SENSITIVITY-CALIBRATION CIRCUIT FOR ABSORPTION ANALYZERS

Abstract: The sensitivity-calibration circuit for absorption analyzers comprises two separate temperature-compensa­ tion circuits (A, B), one (A) for compensating the tem­ perature-drift resulting from the optical system of the analyzer and the other one (B) for compensating the tem­ perature-drift caused by a sample system of the analyzer. The latter mentioned temperature-compensation circuit (B) can be blocked-off from operating on a checking and/or measuring input signal (V IN ) by use of a switch (SW) such that during a measuring state both temperature-compensa­ tion circuits (A, B) are cooperating in sequence whereas during a specific checking state by switching over switch (SW) only said temperature-compensation circuit used in connection with checking the optical system is actively op­ erating on said input signal (V IN ). The advantage over the prior art combination sensitivity-calibration circuits is a more accurate and reliable calibration of the absorption analyzer during both, the measuring state and the checking state of the whole system.

Description: The present invention relates to a sensitivity-calibration circuit for an absorption analyzer, such as non-dispersion type infrared analyzer, colorimetric analyzer and spectrophotometric analyzer, in particular in an absorption analyzer provided with a check-signal generating mechanism so that the sensitivity can be easily checked without requiring the continuous use of a span-calibration sample. FIG. 3(A) of the accompanying drawings shows one example of a fundamental non-dispersion type absorption analyzer for use in gas analysis. A cell b, into which a sample gas and a zero gas are alternately indroduced via a three-way valve d, and a sample-concentration detector c are arranged in an optically linear relation relative to a light source a capable of irradiating infrared light beams and a check-signal generating mechanism f comprising a potentiometric circuit called an elechecker is provided between a preamplifier e of said sample-concentration detector c and a sensitivity-calibration circuit g so that the sensitivity can be easily checked without requiring the continuous use of a span-calibration sample (span gas). The elechecker type check-signal generating mechanism f is adapted to be switched over to a state, in which an output signal (sample-concentration signal V G ) from the preamplifier e of said sample-concentration detector c is sent to the sensitivity-calibration circuit g as it is during the measurement carried out by introducing a sample into said cell b, and a state in which a check signal V c obtained by dividing the output signal from said preamplifier e at an appointed ratio is sent to said sensitivity-calibration circuit g during an easy check process carried out by introducing only a zero gas into said cell b. In addition, also a mechachecker comprising a light-reducing filter and a screen plate, which can be inserted between the light source a and the cell b (or between the cell b and the sample-concentration detector c), as shown in FIG. 3(B), can be used as such a check-signal generating mechanism f in addition to said elechecker comprising a potentiometric circuit. In the case of an absorption analyzer provided with this mechachecker type check-signal generating mechanism f, the sample-concentration signal V G is obtained from the preamplifier e of said sample-concentration detector c to be sent to said sensitivity-calibration circuit g by pulling said light-reducing filter as the mechachecker out of an optical path, as shown by a dotted line in FIG. 3(B). The measurement is carried out by introducing the sample into said cell b while the check signal V c is obtained from the preamplifier e of said sample-concentration detector c to be sent to the sensitivity-calibration mechanism g by inserting said light-reducing filter into an optical path, as shown by a full line in FIG. 3(B), or inserting the screen plate into the optical path to an appointed extent during a so-called "easy check" carried out by introducing only the zero gas into said cell b. The sensitivity-calibration circuit g to which said sample-concentration signal V G or said check signal V c is given as an input signal V IN , comprises an operational amplifier O o and a thermo-sensor Th and is provided with a temperature-compensation circuit A o for compensating a temperature-drift of the input signal V IN on the basis of a result detected by the thermo-sensor Th and a sensitivity-adjustment circuit C containing an operational amplifier O₃ and a sensitivity-adjustment potentiometer VR, as shown in FIG. 4. The conventional sensitivity-calibration circuit according to Fig. 4 has the disadvantage that a single temperature-compensation circuit A₀ is provided for compensation in the measuring state as well as in the checking state. However, in the measuring state the temperature-drifts of the sample system and additonally of the optical system are effective, whereas in the checking state only the temperature-drift of the optical system is effective. Therefore, it is not possible to compensate correctly in both states. To overcome the just mentioned disadvantage, DD-A-132 814 describes a sensivity-calibration circuit for a none-dispersion type infrared gas analyzer having a temperature-compensation means comprising two temperature-compensation circuits and a switching means. The first circuit compensates for the added temperature-drift, as mentioned above, whereas the second temperature-compensation circuit only compensates for the temperature-drift of the optical system. It is