WO1983004101A1

WO1983004101A1 - Oxygen analyzer

Info

Publication number: WO1983004101A1
Application number: PCT/US1983/000704
Authority: WO
Inventors: James A. Hoerlein
Original assignee: Marathon Medical Equipment Corporation
Priority date: 1982-05-07
Filing date: 1983-05-06
Publication date: 1983-11-24
Also published as: EP0108141A1

Abstract

A digital oxygen analyzer is comprised of an oxygen sensor (10) connected to apply input signals to a dividing analog-to-digital converter (14). A thermistor (12) connected in the input circuit of the analyzer compensates for ambient thermal variations, and an absolute pressure transducer (16) coupled to the reference terminals of the analog-to-digital converter compensates for ambient pressure variation. A digital display (15) connected to the analog-to-digital converter indicates relative percentage of oxygen in the vicinity of the oxygen sensor.

Description

OXYGEN ANALYZER

Technical Field The digital oxygen analyzer of the present invention employs a fuel cell for producing a current that varies as a function of the partial pressure of oxygen in the region surrounding the fuel cell. Background Art

The use of such fuel cells for the sensing of oxygen is disclosed, for example, in the U.S. Patent Application 302,602 filed September 15, 1981, by J. A. Hoerlein and assigned to the assignee of the present application. In a fuel cell, the oxygen diffuses through a Teflon membrane and is chemically reduced on the surface of the cathode of the cell. The output of such a cell is limited by the rate at which oxygen diffuses across the membrane and the amount of anode material available for reaction. The rate of reaction in a fuel cell has been found to be temperature dependent such that the output must be temperature corrected as a function of the temperature in the immediate proximity of the fuel cell. This tem¬ perature dependency is recognized in the above-identified application Serial Number 302,602 and accordingly a ther¬ mistor is employed therein for controlling the gain of an operational amplifier coupled to the output of the fuel cell.

Disclosure of Invention The present application is concerned with the provi¬ sion of a portable digital oxygen analyzer that will en¬ able the accurate determination of oxygen content of a gas at any time as opposed to the system of the above application Serial Number 302,602 wherein a fuel cell is employed as a sensor for an alarm device.

Briefly stated, in accordance with the invention, the output of a fuel cell is coupled to the signal input of an analog to digital converter, this input being modi¬ fied by the series or parallel interconnection of a ther- istor for temperature correction. The output of the- analog to digital converter may be employed in a conven¬ tional manner to indicate the partial pressure of oxygen in the gas in the proximity of the fuel cell.

Advantageously, in accordance with a further embodi- ment of the invention, the analyzer is modified to compen¬ sate for ambient pressure in the vicinity of the fuel cell, since such pressure has been found to materially affect the output of the fuel cell. In particularly advantageous embodiment of the invention, the pressure is sensed by a pressure transducer such as a strain gauge, the voltage supply of the strain gauge being adjustable in order to enable calibration of the system. In this embodi¬ ment of the invention the analog-to-digital converter is preferably a dividing analog to digital converter, the out- put of the pressure transducer being applied to reference terminals of the analog to digital converter so that the analyzer may be calibrated to directly display the per¬ centage of oxygen in the gas being tested rather than only the partial pressure of oxygen. The invention thereby enables the provision of an accurate economical portable analyzer enabling the deter¬ mination of oxygen in a gas, such as the percentage of oxygen in atmosphere, the analyzer being readily useable without difficulty by untrained persons. Brief Description of Drawings

In order that the invention will be more clearly understood it will now be disclosed in greater detail with reference to the accompanying drawings, wherein: In the Drawings: FIGURE 1 is a simplified block diagram of a digital oxygen analyzer in accordance with the invention;

FIGURE 2 is a more detailed circuit diagram of the digital oxygen analyzer of FIGURE 1;

