AUTOMATIC MEASUREMENT-RANGE CHANGEOVER DEVICE FOR SURFACE AREA MEASUREMENT APPARATUS

Abstract: This invention consists in an automatic measurement-range changeover device provided for the surface area measurement apparatus. The automatic changeover device is provided with an output value discriminating circuit to operate in such manner that: fed gas quantity detecting signals outputted from fed gas quantity measuring means to detect a gas quantity fed to the sample cell and discharged gas quantity detecting signals outputted from discharged gas quantity detecting means to detect a gas quantity discharged from said sample cell are inputted into the device, and, when differential signals outputted from an amplifier to amplify a difference between fed gas quantity detecting signals and discharged gas quantity detecting signals reach a gain of the amplifier, changeover signals are output for setting a gain of the amplifier at a low level during the period of desorption whereas, when said differential signals outputted from the amplifier is lower than the set value for the gain of the amplifier, changeover signals are output so that the gain of the amplifier is changed over to a high level during the abovesaid period.

Description: The invention relates to a surface area measuring device according to the preamble of claim 1. As shown in Fig. 4, a conventional surface area measuring device has a sample cell 2 mounted detachably in a gas passage 1 which has means A for measuring a flow rate of supply gases to be supplied for measuring a flow rate of measuring gases to the sample cell 2 and means B for measuring a flow rate of discharge gases for measuring an amount of gases discharged from the sample cell 2. A signal G1 for detecting an amount of supply gases generated from the means A for measuring the flow rate of the supply gases and a signal G2 for detecting an amount of discharge gases generated from the means B for measuring the amount of the discharge gases are inputted into an amplifier 3, and the amplifier 3 subtracts the signal G1 for detecting the amount of the supply gases and the signal G2 for detecting the amount of the discharge gases and generates an amplified differential signal G3. In conventional surface area measuring devices of this kind, the signal G2 for detecting the amount of the discharge gases becomes usually smaller than the signal G1 for detecting the amount of the supply gases when the measuring gases are adsorbed on the sample in the sample cell 2 under a cold state, so that the differential signal G3 to be generated from the amplifier 3 is outputted as a negative signal as shown in Fig. 5. The magnitude of the negative differential signal G3 is proportional to the magnitude of the amount of the gases to be adsorbed on the sample. As adsorption of the gases to the sample reaches saturation, the negative differential signal G3 is returned to zero. Thereafter, the cooling of the sample cell 2 is suspended to return the sample cell 2 to ambient temperature. As a result, the gases adsorbed on the sample start desorbing. As shown in Fig. 5, as the gases are desorbed and the signal G2 for detecting the amount of the discharge gases becomes larger than the signal G1 for detecting the amount of the supply gases, the differential signal G3 becomes an increasing positive signal and the magnitude of the differential signal G3 decreases as the desorption gets finished. And the differential signal G3 is returned to zero as the desorption of the gases has been finished. In measuring the surface area, the surface area is calculated by integrating the positive differential signals G3 by means of an arithmetic processing unit 4 in a period of time for desorption of the gases and the calculation result is displayed on the display unit 5 and generated. Such conventional surface area measuring devices of this kind, however, have a switch 6 for shifting gains and the gains of the amplifier 3 are shifted manually. In other words, when the negative differential signal G3 in a period of time for adsorption reaches the set gain of the amplifier 3, the gain of the amplifier 3 is shifted manually to a low level, thereby preventing the positive differential signal G3 from reaching the set gain of the amplifier 3 in a period of time for desorption. If the gain of the amplifier 3 is not shifted, the positive differential signal G3 has reached the set gain of the amplifier 3 in the period of time for desorption, too, as shown in Fig. 6, and the amount of gases desorbed cannot be accurately calculated by integrating the positive differential signals G3. When the magnitude of the negative differential signal G3' during a period of time for adsorption is too small, the gain of the amplifier 3 should be shifted to a gain of a high level because the surface area cannot accurately be calculated even by integrating minute differential signals G3". It is extremely laborious to shift the gains of the amplifier 3 manually and an output should always be monitored during measuring for the surface area. Furthermore, the outcome of measurement may become inaccurate in some cases. The present invention has been accomplished under the circumstances as described hereinabove. More particularly, the object of the present invention is to provide a system for shifting a measuring range of a surface area measuring device so arranged as to be capable of automatically shifting the gain of the amplifier so as to allow the differential signal to reach no set gain of the amplifier during a period of time for desorbing gases, in order to accurately calculate the surface area. In order to achieve the object, the surface area measuring device according to the invention is characterized by the features of claim 1. From the document "Analytical Instrumentation, Vol. 