Self-calibrating carbon dioxide analyzer

• US 4,578,762 A
• Filed: 07/01/1983
• Issued: 03/25/1986
• Est. Priority Date: 07/01/1983
• Status: Expired due to Fees

• Current Assignee: Biochem International, Inc.
• Original Assignee: Tri-Med Incorporated
• Inventors: Wong, Jacob Y.
• Primary Examiner(s): RUGGIERO, JOSEPH F
• Application Number:
  US06/509,920
• Time in Patent Office: 998 Days
• Field of Search: 364/496-499, 364/571, 250/343-345, 250/347-351, 250/353, 422/62, 422/83, 422/84, 422/88, 422/89, 422/92, 422/98, 422/119, 436/900, 128/719, 128/205.28, 128/205.29, 356/51, 356/437, 356/243
• US Class Current: 702/32
• CPC Class Codes: G01J 1/36 using electric radiation detectors
• CPC Class Codes: G01N 21/274 Calibration, base line adjustment, drift correction
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis
• CPC Class Codes: G01N 2201/12723 Self check capacity; automatic, periodic step of checking
• CPC Class Codes: G01N 33/0006 Calibrating gas analysers

Abstract

A carbon dioxide analyzer for medical purposes is rendered self-calibrating by continuously measuring pairs of two or three each of several components as follows:

1st pair: vacuum cell, open hole and sample cell reference cell, open hole and sample cell

2nd pair: vacuum cell, standard cell reference cell, standard cell

3rd pair: vacuum cell, open hole reference cell, open hole

4th pair: vacuum cell, standard cell, sample cell reference cell, standard cell, sample cell

The ratios of these measurement pairs are treated mathematically in a computer or microprocessor to obtain a reading for CO₂ and to correct other readings and to monitor the integrity of the standard cell. A novel two-wheel chopper and mirror arrangement facilitates the measurements.

Claims

1. Optical-mechanical apparatus for a gas analyzer for generating electrical signals for continuously and automatically self-calibrating the electrical output comprising:

(a) a first chopper wheel containing openings occupied by two items of (1) vacuum cell, (2) reference cell, (3) open hole, (4) standard cell;

(b) a second chopper wheel containing openings occupied by the other two items of element "a" above;

(c) means for aligning the openings of the two wheels to define a path;

(d) means for rotating the two wheels in synchronism to align selected openings in the two wheels along said path;

(e) a source of radiant energy disposed at one end of said path;

(f) a sample chamber disposed along said path downstream from said wheels;

(g) an electrical transducer detector of radiant energy disposed at the other end of said path to create changes in electrical quantities,

whereby the detector creates values V_S, V_R, V_S '"'"', V_R '"'"', V_S ", V_R ", V_S^o, and V_R^o, for use in rendering the gas analyzer self-calibrating, and wherein these values are;

V_S ;

vacuum cell and open hole and sample cell,

V_R ;

reference cell and open hole and sample cell,

V_S '"'"';

vacuum cell and standard cell,

V_R '"'"';

reference cell and standard cell,

V_S ";

vacuum cell and open hole,

V_R ";

reference cell and open hole,

V_S^o ;

vacuum cell and standard cell and sample cell,

V_R^o ;

reference cell and standard cell and sample cell.

2. Apparatus as set forth in claim 1 wherein a filter is disposed in the path to pass only a selected portion of the electromagnetic spectrum.

3. Apparatus as set forth in claim 1 wherein the two wheels are mounted on the same axle.

4. Optical-mechanical apparatus for a gas analyzer for generating electrical signals for continuously self-calibrating the analyzer output comprising:

(a) a source of radiant energy radiating energy along a first path;

(b) an electrical transducer detector for receiving energy at one end of a second radiant energy path;

(c) a first mirror disposed at the other end of said first path and directing energy on said detector;

(d) a second mirror disposed at the other end of said second radiant energy path;

(e) a first chopper wheel containing openings and rotatable to dispose selected openings in said first path, said openings occupied by two items of (1) vacuum cell, (2) reference cell, (3) open hole, (4) standard cell;

(f) a second chopper wheel containing openings occupied by the other two of said items of "e" above and also containing mirrors and rotatable to dispose selected openings on both paths, said chopper wheel mirrors being so disposed that energy in the first path is directed to the second mirror in the second path;

(g) a sample chamber disposed in said first path downstream from the chopper wheels; and

(h) means for synchronizing the rotation of the two wheels to dispose selected openings simultaneously in the two paths;

whereby said detector generates electrical signals of V_S, V_R, V_S '"'"', V_R '"'"', V_S ", V_R ", V_S^o, and V_R^o, and wherein these signals result from radiant energy passing through the following wheel openings;

V_S ;

vacuum cell and open hole and sample cell,

V_R ;

reference cell and open hole and sample cell,

V_S '"'"';

vacuum cell and standard cell,

V_R '"'"';

reference cell and standard cell,

V_S ";

vacuum cell and open hole,

V_R ";

reference cell and open hole,

V_S^o ;

vacuum cell and standard cell and sample cell,

V_R^o ;

reference cell and standard cell and sample cell.

5. Apparatus as set forth in claim 4 wherein a beam splitter is disposed in the second path downstream from the second chopper wheel and the first mirror directs energy on the beam splitter to thereby direct energy from the first path to the detector.