FIGURE 3 is a circuit diagram of a modification of the analyzer of FIGURE 2; FIGURES 4 and 5 are graphs illustrating the effect of the series and parallel interconnections of a thermistor in the system of the invention;

FIGURES 6 and 7 are plan and side views of the fuel cell and thermistor of the analyzer of the invention;

FIGURE 8 is a perspective view of one embodiment of a digital oxygen analyzer in accordance with the invention. Best Mode for Carrying Out the Invention: Referring now to FIGURE 1, therein is illustrated a simplified diagram of an oxygen analyzer in accordance with the invention. The analyzer includes an oxygen sen- . sor 10, which may be conventional fuel cell. For this .purpose it has been found satisfactory to employ a Teledyne Class C-l fuel cell manufactured by Teledyne Analytical Instrument Company. The oxygen' sensor 10 produces an elec¬ trical current proportional to the partial pressure of oxygen in the space surrounding the cell. The gas diffuses through a Teflon membrane and is chemically reduced on the surface of the cathode, while a corresponding oxidition occurs at the anode of the cell. The output of the cell is limited by the rate at which oxygen can diffuse across the membrane, and also by the amount of anode material available for reaction.

The output of the oxygen sensor 10, i.e., an electrical current proportional to the partial pressure of oxygen, is supplied to a signal conditioner 11.

Since the rate of reaction in the fuel cell is tem¬ perature dependent, the output current from the cell is therefore also temperature dependent. It is therefore necessary to provide means for compensating for the effect of temperature on the output of the fuel cell. For this purpose, a temperature sensor 12, for example a thermistor, is mounted in very close proximity to the fuel cell and also coupled to the signal conditioner 11 for modification of the output of the fuel cell. The signal conditioner 11

O PI hence provides a temperature independent output dependent upon the partial pressure of oxygen surrounding the fuel cell. This output is applied as an input signal to an analog-to-digital converter 14 for producing a digital 5 signal for display on the display device 15.

Since the output of the fuel cell is directly depen¬ dent upon the ambient pressure, on some occasions it is also desirable to compensate the measurements of the ana¬ lyzer for pressure variations. For this purpose, an absolute

10 pressure transducer 16 is provided in the proximity of the oxygen sensor, the pressure transducer providing an output that serves as a reference level for the analog-to-digital converter. If the analog-to-digital converter is inter¬ connected to divide the inputs applied thereto, the display

15 15 may conveniently show the percentage of oxygen in the atmosphere. The display may, of course, also be calibrated to show percentage of oxygen in the atmosphere even though pressure compensation is not provided, under the assumption of a determined ambient pressure.

20 In the more detailed block diagram of FIGURE 2, a ther¬ mistor 20 is physically mounted on the fuel cell 10, and connected in parallel therewith. This assembly is con¬ nected, by way of a connector 21 to the input terminals of a type 7106A/D converter 22. A calibration resister

25 23 may also be connected in parallel with the input leads, this resister having a value calculated to minimize the variation of the input signal temperature in combination with the selected thermistor 20. The resister 23 need not be in the proximity of the fuel cell.

30 The ambient pressure compensation system is comprised of an absolute pressure transducer 25, such as a strain gauge. The output of this transducer is connected to the reference terminals of the A/D converter, whereby the output of the A/D converter is proportional to the ratio

35 of the signal input (oxygen partial pressure) and the reference input (absolute ambient pressure).

As illustrated in FIGURE 2, the pressure transducer 26 is comprised of a strain gauge bridge, preferably a 15 PSIA absolute pressure transducer, the signal output dia- gonals of the bridge being connected to the reference voltage terminals of the A/D converter 22. One of the supply terminals of the bridge is connected to the posi¬ tive supply and the other is connected to the output of an operational amplifier 27. The operational amplifier 27 is connected as an inverting amplifier, with the in¬ verting input being coupled to a potentiometer 28 connected between the positive supply and the common terminal of the A/D converter. The potentiometer 28 thereby enables con¬ trol of the operating supply connected to the strain gauge, and hence the scale factor between the signals input and reference input. This scale factor is preferably set, for example, to be 100.