15, No. 3, 1986, New York, US, pages 241 to 257", an autoranging amplifier is known as such. The document mentions the use in electrochemical instruments. The US-A 3 211 006 discloses a continuous flow method and apparatus for determining absorption isotherms of solid material. In accordance with the present invention, the differential signal outputted from the amplifier is outputted to the output value determining circuit during a period of time for adsorbing gases. The output value determining circuit generates a signal for reducing a level of the gain for altering the gain of the amplifier to a low level by comparing the set gain or cutoff value of the amplifier with the differential signal to be inputted, while generating the signal for reducing the level of the gain to the shift switch for shifting the level of the gain, when it is detected that the differential signal reaches zero from a negative value. Further, the output value determining circuit monitors to determine if the differential signal has a constant output during a predetermined period of time. For instance, it monitors to determine if the differential signal exceeds 50% of the gain, and it generates a signal for increasing the level of the gain when the differential signal does not exceed 50% of the gain while it outputs the signal for increasing the level of the gain to a switch for shifting the level of the gain. The differential signal does not always exceed the gain of the amplifier during a period of time for desorbing gases and the differential signal does not become too small with respect to the gain of the amplifier, so that the surface area can accurately be calculated by integraing the differential signals. Fig. 1 is a circuit diagram showing an embodiment according to the present invention. Fig. 2 is a graph showing a wave form indicating the action of the present invention. Fig. 3 is a circuit diagram showing an example of the structure of the output value determining circuit in the aforesaid embodiment of the present invention. Fig. 4 is a circuit diagram showing a conventional device. Fig. 5 is a graph showing a wave form indicating a state of signals generated from the amplifier when measuring gases are adsorbed on a sample in the sample cell and when the measuring gases adsorbed are desorbed. Fig. 6 is a graph showing a wave form indicating a signal to be outputted from the amplifier for a conventional device. BEST MODES FOR CARRYING OUT THE INVENTION The present invention will be described more in detail by referring to an example. It is to be noted herein that the present invention is construed as being not limited to the following embodiments and it can appropriately be modified without departing from the scope of the present invention. As shown in Fig. 1, the surface area measuring device is such that a sample cell 11 is detachably connected to a passage 10 for measuring gases, means for measuring a flow rate of supply gases, i.e. a first gas flow meter A of thermal conductivity type, which generates the signal G1 for detecting an amount of supply gases by measuring a flow rate of the supply gases to be supplied to the sample cell 11, is disposed in the gas passage 10 for the measuring gases, and means for measuring a flow rate of discharge gases, i.e. a second gas flow meter B of thermal conductivity type, which generates the signal G2 for detecting an amount of discharge gases by measuring a flow rate of the discharge gases to be discharged from the sample cell 11, is disposed in the gas passage 10 for the measuring gases. This surface area measuring device further has an amplifier AMP, a shift switch SW for shifting gains of the amplifier, an output value determining circuit 12, arithmetic means 13, and a display unit 14. The amplifier AMP is so arranged as to receive the signal G1 for detecting the amount of supply gases to be generated from the first gas flow meter A of thermally conductive type and the signal G2 for detecting the amount of discharge gases to be generated from the second gas flow meter B of thermally conductive type, to subtract one signal from the other signal, and to amplify and generate the differential signal G3. The shift switch SW for shifting the gains of the amplifier AMP is so arranged as to automatically shift the gains of the amplifier AMP between a high level RH and a low level RL in response to a shift signal G4 to be generated from the output value determining circuit 12. As shown in Fig. 3, the output value determining circuit 12 comprises a circuit 12a for generating a signal for lowering the level of a gain, a circuit 12b for generating a signal for detecting a zero level, a circuit 12c for generating a signal for expanding the level of a gain, a first AND circuit 12d, a second AND circuit 12e, and an OR circuit 12f. The circuit 12a for generating the signal for lowering the level of the gain is so arranged as to receive the differential signal G3 to be outputted from the amplifier AMP, to compare the differential signal G3 with the set gain of the amplifier AMP, and to generate and output a signal GL for lowering the level of the gain when the differential signal G3 agrees with the set gain of the amplifier AMP. The circuit 12b for generating the signal for detecting the zero level is so arranged as to receive the differential signal G3, to detect the time at which the differential signal G3 turns from its negative value to zero, and to output a signal Go for detecting the zero level. The circuit 12c for generating the signal for expanding the level of the gain is so arranged as to receive the differential signal G3, to compare the differential signal G3 with a reference value predetermined with respect to the set gain, i.e. 50% of the set gain, and to detect that the differential signal G3 does