6. Apparatus as set forth in claim 4 wherein a filter is disposed immediately upstream from the detector.

7. In a gas measurement apparatus employing sample chamber, a reference cell and a vacuum cell to obtain V_R /V_S measurement, a standard cell for calibration and a computer, the method of detecting leakage in the standard cell comprising:

(a) storing in permanent memory at least three numerical coefficients A₀, A₁, and A₂ to represent the calibration S-curve;

(b) storing in nonvolatile but erasable memory Q₀, Q₁, and Q_S^o ;

(c) continuously measuring V_R when the reference cell and the sampling chamber are in series;

(d) continuously measuring V_S when the vacuum cell and the sampling chamber are in series;

(e) continuously ratioing said V_R over said V_S to obtain Q(Q=V_R /V_S);

(f) continuously measuring V_R^o when the reference cell, standard cell, and sampling chamber are in series;

(g) continuously measuring V_S^o when the vacuum cell, standard cell, and sampling chamber are in series;

(h) continuously ratioing said V_R^o over said V_S^o to obtain Q_I (Q_I =V_R^o /V_S^o);

(i) continuously calculating S, S_S, and S_I from the measured values of Q and Q_I and Q₀, Q₁ and Q_S^o values retrieved from erasable memory;

(j) storing in permanent memory a constant value Δ

I representing a tolerance between the measured S_I (calculated directly from Q_I) and the calculated new S_I or S_I *;

(k) continuously calculating S_I * in accordance with the formula ##EQU12## (l) calculating the difference between S_I and S_I *;

(m) comparing the absolute value of the difference between S_I and S_I * to the tolerance Δ

I; and

(n) energizing an alarm after a selected time delay if the absolute value of the difference between S_I and S_I * exceeds the tolerance Δ

I.

8. In a gas measurement apparatus employing a sample chamber, a reference cell and a vacuum cell to obtain a V_R /V_S measurement, a standard cell for calibration and a computer, the method of checking for shifts in Q_A^o and Q_S^o comprising:

(a) storing in nonvolatile but erasable memory Q_A^o and Q_S^o ;

(b) storing in permanent memory a tolerance Δ

A;

(c) storing in permanent memory a tolerance Δ

S;

(d) continuously measuring Q_A and Q_S ;

(e) continuously computing a new Δ

Q_A according to the formula

space="preserve" listing-type="equation">Δ

Q.sub.A =Q.sub.A -Q.sub.A.sup.o

(f) continuously computing a new Δ

Q_S according to the formula

space="preserve" listing-type="equation">Δ

Q.sub.S =Q.sub.S -Q.sub.S.sup.o

(g) continuously comparing Δ

Q_S with Δ

S stored and comparing Δ

Q_A with Δ

A stored; and

(h) continuously correcting Q_A^o and Q_S^o by Δ

Q_A and Δ

Q_S, respectively, in erasable memory if either Δ

Q_S or Δ

Q_A exceeds the tolerances.

9. In a gas measurement apparatus employing a sample chamber, a reference cell and a vacuum cell to obtain V_R /V_S measurement, a standard cell for calibration and a computer having erasable memory and permanent memory, the method of continuously self-calibrating the S-curve as components change with aging, temperature, and other causes, comprising:

(a) storing A₀, A₁, A₂, Δ

A, Δ

S, K₁, and K₂ in permanent memory;

(b) storing Q₀, Q₁, Q_S^o, and Q_A^o in nonvolatile but erasable memory;

(c) continously measuring and calculating Q_A and Q_S ;

(d) continuously computing Δ

Q_A according to the formula Δ

Q_A =Q_A -Q_A^o ;

(e) continously computing Δ

Q_S according to the formula Δ

Q_S =Q_S -Q_S^o ;

(f) continously comparing Δ

Q_A with Δ

A stored;

(g) continously comparing Δ

Q_S with Δ

S stored;

(h) if either Δ

Q_A or Δ

Q_S exceeds the tolerance Δ

A or Δ

S respectively, continuously computing Δ

Q₀ according to the formula ##EQU13## (i) if either Δ

Q_A or Δ

Q_S exceeds the tolerance Δ

A or Δ

S respectively, continuously computing Δ

Q₁ according to the formula ##EQU14## (j) algebraically adding calculated Δ

Q_A from stored Q_A^o to obtain Q_A^o, new and store the result in erasable memory;

(k) algebraically adding calculated Δ

Q_S from stored Q_S^o to obtain Q_S^o, new and store the result in erasable memory;

(l) continuously calculating Q₀^new according to the formula

space="preserve" listing-type="equation">Q.sub.0.sup.new =Q.sub.0 +Δ

Q.sub.0

(m) continuously calculating Q₁^new according to the formula

space="preserve" listing-type="equation">Q.sub.1.sup.new =Q.sub.1 +Δ

Q.sub.1

(n) erasing from memory the prior values of Q₀, Q₁, Q_A^o, and Q_S^o and substituting the values of Q₀^new, Q₁^new, Q_A^o, new, and Q_S^o, new ; and

(o) continuously calculating S values from measured Q values using Q₀^new and Q₁^new according to the formula

space="preserve" listing-type="equation">S=(Q-Q.sub.0.sup.new)/(Q.sub.1.sup.new -Q.sub.0.sup.new)

10. In a gas measurement apparatus employing a sample chamber, a reference cell and a vacuum cell to obtain a V_R /V_S measurement, a standard cell for calibration and a computer, the method of calculating the partial pressure of the gas comprising:

(a) storing in permanent memory the values of A₀, A₁ and A₂ representing the S calibration curve;

(b) continuously measuring, correcting, and storing in erasable memory Q₀ and Q₁ ;

(c) continuously computing Q from the formula Q=V_R /V_S

(d) continuously calculating S from the formula

space="preserve" listing-type="equation">S=(Q-Q.sub.0)/(Q.sub.1 -Q.sub.0)

(e) calculating the partial pressure of the gas from the formula

space="preserve" listing-type="equation">pCO.sub.2 (mmHg)=A.sub.0 +A.sub.1 S+A.sub.2 S.sup.2

(f) and delivering the result to an intelligible display device.