the object of the present invention to provide a sensitivity-calibration circuit of simple construction but having satisfying temperature-compensation properties in the measuring state as well as in the checking state. The sensitivity-calibration circuit according to the present invention for an absorption analyzer provided with the check-signal generating means comprises: a first, separate temperature compensation circuit within a temperature compensation means for compensating the temperature-drift resulting from the optical system of the analyzer, a second, separate temperature-compensation circuit within a temperature compensation means for compensating the temperature-drift resulting from the sample system of the analyzer, and a switch means being arranged to connect in a checking state only said first temperature-compensation circuit to the output terminal of said temperature compensation means, and to connect in a normal measuring state a series connection of said first and said second temperature-compensation circuits to said output terminal. Embodiments of the invention will now be described with reference to the accompanying drawings. In the drawings: Fig. 1(A) is a block-circuit diagram depicting a fundamental construction of the sensitivity-calibration; Fig. 1(B) shows another possible embodiment of the constructional features of a sensitivity calibration circuit according to the invention; Fig. 2 is a block circuit diagram of a preferred embodiment of a sensitivity-calibration circuit comprising features of the invention; Fig. 3(A) and 3(B) is a principle block diagram of an absorption analyzer, respectively, already discussed above, and Fig. 4 depicts a block circuit diagrame likewise described above of a conventional sensitivity-calibration circuit in an absorption analyzer. In Fig. 1(A) and Fig. 1(B), C designates a sensitivity-adjustment circuit, reference characters O₁, O₂, O₃ designate operational amplifiers, respectively, Th o , Th G refer to thermo-sensors, respectively, and VR designates a sensitivity-adjustment potentiometer. The effects exhibited by the above described characteristical construction are as follows: With a sensitivity-calibration circuit in an absorption analyzer according to the present invention, since a temperature-drift resulting from the sample system and a temperature-drift due to the optical system are not collectively compensated as in a sensitivity-calibration circuit of conventional construction, a normal temperature-compensation can be achieved in not only a regular sensitivity-calibration and during a usual measurement using a span-calibration sample but also during an easy check without using the span-calibration sample. The circuit comprises a temperature-compensation circuit A for use in an optical system and a separate temperature-compensation circuit B for use in a sample system. A sample-concentration signal V G as an input signal V IN can be passed through both said temperature-compensation circuit A and said temperature-compensation circuit B via a two positions switch SW. In a first switching position X, V IN is applied to the so-called "concentration measuring side" for a regular sensitivity-calibration and during a usual measurement. In a second switching position y, a check signal V c can be passed as the input signal V IN only through said temperature-compensation circuit B for use with a sample system simultaneously bypassing said temperature-compensation circuit A. The switching position y is also called the "check side" used during an easy check without using the span-calibration sample. With a circuit arrangement of this type, an accurate sensitivity check or a sensitivity-calibration can be always achieved without the above discussed uncertain reverse effects inherent to the prior art sensitivity-calibrationcircuits described in connection with Figs. 3(A), 3(B) and Fig. 4. The specific advantage achieved by the two separate compensation circuits A and B may be described by equations similar as above. Provided that a temperature-drift function of the optical system is f(t) and a temperature-drift function of the sample system is g(t), the sample-concentration signal V G and the check signal V c may be expressed by the following equations: V G = c₁ · f(t) · g(t) V c = c₂ · f(t) wherein c₁, c₂ are constants. If the gain of said temperature-compensation circuit A for use in the optical system is adjusted to K₁/f(t) and the gain of said temperature-compensation circuit B for use in the sample system to K₂/g(t) in this case, and with the gain G(VR) of the sensitivity-adjustment circuit C, the total gain G TS of the sensitivity-calibration circuit is defined by the following equation in the case where the switch SW is switched to position x. G TS = G(VR) · K₁ · K₂/{f(t) · g(t)} Consequently, the output signal V OUT of the sensitivity-calibration circuit during a regular sensitivity-calibration and a usual measurement process using a span-calibration sample is expressed by the following equation: whereby an influence by temperature is eliminated. On the other hand, the total gain G TC of the sensitivity-calibration circuit in the case, where the switch SW is switched to the check side (position y) and the temperature-compensation circuit B is not used, is defined by the equation: G TC = G(VR) · K₁/f(t) Accordingly, the output signal V OUT of the sensitivity-calibration circuit during an easy check process without using the span-calibration sample is determined by the following equation: Also in this case, the temperature influence is eliminated. The preferred embodiment of the present invention will be below described