not reach 50% of the set gain for a predetermined period of time, thereby outputting a signal GH for expanding the level of the gain. The first AND circuit 12d is to perform AND operation between the signal GL for lowering the level of the gain and the signal Go for detecting the zero level. The second AND circuit 12e is to perform AND operation between the signal GH for expanding the level of the gain and the signal Go for detecting the zero level. The OR circuit 12f is to perform OR operation between the signal outputted from the first AND circuit 12d and the signal outputted from the second AND circuit 12e. The arithmetic unit 13 is so arranged as to calculate the surface area of a sample by integrating the differential signals G3 during the period of time for desorption of the gases, a differential signal being outputted from the amplifier AMP. The arithmetic unit 13 may be composed of a CPU. The display unit 14 is a printer or a CRT unit for outputting or displaying data from the arithmetic unit 13. When the measuring gases are adsorbed on the sample in the sample cell 11 in the device having the aforesaid structure, the differential signal G3 to be outputted from the amplifier AMP is a negative signal as shown in Fig. 2. This negative signal is inputted into the output value determining circuit 12 which in turn is so arranged as to allow the negative differential signal G3 to be inputted into the circuit 12a for generating the signal G3 for lowering the level of the gain, the circuit 12b for generating the signal Go for detecting the zero level and the circuit 12c for generating the signal GH for expanding the level of the gain. It is noted herein that, for instance, if the differential signal G3 is a signal which exceeds the gain, the signal GL for lowering the level of the gain is outputted from the circuit 12a for generating the signal for lowering the level of the gain. The signal GL for lowering the level of the gain is inputted into the first AND circuit 12d. On the other hand, the extent to which the measuring gases are adsorbed on the sample approaches its saturated state, the differential signal G3 to be outputted from the amplifier AMP increases from the negative value to the zero level and, as the measuring gases are adsorbed on the sample to a saturated extent, the differential signal G3 is turned to the zero level. At this time, the circuit 12b for generating the signal for detecting the zero level disposed in the output value determining circuit 12 detects the zero level to which the differential signal G3 has reached and generates the signal Go for detecting the zero level. The signal Go for detecting the zero level is outputted into the first AND circuit 12d which in turn outputs the signal GL for lowering the level of the gain through the OR circuit to the shift switch SW for shifting the gain of the amplifier AMP, in response to the signal Go for detecting the zero level, thereby lowering the gain of the amplifier AMP. As a consequence, as shown in Fig. 2, the differential signal G3 to be outputted from the amplifier AMP can be outputted from the amplifier AMP without exceeding the set gain or cutoff value (dashed lines in Fig. 2) of the amplifier although the differential signal G3 would otherwise exceed the set gain if the gain would not be lowered. The differential signal G3 is then outputted and added up in the arithmetic unit 13, thereby operating the amount of the gases adsorbed on the sample and generating the operation result into the display unit 14. On the other hand, when the differential signal G3 to be outputted from the amplifier AMP is too small with respect to the set gain (dashed lines in Fig. 2) as shown in Fig. 2, the signal GH for expanding the level of the gain is generated by the circuit 12c for generating the signal for expanding the level of the gain and it is outputted into the shift switch for shifting the level of the gain through the OR circuit by the second AND circuit 12e for inputting the signal GH for expanding the level of the gain and the signal Go for detecting the zero level, at the time when the period of time for adsorbing the gases is finished, thereby expanding the gain of the amplifier AMP. As a result, as shown in Fig. 2, the differential signal G3 to be outputted from the amplifier AMP can be outputted as an appropriate output value from the amplifier AMP without exceeding the set gain although the differential signal G3 would otherwise be considerably small during the period for desorption of the gases if the gain would not be expanded. The differential signal G3 is then outputted and added up in the arithmetic unit 13, thereby operating the amount of the gases adsorbed on the sample and generating the operation result into the display unit 14. As described hereinabove, the differential signal G3 during the period for desorption of the gases is adjusted appropriately by determining the output state of the differential signal G3 during the period for absorption of the gases, so that the device according to the embodiment of the present invention can always measure the amount of gases to an accurate extent. FIELD OF INDUSTRIAL UTILIZATION The system according to the present invention can automatically adjust the gain so as for the signal outputted from the amplifier during the period for desorption of the gases not to exceed the set gain or cutoff value of the amplifier or to become too small with respect to the gain of the amplifier by determining the state of the signal to be outputted from the amplifier during the period for adsorption of the gases. Hence, the surface area measuring device having the system for automatically shifting the measuring ranges can measure the surface area of the sample with accuracy, although the system is of a simple structure.

