How and when to apply calibration to my pressure measurements (calibration factor is available)?

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Hi all,
At the moment I am processing pressure data. I am using a 50Hz Notch filter for power line artefacts, then I calibrate and after that I use a low pass butterfilter to eliminate high frequency stimulation noise.
The code is below and I attached the variables. I don't really feel like the values I am getting are correct, as they are very negative and the researcher that supplied the data claims it should be positive in a 0 - 40 cmH20 range. Am I doing something wrong?
pres = cell(2,num_pres) ; % first raw, second modified
for nn = 1:num_pres
pres(1,nn) = {data.pressure(:,nn)} ;
pres(3,nn) = {fs_s} ;
% Notch filter 50Hz
net_filter = designfilt('bandstopiir','FilterOrder',2, ... % create notch filter for 50 Hz (48-52 Hz)
'HalfPowerFrequency1',49.9,'HalfPowerFrequency2', ...
pres(2,nn) = {filtfilt(net_filter,pres{1,nn})} ;
% Calibration
pres(2,nn) = {((1.13*pres{2,nn})-31.4)/au}
% Lowpass filter
pres(2,nn) = {filtfilt(b,a,pres{2,nn})};

Accepted Answer

dpb on 4 Aug 2022
Calibrate/convert the time signal to pressure units as the very first step.
dpb on 5 Aug 2022
Edited: dpb on 5 Aug 2022
pres(2,nn) = {((1.13*pres{2,nn})-31.4)/au}
will result in values <0 unless
1.13*pres(2,:) > 31.4/au
It would appear that must be happening but we don't know the magnitude of the input signals nor what the value of au is to be able to see.
IOW, the calibration must not be what you think it is.
Plot and verify w/ your researcher the values of the actual pressures you're computing first before even thinking about anything else...unless and until those are correct, everything else is meaningless.

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