with reference to the drawing of Fig. 2. The sensitivity-calibration circuit for an absorption analyzer according to the present preferred embodiment comprises said temperature-compensation circuit B for use in a sample system and said sensitivity-adjustment circuit C collectively having the fundamental construction shown in FIG. 1(A) or FIG. 1(B). One operational amplifier can be saved by using the operational amplifier O₂ in said temperature-compensation circuit B also as the sensitivity-adjustment amplifier O₃ in said sensitivity-adjustment circuit C. The sensitivity-adjustment potentiometer VR in said sensitivity-adjustment circuit C is divided into a main sensitivity-adjustment potentiometer VR₁ and an auxiliary sensitivity-adjustment potentiometer VR₂. The other circuit parts are constructed similar to those shown in FIG. 1(A) or FIG. 1(B). At first during the regular sensitivity-calibration using the span-calibration sample the switch SW is positioned at the concentration-measuring side x. In this case, the sample-concentration signal V G , in short V G = c₁ · f(t) · g(t) is supplied as the input signal V IN , wherein c₁ is a constant. Provided that the temperature-drift function of the optical system is f(t) and the temperature-drift function of the sample system is g(t), the gain of temperature-compensation circuit A is expressed by K/f(t) while the gain of the temperature-compensation circuit B and the sensitivity-adjustment circuit C is expressed by G₁(VR₁)/g(t), then the total gain G TS of the sensitivity-calibration circuit under this condition is defined by the following equation: G TS = K · G₁(VR₁)/{f(t) · g(t)} Accordingly, the output signal V OUT of the sensitivity-calibration circuit during the regular sensitivity-calibration using a span-calibration sample is expressed by the following equation: Consequently, the temperature influence is eliminated, and the output signal V OUT becomes constantly independent of temperatures. The main sensitivity-adjustment potentiometer VR₁ serves to adjust G₁(VR₁) so that the output signal V OUT corresponds to the concentration of the span-calibration sample used. On the other hand, during the first easy check without using the span-calibration sample switch SW is positioned at the check side y. In this case, the check signal V c , in short V c = c₂·f(t) is supplied as the input signal V IN , wherein c₂ is a constant. Since the thermo-sensor TH G in the temperature-compensation circuit B is disconnected and the auxiliary sensitivity-adjustment potentiometer VR₂ is used, the gain of the temperature-compensation circuit B and the sensitivity-adjustment circuit C is expressed by G₁(VR₁·G₂(VR₂)/f(t) and the total gain of the sensitivity-calibration circuit under this condition is defined by the following equation: G TC = K·G₁(VR₁)·G₂(VR₂)/f(t). Accordingly, the output signal V OUT from the sensitivity-calibration circuit during the easy check process not using a span-calibration sample is expressed by the following equation: Consequently, also in this case, the temperature influence is eliminated, and the output signal V OUT becomes constantly independent of temperatures. The auxiliary sensitivity-adjustment potentiometer VR₂ is operated to adjust and fix G₂(VR₂) so that the output signal V OUT may assume the same value as during the above described regular sensitivity-calibration. In addition, since the ratios of said constants c₁, c₂ for changing the sensitivity can be made identical by a corresponding adjustment of the auxiliary sensitivity-adjustment potentiometer VR₂ , for a subsequent easy check it is only necessary to operate said main sensitivity-adjustment potentiometer VR₁. However, since also usual measurements are carried out under the condition that said switch SW is positioned at the concentration-measuring side x, it goes without saying that the output signal V OUT of the sensitivity-calibration circuit becomes constantly independent of temperatures in the same manner as with the above described regular sensitivity-calibration. Furthermore, as the expert will realize, the above described sensitivity-calibration circuit according to the present invention can be applied to both cases, i.e. where the sample is a gas (gas analyzer) and the case, where the sample is a liquid (liquid analyzer).

Claims: A sensitivity-calibration circuit for an absorption analyzer provided with a check-signal generating means, said sensitivity-calibration circuit comprising a temperature-compensation means switchable by a switch means between a measuring state and a checking state, said temperature-compensation means comprising: a first, separate temperature-compensation circuit (A) for compensating the temperature-drift resulting from the optical system of the analyzer and a second, separate temperature-compensation circuit (B) for compensating the temperature-drift resulting from the sample system of the analyzer and said switch means (SW) which are arranged to connect in said checking state only said first temperature-compensation circuit to the output terminal of said temperature-compensation means, and to connect in said measuring state a series connection of said first and said second temperature-compensation circuits (A + B) to said output terminal. The sensitivity-calibration circuit as said forth in claim 1, characterized by an amplifier in said second, separate temperature-compensation circuit (B) which is bifunctionally used also as a sensitivity-adjustment amplifier (O₃).

Inventors: ASANO, ICHIRO, KIHARA, NOBUTAKA, KOJIMA, KENNOSUKE
Applicants: HORIBA, LTD.
Application year: 1986
Main classification: G01N21
IPC codes: G01N21