Gasless calibration in metabolic gas analyzers
View Patent ↗A method of calibrating a metabolic analyzer incorporating an oxygen analyzer and a NDIR carbon dioxide analyzer in the field that does not require the use of gas cylinders containing gases of known concentration is described. In calibrating the CO 2 detector, at the time of factory setup, the detector output for a gas of a known concentration is measured and stored in the memory of the metabolic analyzer's microprocessor, as is the detector output voltage when the IR source is dimmed by a known percentage. Subsequently, in the field, CO 2 levels in ambient air and cell pressure are measured at two different flow rates through the sample chamber and the IR source is again dimmed by the same percentage as had been used at the time of factory setup. Based upon the resulting readings, both the zeroing and span adjustment factors can be computed.
1. A method of calibrating a metabolic analyzer having a sample line adapted to receive inspiratory and expiratory respiratory gas therein, the sample line connected to a non-dispersive infrared carbon dioxide sensor having an IR emitter and an IR detector spaced from one another in the sample line, an oxygen sensor in series with said carbon dioxide sensor, means for drawing the respiratory gases through the carbon dioxide sensor and the oxygen sensor, a pressure sensor for sensing absolute pressure in the sample line and a flow regulator controlling the rate that the respiratory gases are drawn and a microprocessor based controller operatively coupled in controlling relation to the IR emitter, the IR detector, the oxygen sensor, the pressure sensor and the flow regulator comprising the steps of:
(a) at the time of factory set-up:
(i) introducing two samples of ambient air of differing CO 2 concentration into said sample line and storing resulting CO 2 detector output signals in a memory of the microprocessor based controller;
(ii) calculating an ambient CO 2 gain factor corrected to normal temperature and pressure and store resultant value;
(iii) measuring the CO 2 concentration and pressures in the sample line at two discrete flow rates of predetermined values and storing the difference in CO 2 detector output signal values and pressure sensor output signal valves;
(iv) reducing current to IR emitter by a fixed percentage and record resulting CO 2 detector output signal;
(v) computing and storing ratio of detector output signal due to dimming to detector output signal due to introduction of a known span gas introduction in step (a)(i) corrected to normal temperature and pressure;
(b) in the field:
(i) measuring CO 2 concentration in ambient air sample drawn into sample line at a first flow rate and at a second flow rate;
(ii) determining the difference between the ratio established in step (a)(iii) and step (b)(i) as the zero calibration factor;
(iii) reducing the current to the IR emitter by the same fixed percentage as used in the step (a)(iv);
(iv) comparing the ratio of outputs as established in step (a)(v) to those achieved in step (b)(iii); and
(v) computing a span calibration factor as:
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2. The method of claim 1 and further including a step of calibrating the oxygen sensor by:
(a) at the time of factory setup:
(i) storing a psychometric table in the memory of the microprocessor based controller whereby percentage water vapor in ambient air can be derived from measured values of relative humidity, barometric pressure and temperature;
(b) in the field:
(i) measuring relative humidity, temperature, barometric pressure and the CO 2 concentration at the site of the field calibration;
(ii) performing a table look-up operation to obtain percent water vapor in ambient air sample;
(iii) computing correction factor to be applied to percentage of O 2 in dry air to arrive at percentage of O 2 in ambient air due to water vapor and level of CO 2 and N 2 in sample of ambient air; and
(iv) using computed correction factor as a span adjustment factor and zero as an offset value.