The offset of the pressure compensation may be adjusted by means of a potentiometer having its ends connected be- tween the signal outputs of the pressure transducer bridge and its arm connected to the positive supply. This may be set, for example, by adjustments of the potentiometer until a zero output is obtained from the transducer when a vacuum is applied thereto. A fuel cell of the above type has a cell life dependent upon the number of hours of operation and the percent of ^• oxygen detected. Thus, the Class C-l cell has a nominal fuel cell life of 240,000 percent hours of oxygen. This corres¬ ponds to one hundred days of constant exposure to 100 per- cent oxygen. In order to extend the life of the cell, it is hence desirable to expose the cell to oxygen enriched gas only when measurements are to be taken.

For the purpose of maximizing battery life, as illus¬ trated in FIGURE 2, the battery 30, such as a nine volt battery, which powers the operating system of the anlyzer is connected to the circuit by way of a pushbutton switch 31, so that the analyzer is operative only when the switch is manually depressed.

It is also desirable to provide an indication when the battery needs replacement and for this purpose, a type 8211 low voltage detector 35 is connected to pro¬ vide an output to a low battery indication on the display 15. The display 15 may be a seven segment display, for example, a type FED0203. On some occasions it may be unnecessary to provide compensation for pressure changes. In this case, a simpler analyzer may be provided, as illustrated in FIGURE 3, wherein the pressure transducer is deleted, the negative reference terminal is connected to the com- mon terminal of the A/D converter, and the positive re¬ ference terminal is connected to a calibration potentio¬ meter circuit 40.

When the temperature dependent resistor or thermistor is connected in series with the fuel cell, as illustrated in the graph of FIGURE 4, the signal input to the A/D pro¬ vides better correction for temperature changes than when the temperature dependent resister is connected in parallel with the fuel cell, as shown in the graph in FIGURE 5. It is thus preferable to interconnect the thermistor in series with the fuel cell. In the embodiment of the in¬ vention illustrated in FIGURES 2 and 3, however, the parallel interconnection of the thermistor has been em¬ ployed in view of the impracticability of achieving the required intimate contact between the fuel cell and the thermistor when the thermistor is connected in a series with the fuel cell. Thus, when a type C-l fuel cell is employed as illustrated in FIGURES 6 and 7, the thermistor was taped or otherwise held or bonded to the casing of the fuel cell, employing a suitable thermal compound. -A series electrical connection could not readily be achieved with

Λ —2__2 this arrangement.

The oxygen analyzer in accordance with the invention is readily adaptable to portable use. Thus, as illus¬ trated in FIGURE 8, the analyzer may be enclosed in a casing 50, for example about 2 1/2 inches wide, 1 1/2 inches deep and 4 1/2 inches high. The LCD display 15 is mounted to be visible behind a cover 51 of the casing. The push-button switch 31 extends through a hole in the front of the cover 51, or side of the casing 50. An in- let hole 52 for air to be measured is provided in the top of the casing 50, this hole having a diameter of about 0.39 inches. An exhaust hole (not shown) with a diameter of about .3 inches is provided in the rear of the casing. It will of course be apparent that other physical arrangements may be provided for the analyzer of the invention.

If desired, a further hole may be provided in the casing to allow access to the calibration potentiometer. It is of course apparent that many other variations may be provided in accordance with the invention. Thus, the oxygen input for the casing may be in the form of a quarter inch hose barb located on the top of the case. A restrictive inlet orifice may be used to connect to pressurized gas sources, or to continuously monitor the outputs of an oxygen concentrator or like equipment.

Instead of a type 7016 dividing analog-to-digital conver¬ ter, a type 7126 analog-to-digital converter may be alternatively employed for a longer battery life.

