WO1998046277A2

WO1998046277A2 - Humidity compensation for fuel cell zeroing in respiratory gas measurement

Info

Publication number: WO1998046277A2
Application number: PCT/US1998/006254
Authority: WO
Inventors: Kathleen Bergeron Gull
Original assignee: Ohmeda, Inc.
Priority date: 1997-04-16
Filing date: 1998-03-30
Publication date: 1998-10-22
Also published as: AU6872698A; WO1998046277A3

Abstract

The accuracy of respiratory gas measurements taken by electrochemical fuel cells is affected by changes in humidity. Such changes become more apparent after a monitoring bench containing fuel cells has been zeroed, and then returned to the sampling mode where a measurement is taken of a respiratory gas sample. The difference in humidity between the sampling gas stream, and the zeroing gas stream causes a zero drift in the fuel cell readings. A piece of NAFION tubing (50) is used after the pneumatics manifold (10) which switches between the two gas streams (30, 40) and the fuel cell bench (20) in order to equilibrate the humidity in both the sample (40), and zero gas streams (30).

Description

HUMIDITY COMPENSATION FOR FUEL CELL ZEROING IN RESPIRATORY GAS MEASUREMENT

BACKGROUND OF THE INVENTION In respiratory gas monitors electrochemical fuel cells are used in conjunction with additional circuitry to measure the amount of a specific gas within a respiratory gas stream. For example, in a monitor for measuring the concentration of nitric oxide and nitrogen dioxide electrochemical fuel cells are used to generate an electrical signal indicative of the respective concentrations of nitric oxide and nitrogen dioxide. Such a monitor is useful during the administration of nitric oxide to a patient for medical purposes. For example, see United States Patent No. 5,485,827, Zapol et al., which discusses the method of treating patients with nitric oxide and United States Patent No. 5,558,083, Bathe et al., which discloses a nitric oxide delivery system of which a gas monitor is one element.

SUMMARY OF THE INVENTION

The accuracy of respiratory gas measurements taken by electrochemical fuel cells is affected by changes in humidity. Such changes come more apparent after a monitoring bench containing fuel cells has been zeroed and then returned to the sampling mode where a measurement is taken of a respiratory gas sample. The difference in humidity between the sampling gas stream and the zeroing gas stream causes a zero drift in the fuel cell readings. A six inch piece of Nafion® tubing is used in the present invention after the pneumatics manifold which switches between the two gas streams and the fuel cell bench in order to equilibrate the humidity in both the sample and zero gas streams. The addition of this Nafion® tubing eliminates the zero drift between the two gas sources allowing for an accurate zeroing of the nitrogen oxide and nitrogen dioxide fuel cells.

One of the problems associated with respiratory gas monitors is the excess humidity in the sample gas stream. United States Patent No. 5,042,500 to Norlien et al. discloses the use of Nafion® tubing placed between the patent and a respiratory gas monitor to dry the exhaled breath of patient. However, this does not eliminate the zeroing problem described above which is due to discrepancies in humidity between the sample gas (exhaled breath) and the zero gas streams. Therefore, it is an object of this invention to provide for a novel apparatus and method for the removal of humidity differences between the sample and zero gas streams in order to reduce or eliminate monitoring inaccuracies due to zero drift.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram of the gas stream flow components of a respiratory gas monitor according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 depicts the major components of a respiratory gas monitor according to the present invention through which the sample and zero gases flow. The pneumatic manifold 10 controls the source of the gas stream input to the bench 20 containing the electrochemical fuel cells for the measurement of nitric oxide, nitrogen dioxide and oxygen. The manifold 10 enables the monitor control to select between inputting a sample of the respiratory gas stream into the sample inlet 40 or a sample of a calibration gas into scrubber 30. A Nafion® tube or other hydrophilic tube 50 is placed between manifold 10 to bench 20 to reduce the amount of moisture in the gas stream. Vacuum pump 60 provides the Vacuum necessary to draw the sample or calibration (zeroing) gases through the system.

