Automatic scale contraction for the SMA 12-60.

Tammes, A. An electronic device which automatically halves "off-sca'.e" signal voltages of the SMA 12/60 is described. Presetting permits any electronic dilution desired. It may be constructed from commercial components by a reasonably proficient electronic technician. The device is placed "in-line" at the terminal box of the recorder without further modification of the SMA 12/60. It senses and modifies the steady state peak voltage differential. The chemical reactions of nine determinations provide linearity in excess of twice the maximum indicated on available chart paper. The operator is alerted to "off-scale" values by an event marker. Phosphorus presently is nonlinear beyond 6.5 nig. per 100 ml.; total protein and albumin were not tested. The capability to estimate with accuracy "off-scale" test values has proven very useful. SAMPLES OF BLOOD from hospitalized patients often show off-scale elevations of at least one channel when tested on a Sequential Multiple AutoAnalyzer 12/60.* In an earlier report 1 a simplified, manually activated scale halver adaptable to individual channels of the SMA 12/30 was described. Limitations intrinsic in the manual modification include the need to standardize respective channels at frequent intervals and to monitor each channel continuously. In addition, the shortened sampling time of the SMA 12/60 makes manual operation and accurate single channel standardization extremely difficult. The need for an automatic scale contractor is apparent. The device precludes many manual dilutions, eliminates some venipunctures to obtain more samples, estimates off-scale values with greater accuracy , and functions without close operator attention. Materials and Methods Figure 1 is a block diagram of the location of the automatic scale contractor. "On-scale" signals (< 50 mv.) reach the Bristol amplifier unaltered; "off-scale" signals (> 50 mv.) activate a switch, diverting the signal through the "divide by two circuits," which delivers half-voltage to the Bristol amplifier. If desired, the signal could be reduced by a higher factor. Figure 2 shows the detailed circuit schematic for the device. Briefly, the components serve the following functions: R x and R 2 serve as the input zero adjustment of buffer amplifier A,. R 7 and R 0 permit output zero adjustment. C 1; R 5 , and R 3 compensate frequency and voltage of A x. D t and D 4 provide stable voltage to amplifier A 2. D 3 and D 4 protect A 2 against excess signal surge. R 9 and R 10 control reference voltage for the comparator A …