Patent GB-1423877-A (GB)
Title: Measuring apparatus for two-variable function signal system
IPC: G01N27/00
Filed: 1973-09-06
Abstract: 1423877 Absorption measurement YOKOGAWA ELECTRIC WORKS Ltd 6 Sept 1973 [22 Sept 1972] 41972/73 Heading G1A [Also in Division G4] An apparatus for evaluating a desired variable from a signal corresponding to an exponential function of the desired variable and another independent variable, comprises means for observing the difference between a reference signal and the product of the reference signal and the exponential function signal, means for linearizing this difference, means for subtracting from the linear signal a signal corresponding to the product of the reference value of the desired variable and the difference between the other variable and the reference value of the other variable, and a division circuit for dividing the output of the last mentioned subtraction means by a signal corresponding to the difference between the other variable and the reference value of the other variable. As shown, the difference between the output I x  of an absorption cell 2 for petroleum and a reference signal I 0  (variable) is fed to amplifier Al to provide output El. The signal I x  may be expressed as: where a = Á S  + Á CH  and b = Á CH  and Á S , Á CH  are mass absorption coefficients of sulphur and carbohydrate respectively, p is the density of the petroleum and t a parameter of the cell 2. C S  is the sulphur concentration, which it is desired to measure. I' o  is the output of the empty cell. The equation may be written in the form I x  = I o  (1-x) when the terms I o  and x are composed of exponential expressions. Now El = R S .I O  x which, after inversion and level-adjustment in amplifier A2, yields E2 =-AR S .I OX  =-E 0 x, (E 0  = -AR S .I O  and is the reference voltage). The circuit 11, including divider 15, and two D.C. sources of value Eo, is a logarithmic converter, the output of the divider being E 5  = Eo/1 -#x where X is arranged to be approximately equal to 1/#2. A signal provided at the point d 2  of potentiometer 14 can be shown to be equal to -(1-#). E 0 x and to this is added the reference voltage Eo to give: E 6  =-E 0 {-1 + (1-#) x}. The subtraction unit 16 performs the operation E 7  = E 5 -E 6  and its output is equal to:  and it can be shown that if X # 1#2, the expression in the brackets is equal approximately to -log(1-x). Therefore E 7 #-E 0  log (1-x) and substituting for x the expression given by the first two equations we have: A density compensation signal e 6  equal to the last term of the expression for E 7  is produced from a density transducer 17 in circuits 19, 20, 21, 23, 25. These circuits provide zero and range adjustment, standardization with respect to the temperature in the transducer 17 and compensation for the temperature in the cell 2. The signal applied to amplifier A3 is Eg = E 7 -e 6  = E 0 #ta (C S -C So ) and this is adjusted for slope and intercept by circuits 29, 30 to vary between IV and 5V D.C., giving: Signal E 9  is fed to a divider 31 having a divisor input: derived from the density measuring circuits 19, 20, 23, 25. The divider gives a signal: which is proportional to the sulphur concentration.