Volume is computed by integrating the flow rate. Any time that data are integrated, decisions must be made about when to “re-set” the integral. That is, at some point it should be declared that the volume is zero. Typically the flow signal itself is used to re-set the integral. When the flow drops below a certain value, the volume calculation channel is re-set to zero, and subsequent flow is integrated until the flow becomes less than the re-set threshold again, at which point the volume is declared to be zero again. When no subject is breathing through the flow transducer, the signal will, ideally, be very close to zero. In this case, integrating the flow is of no consequence, especially if the signal frequently crosses zero (so that sometimes ‘volume’ is added and sometimes subtracted) or the re-set threshold (so that the volume keeps getting set back to zero). However, if there is a slight offset in the transducer – if a non-zero signal is consistently indicated when there is no flow, the volume calculation channel will continuously add (or subtract) that apparent flow.
The TSD117/SS11L(A) is sensitive to orientation. When calibrated, it should be held in a perfectly vertical position, and more importantly, the orientation during measurements should be the same as it is during calibration. The transducer has a threaded hole in the bottom that can be used to mount the device to a tripod. Maintaining this stability should help to hold the signal at zero when there is no flow through the transducer.
Page last modified 19Jan2015
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