Claims: A surface area measuring device comprising: a gas passage (10); a sample cell (11) mounted detachably in said gas passage (10), said sample cell (11) containing a sample, the surface area of said sample to be measured by the amount of gases adsorbed by said sample; first measurement means (A), located in said gas passage (10) and connected to a first end of said sample cell (11) to which gases are supplied through said gas passage (10), for detecting a flow rate of gases to be supplied to said sample cell (11) to measure an amount of supplied gases and generating a supply-gases-flow-rate representing signal (G1); second measurement means (B), located in the gas passage (10) and connected to a second end of said sample cell (11) from which said supplied gases are discharged, for detecting a flow rate of gases discharged from said sample cell (11) in order to measure an amount of discharged gases and generating a discharge-gases-flowrate representing signal (G2); an amplifier (AMP) electrically connected to said first measurement means (A) and to said second measurement means (B) for receiving said supply-gases-flow-rate representing signal (G1) and said discharge-gases-flow-rate representing signal (G2), amplifying a difference between said supply-gases-flow-rate representing signal (G1) and said discharge-gases-flow-rate representing signal (G2) and generating a differential signal (G3), said amplifier (AMP) being capable of varying a gain; arithmetic means (13), electrically connected to said amplifier (AMP) to receive said differential signal (G3), for calculating an amount of gases adsorbed on said sample by integrating said differential signal (G3); characterized in that said measuring device further comprises: output value determining circuit electrical means (12) comprising: first circuit means (12a) for generating a signal G L for lowering the level of said gain, said first circuit means (12a) electrically connected with said amplifier (AMP), for receiving said differential signai (G3) generated from said amplifier (AMP), for comparing said differential signal (G3) with a set gain of said amplifier, and for outputting said signal G L when said differential signal (G3) reaches said set gain; second circuit means (12b) for generating a zero level informing signal Go, said second circuit means (12b) electrically connected with said amplifier (AMP) for receiving said differential signal (G3), for detecting a time at which said differential signal (G3) turns from a negative value to zero, and for outputting said zero level informing signal Go; third circuit means (12c) for generating a signal G H for expanding the level of said gain, said third circuit means (12c) electrically connected with said amplifier (AMP), for receiving said differential signal (G3), for comparing said differential signal (G3) with a value predetermined with respect to said set gain, and for detecting whether said differential signal (G3) does not reach said value for a predetermined period of time, thereby outputting said signal G H ; first AND circuit means (12d), electrically connected with said first circuit means (12a) and said second circuit means (12b) for receiving said signal G L for lowering said level of said gain and said digital zero level informing signal Go, for performing AND operation between said digital signal G L for lowering said level of said gain and said digital zero level informing signal Go and outputting a signal; second AND circuit means (12e), electrically connected with said third circuit means (12c) and said second circuit means (12b), for receiving said signal G H for expanding said level of said gain and said digital zero level informing signal Go, for performing AND operation between said signal G H for expanding said level of said gain and said zero level informing signal Go and outputting a signal ; and OR circuit means (12f), electrically connected with said first AND circuit means (12d) and second AND circuit means (12e), for receiving said signal outputted from said first AND circuit means (12d) and said signal outputted from said second AND circuit means (12e), for performing OR operation between said signal outputted from said first AND circuit means (12d) and said signal outputted from said second AND circuit means (12e), and outputting a shifting signal (G4); and shift switch means (SW) electrically connected with said output value determining circuit means (12) and said amplifier (AMP), for receiving said shift signal (G4) and switching said gain of said amplifier (AMP) in accordance with said shift signal (G4).

Inventors: TAKEUCHI MINORU, TAKEUCHI, MINORU, TAKEUCHI, MINORU NIKKISO COMPANY LIMITED
Applicants: NIKKISO CO LTD, NIKKISO CO., LTD.
Application year: 1990
Main classification: G01N15
IPC codes: G01N15