Patent GB-1297542-A (GB)
IPC: G06J1/00
Filed: 1970-03-02
Abstract: 1297542 Voltage measuring PERKIN-ELMER CORP 2 March 1970 [3 March 1969] 9903/70 Heading G1U [Also in Division G4] Apparatus for use in spectrophotometry, in which a mathematical relationship between an input sample signal I on lead 10 and a reference signal I 0  on lead 20, preferably representing the reference beam intensity, is computed, is calibrated by periodically comparing the reference signal with the signal arising from a blank sample applied to the lead 10. As described the blank sample signal 1 A  and the reference signal I 0  are integrated by integrators 17, 27 for a time determined by the opening of switches 14, 22 these integrated values being held and compared with a linearly decreasing ramp signal (derived by energizing relay 83) clock pulses being accumulated in counter 291, 311 through the time interval between equalities of the blank and reference signals and the ramp signal. This results in the series of counter 291, 311 being advanced relative to that of counter 282, 310 (both counters receiving clock pulses on lead 185<SP>1</SP>) so that the on/off time of bi-stable 320 controlling the duty cycle of switch 22 is varied to bring the integrated values of the two signals into agreement. This calibration cycle may then be repeated 10 or 100 times and the average value taken. A sample I is measured by comparing the value of the sample integrated for a time S 1  and the reference signal integrated by a time during the interval S 1  determined by the count derived during the calibration cycle with a exponentially decreasing ramp signal. The time interval between the two signals being equal to the ramp represented by a pulse on lead 132 and on lead 131 may be used to control an analog chart type recorder by applying the pulse to integrator 154 the output of which is connected via scaling potentiometer to recorder amplifier 172. Alternatively it may be used to enable gate 138 so that clock pulses are accumulated to represent the required logarithm i.e. log Io/eI. The binary   coded decimal count in counter 450 may be displayed on Nixes, printed or sent to a data processor. Switch 230 may be operated to average 10 or 100 samples. In the unlikely event of the integrated value of the sample I exceding the integrated value of the reference the count is alternatively accumulated in counter 451. To ensure that I o  is always greater than I however the input resistor 11 may have a higher value than the input resistor 21 so that when the two signals are equal switch 22 is only on for a percentage of its duty cycle. Potentiometers 108, 110, 112 may be adjusted so that the computed value is exactly equal to log Io/I. A curvature current generator allows an intentional deviation to be introduced into the ramp signal to compensate for the fact that in atomic absorbtion double beam spectrophotometry the actual concentration of the sample component deviates slightly from the Lambert-Beer Law. Modification. A linear ramp may be used for affecting the measurement to give the output as linear absorption. This is effected by operating switch 612 to supply a constant current to amplifier 98 during the discharge time. Instead of being derived from the spectrophotometer I 0  may be a fixed reference value.