990 CLD Autoline™ Leak Detector
A-2
DRAFT 2/2/04
Another way of expressing this type of output voltage is to say that the most sensitive decade
produces an output voltage of between 1 and 10 mV (0.001 to 0.01 V), the next least
sensitive decade 10 mV to 100 mV (0.01 to 0.1 V), the next least sensitive decade 0.1 to 1
V, and the least sensitive decade 1 to 10 V.
Remember, the 990CLD covers four decades of sensitivity; selecting which of the four
decades to which the machine can be calibrated is a function of mechanical pump speed
and high vacuum pump speed setting.
Why Use a Linear Analog Output?
The linear output is useful in two types of applications: 1) when the testing takes place in
only one decade, near the least sensitive range of the leak detector, and 2) when a simple
mathematical function is desired to relate output voltage to leak rate.
Use of the least sensitive range of the leak detector is desirable with the linear output to
obtain the highest possible resolution from the Analog to Digital converter in your
Programmable Logic, Controller or computer. Many PLCs have A-to-D converters with 12
bits of resolution and a maximum input voltage of 10 V. This means that over this 10 V
range, the converter resolves 1 part in 4096, or about 2.4 mV. If you want to test for leaks of
1 x 10
-1
std cc/sec with the RANGE SELECT switch set at 2, then you are operating in the
least sensitive decade of the leak detector. The resolution, in that decade, with a 12-bit
A-to-D converter is:
(0.0024 V) I (10 V for the least sensitive decade) x 100 = 0.024 percent
Now, try looking for a leak of 1 x 10
-4
with the same setup, and you are operating in the
most sensitive decade of the leak detector. The output voltage in that situation is only 1 mV
and the resolution for that decade is:
(0.0024) I (0.01 V for the least sensitive decade) x 100 = 24%.
Obviously, it is undesirable to test in this range using a linear analog output fed into the PLC
or computer. It is important to remember that percentage resolution per decade is not
constant for the linear analog output. Resolution for the least sensitive decade (with a 12-bit
A-to-D converter) is 0.024 percent, for the next most sensitive decade, 0.24 percent, for the
next most sensitive decade, 2.4 percent, and for the most sensitive decade, 24 percent.
Although the output does go to zero with a zero leak rate displayed, use of this level should
be avoided due to the small voltages encountered.
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