Respiratory gas analysis instrument having improved volume calibration method and apparatus

In a gas analysis instrument of the type having at least one gas analyzer for measuring the concentration of a compound of interest in human breath, and a gas turbine for producing an output signal that varies in accordance with the volume and rate of flow of breath therethrough, the improvement comprising:

• (a) first means for storing the information necessary to make available a piecewise linear representation of the response of the gas turbine, said representation relating the volume of breath flowing through the turbine to the rate of flow of that breath therethrough,
• (b) a breath switch connected in series with the gas turbine to generate an output signal indicative of the duration of a breath,
• (c) second means responsive to the output signal of the turbine and the output signal of the breath switch for determining the rate of flow of a breath, and
• (d) third means responsive to the first means and the second means for determining volume data for a breath.

A method for volume calibrating a gas analysis instrument of the type having (a) a source for providing a known volume of calibration gas, and (b) a gas turbine for providing an output signal that varies in accordance with the volume and rate of flow of gas therethrough, said method including the steps of:

• (a) storing a first piecewise linear approximation of the output response of a typical turbine of the class of turbine to which said gas turbine belongs,
• (b) directing said known volume of gas through the turbine at a plurality of rates of flow which correspond to a plurality of the linear segments of the piecewise linear approximation and storing data indicative of the resulting turbine output signals,
• (c) combining the approximation of step (a) with the data stored during step (b) to produce a second piecewise linear approximation of the output response of the gas turbine, and
• (d) making the second piecewise linear approximation available for use in interpreting measurements made by the instrument.

In a gas volume calibration apparatus for respiratory gas analysis instruments of the type including (a) a calibration syringe having a housing and a piston slidably mounted therein, said syringe being adapted to provide a known volume of gas as the piston is moved between predetermined first and second positions, and (b) a gas turbine for providing a number of output pulses that varies in accordance with the rate at which said volume of gas flows therethrough, the improvement characterized by:

• (a) a breath switch, connected in series with the turbine, for providing an output signal indicative of the duration of a stroke of the piston,
• (b) first means for receiving the turbine output pulses produced during a stroke of the piston and the output signal of the breath switch and for generating an actual flow rate signal indicative of the speed of the stroke,
• (c) second means for storing maximum and minimum acceptable values for said flow rate signal, and
• (d) third means for comparing the actual flow rate signal with said maximum and minimum acceptable values and for accepting the actual flow rate signal for use in calibrating the instrument only if it compares favorably with said maximum and minimum values.

A method for providing volume calibration data to a gas analysis instrument of the type including (a) a calibration syringe for providing a known volume of gas, said syringe having a housing and a piston slidably mounted therein, and (b) a gas turbine for providing a number of output pulses that varies in accordance with the volume and rate of flow of gas therethrough, said method including the steps of:

• (a) establishing a plurality of pairs of maximum and minimum acceptable pulses-per-second values corresponding to a pluraliy of flow rates for said known volume of gas through the turbine,
• (b) moving the piston in a succession of strokes to cause said known volume of gas to flow through the turbine at a first estimated flow rate,
• (c) calculating the pulses-per-second values associated with the strokes of step (b),
• (d) comparing the resulting pulses-per-second values to a first pair of maximum and minimum acceptable pulses-per-second values,
• (e) storing those pulses-per-second values which compare favorably with said first pair of maximum and minimum acceptable values for use in calibrating the instrument, and
• (f) repeating steps (b) through (e) for at least a second estimated flow rate and a second pair of maximum and minimum acceptable pulses-per-second values.