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

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', ...
                       50.1,'DesignMethod','butter','SampleRate',fs_p);  
    pres(2,nn) = {filtfilt(net_filter,pres{1,nn})} ; 

    % Calibration
    pres(2,nn) = {((1.13*pres{2,nn})-31.4)/au}

    % Lowpass filter
    [b,a]=butter(4,f_filt/(0.5*fs_p));  
    pres(2,nn) = {filtfilt(b,a,pres{2,nn})};

Answer 1

[dpb]

Calibrate/convert the time signal to pressure units as the very first step.

[Yasmin Ben Azouz]

@dpb Thank you for your answer, the values are still nearly the same and maybe even a bit more negative. They now have a range of -4 to -24. Do you have any other idea what might be the issue?

[Yasmin Ben Azouz]

Or am I not understanding you correctly? I swapped the notch filter and the conversion.

[dpb]

    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.