-2 but is only 9.763 ms-2 at the 5895 m summit of Mount Kilimanjaro located almost on the equator.
This document assumes that the accelerometer recalibration is being undertaken for high-precision calculation of roll and pitch orientation angles from the ratios of accelerometer channel readings. In this case the precise apparent gravitational field at the recalibration site cancels in the mathematics and is simply assumed to be '1g'.
If, however, the recalibration is being performed to produce an absolute estimate of gravity or linear acceleration in units of ms-2 then the apparent gravitational field at the recalibration site must be known and stored on the product for a simple final multiplication from '1g' to the required gravity or acceleration estimate measured in m s-2
The rest of the paper explains how to recalibrate the accelerometer using trig and matrix algebra as well as using least squares to average out the results, which is beyond the scope of 99.9 % of model rocketeers including yours truly.