to change possibly resulting in signal divergence this will result in flow direction 64 misidentification and flow measurement inaccuracies because of the nature of respiratory therapy it is not uncommon for sensing elements 62 to be replaced periodically for cleaning this may occur when the patient is connected to the ventilator a new or cleaned sensing element 62 with dac values associated with a previous sensing element in another system will likely result in a mismatch between the sensing element 62 and subsequent flow measurement inaccuracies as described above in all the cases of measurement inaccuracies of the prior art a periodic zeroing procedure during therapy would greatly alleviate the inaccuracies in this application however the ventilator control computer cannot create a guaranteed zero flow condition at the wye piece of the system where the sensing elements 62 are positioned eliminating the possibility of forced zeroing during therapy this inability to create a zero flow condition is usually due to patient interaction such as spontaneous breathing however at the wye piece the sensing elements 62 are not subjected to base flow that the ventilator may be delivering to the rest of the system as a result a zero gas flow condition will exist during each breath cycle referring now to fig4 an inspiration expiration analysis 70 corresponding to the operation of the present invention is depicted in this inspiration expiration analysis 70 the inspiration signals 72 represent the voltage over the sensing elements while the patient is inhaling and the expiration signals 74 indicate the voltage drop of the sensing elements while the patient is exhaling the net flow 76 in liter min shows the difference between the inspiration signal 72 and the expiration signal 74 the zero flow condition.