In an actual embodiment of the invention, the re- quired intimate thermal contact between the thermistor and the fuel cell was effected by employing a YSI type 44001 thermistor mounted in a 0.1 inch diameter hole drilled 0.15 inches deep in the side of the fuel cell. The thermistor was dipped in a thermal compound before. insertion into the hole, and the bare thermistor wires

OMPI _ were routed up the side of the fuel cell and held in place with tape. These wires were then soldered to the fuel cell contact areas, employing suitable electrical insulation. The use of a series connected thermistor would have required either that the resistor be a part of the fuel cell/thermistor assembly or that three wires be connected between the fuel cell/thermistor assembly and circuit board. Either of these latter solutions would have required an insulated junction in the fuel cell assembly between the very fine thermistor wire and a comparatively coarse wire, thereby resulting in a fra¬ gile assembly.

The oxygen analyzer in accordance with the invention has been found to have an accuracy of the displayed oxy- gen concentration within ^•*- l percent of the reading, with the oxygen concentration displayed being within 1 percent of its final value within 60 seconds after the sensor is exposed to the gas. The analyzer has an accuracy of 1 percent in the temperature range 68 to 85 F, and compen- sation for pressure variation is effective from 19.0 inches of mercury to 32.1 inches of mercury. While the invention has been described in conjunction with oxygen concentration measurement, it is understood that the in¬ vention can be employed to measure any constraint of a gas, with the appropriate sensor.

While the invention has been disclosed and described with reference to a limited number of embodiments, it is apparent that variation and modification may be made there¬ in, and it is therefore intended in the following claim to cover each such variation and modification as falls within the true spirit and scope of the invention.

Claims

1. An analyzer for measuring a constituent of a gas, comprising a sensor, an analog-to-digital converter, means coupling the output of said fuel cell to the input of said analog-to-digital converter, and thermistor means thermally connected to said sensor and electrically con¬ nected to modify the output of said fuel cell that is applied to said analog-to-digital converter, for compen¬ sating for the temperature dependent variation in the out- put of said sensor.

2. The analyzer of claim 1 whereas said thermistor means is connected in parallel with the output of said sensor.

3. The analyzer of claim 1 further comprising a pressure transducer for measuring the ambient pressure in the vicinity of said sensor, and means for modifying the output of said sensor, and means for modifying the output of said analog-to-digital converter in response to said pressure transducer.

4. The analyzer of claim 3 wherein said analog-to- digital converter comprises a dividing analog-to-digital converter connected to provide an output that is the ratio of the input applied from said sensor and the out¬ put of said pressure transducer.

5. The analyzer of claim 4 wherein said pressure transducer comprises a strain gauge connected as a resis¬ tor bridge.

6. The analyzer of claim 5 further comprising potentiometer means connected to enable adjustments of operating voltage applied to said strain gauge, for cali¬ brating said analyzer.

7. The analyzer of claim 1 further comprising a source of operating potential for said analyzer and con¬ nected thereto by way of a momentarily operable switch, whereby said analyzer is energized only when measurements are to be taken.

8. An analyzer for measuring the oxygen content of a gas, comprising a fuel cell, a dividing analog-to- digital converter, means applying the output of said fuel cell as a signal input to said analog-to-digital converter, thermistor means thermally connected to said fuel cell and electrically connected to modify the current of said cell that is applied to said analog-to-digital converter, for compensating for temperature dependent variations in the output of said cell, said analog-to-digital converter having reference terminals, pressure transducer means connected to said reference terminals for applying a voltage thereto responsive to pressure in the vicinity of said fuel cell, said dividing analog-to-digital converter providing the output proportional to the ratio of the signal input and the voltage applied to the reference ter¬ minals thereof, and digital indicator means coupled to display the output of said analog-to-digital converter means.

9. The analyzer of claim 8 further comprising means coupled to said pressure transducer for applying an adjust¬ able voltage thereto for calibrating said analyzer.

OMH