2.1. Chair-specific sensor mat calibration
As pressure mats are known to be influenced by the surface area,
material properties as well as the geometry of the analysed product,
the sensor mat of each seat pan was calibrated prior to subject
measurements. Here, the pressure distribution of the buttocks of a
sitting calibration dummy loaded at the centre of mass of the
unloaded buttocks with different weights was analysed (5 s, 10 Hz)
for each office chair three times (Fig. 1). The buttocks were modified
with modelling clay and foam material as well as clothed with
shorts in order to imitate the pressure mat response of a human
buttock as closely as possible. The total weights of the calibration
dummy were 4.40 kg, 6.41 kg, 9.30 kg, 14.20 kg, 24.30 kg and
44.20 kg in order to calibrate the pressure sensor mat for the full
pressure range of our subject measurements. Attention was paid to
ensure that the contact surface of the calibration dummy remained
within the sensitive area of the pressure mat. The relationship
between the mean pressure values was then assessed using the
pressure on the mat measured using the calibration dummy with
the different additional weights (pmat) and the theoretical pressure
value calculated using the known weights of the calibration dummy
divided by the assessed contact area (pcalib). In order to determine
each chair's pressure correction factor, fcorr, based on the assessed
pressure (pmat), fcorr was defined as pcalib divided by pmat. Finally,
xey plots of the calibration measurements (pmat against fcorr)
clearly indicated an exponential function through the values of fcorr
and pmat with three parameters a1, a2 and a3 (1). This function was
then used to correct the pressure values of the seat pan mat at each
time point.