A series of tests was conducted using a respiratory gas monitor having electrochemical cells for the measurement of nitric oxide, nitrogen dioxide and oxygen. Differing amounts of Nafion® tubing were placed between the scrubber 30 and the manifold 10 and the relative humidity of the gas was measured on the opposite side of the bench 20 as set forth in the table below:

Table 1

It appears from the table above that there is an increase in drying efficiency with an increase in the length of the Nafion tubing placed after the scrubber. However the gas concentrations measured by the cells did not change appreciably with the change in relative humidity in this dry gas sample. Therefore, there is no advantage to adding Nafion to the scrubber side of the system.

The next configuration tested used humidified sample gas and dry room air gas. The system was zeroed in each of four configurations: (1) no Nafion; (2) 4 inches of Nafion; (3) 6 inches of Nafion; and (4) 16 inches of Nafion. After zeroing the system was switched to humid sample gas and the variance in the relative humidity of the gas readings taken on the far side of the bench was recorded as set forth in the following Table 2.

Table 2

These readings show that the difference in humidity between the sample side and the room air side affects the cell readings. Although the Nafion appeared to be working on the sample side because the original gas entering the sample line had a relative humidity of about 85% it did not reduce the humidity level in that line to that of ambient. The nitrogen dioxide and the oxygen cells were more susceptible to the swing in humidity than the NO cell. In view of this experimental data it can be seen that the use of Nafion tubing at the scrubber does not solve the problem of humidity in the system.

Six inches of Nafion tubing was placed in front of bench 20 after the manifold 10. This length was chosen because it was minimally difficult to package and the prior test showed no marked improvement for much longer lengths of tubing. The system was then switched to a dry sample line and, thereafter, a humidified sample line resulting in the following:

Table 3 Thus, six inches of Nafion tubing in front of the bench nearly eliminates the drift phenomenon. Although it does not completely eliminate the difference in relative humidity between the sample side of the system and the ambient air side of the system, the differences are lower than previously measured.

Claims

I claim:

1. An apparatus for the measurement of the concentration of a predetermined respiratory gas in a respiratory gas sample comprising: a respiratory gas sample inlet for inputting said respiratory gas sample; a scrubber for inputting a calibration gas; a respiratory gas measurement bench for measuring the concentration of said predetermined respiratory gas in said respiratory gas sample or said calibration gas; a manifold for selecting between input of a portion of said respiratory gas sample or a portion of said calibration gas into the respiratory gas measurement bench; a means between said manifold and the respiratory gas measurement bench for reducing the difference in humidity between said portion of said respiratory gas sample and said portion of said calibration gas sample.

2. The apparatus of Claim 1 wherein said means for reducing the difference in humidity comprises a length of tubing made from a hydrophilic material.

3. The apparatus of Claim 2 wherein the length of tubing is made from Nafion(R).

4. The apparatus of Claim 2 wherein the length of tubing is approximately six inches.

5. The apparatus of Claim 1 further comprising a vacuum pump for drawing either of said portion of said respiratory gas sample or said portion of said calibration gas through the respiratory gas measurement bench.

6. The apparatus of Claim 1 wherein said predetermined gas sample is nitric oxide.

7. The apparatus of Claim 1 wherein said predetermined gas sample is nitrogen dioxide.

8. A method of measuring the concentration of a predetermined respiratory gas in a respiratory gas sample comprising: placing a respiratory gas sample onto a sample inlet; placing a calibration gas into a scrubber; selecting between input of a portion of said respiratory gas sample and said calibration gas into a respiratory gas measurement bench; equilibrating the humidity present in said selected portion of said respiratory gas sample or said calibration gas sample prior to input of said selected portion into said respiratory gas measurement bench; measuring the concentration of said predetermined respiratory gas in said selected portion using said respiratory gas measurement bench.

9. The method of Claim 8 wherein said controlling step comprises flowing said selected portion through a length of tubing made from a hydrophilic material.

10. The method of Claim 9 wherein said length of tubing is made form Nafion(R).

11. The method of Claim 9 wherein said length of tubing is approximately six inches.

12. The method of Claim 8 wherein said predetermined gas is nitric oxide or nitrogen dioxide.