990 CLD Autoline™ Leak Detector
DR
AFT 2/2/04
A-3
Logarithmic Analog Output Voltage
The 990 CLD leak detector offers two logarithmic analog output voltages: 2 V/decade, and
3 V/decade. We will focus on 2 V/decade in this description. Figure A-2 on page A-6
labeled Logarithmic Output Voltage is the graphic conversion chart for 2 V/decade
reference, while Figure A-3 on page A-7 covers three V/decade. Three V/decade is useful to
obtain higher resolution per decade, but reads only part of the least sensitive decade due to
output voltage clipping set at about 10.7 V.
Again, as in the linear output conversion chart, there are six lines on the chart
corresponding to the six RANGE SELECT switch settings.
The logarithmic analog output voltage is derived from an amplifier whose output is a
logarithm of the input voltage. The output voltage is obtained by first multiplying the front
panel leak rate display by 10 raised to the RANGE SELECT switch s3. The logarithm
(base 10) of this result is then taken, and multiplied by 2 (for 2 V/decade) or 3 (for 3
V/decade). Using the same setup as in the first linear output example, the analog output
voltage corresponding to a leak of 1 x 10
-3
std cc/sec with the RANGE SELECT switch set to
2 would be:
2 x Log((1 x 10
-3
) x 10(2 + 3)) = 4.00 VDC
If you needed to read leaks at 1 x 10
-5
and 1 x 10
-1
std cc/sec, as in the second and third
linear output examples, the voltages would be:
2 x Log((1 x 10
-5
) x 10(2 + 3)) = 0.00 VDC
and
2 x Log((1 x 10
-1
) x 10(2 + 3)) = 8.00 VDC
respectively.
Note that while the Logarithmic output is 2 V/decade covering the full decade, the output is
not linear within the decade. For example, assume the system is set up to read a full scale
leak of I1x10
−2
std cc/sec in the most sensitive decade with the RANGE SELECT switch set
to 0. The output in this situation would read 2 V. If you read an output of 1 V instead,
it is
not half that leak rate.
The leak rate with 1 V output in this setup is:
10(Output Voltage/2)/10(Range Switch S3) = 10(1/2)/ 10(0+3) = 3.1x10
-3
std cc/sec
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