Chapter Four: Functional Overview The [AUTO] and Ran
e Ke
s
65
significant digit to the display. The full scale voltage on the x0.1 range is 10 Volts. The range
remains at x0.1 for pressure readings between 105% of the
current
range (10.5 Torr) and 9% of
the
current
range (0.9 Torr). Once the pressure reading drops below 9% of the voltage range
(0.9 Torr), the 670 instrument applies the x0.01 range. The range remains at x0.01 for pressure
readings between 105% of the
current
range (1.05 Torr) and zero (0.00 Torr). This range
increases the number of significant digits by one.
A differential transducer may generate negative pressure readings. The autoranging sequence is
repeated for negative pressure values, up to -105% F.S. Using the example system in Figure 14,
page 64, the range remains at x0.01 when the pressure reading drops below zero to -105% of the
current
range (-1.05 Torr). When the pressure drops below -1.05 Torr, the range switches to
x0.1. The 670 instrument holds the range at x0.1 for pressure readings between -9% of the
current
range (-0.9 Torr) and 105% of the
current
range (-10.5 Torr). Once the pressure
reading drops below -105% of the x0.1 range (-10.5 Torr), the range switches to the x1. The
range remains at x1 for pressure readings between -9% of full scale (-9 Torr) and -105% of full
scale (-105 Torr).
The display reads
UNDERRANGE
when the pressure is less than -105% of full scale (105 Torr)
and
OVERRANGE
when the pressure exceeds +105% of full scale (+105 Torr).
Note
In the autoranging mode, the display may list
UNDERRANGE
or
OVERRANGE
while chan
in
ran
es since the 670 instrument uses all the
previous data to calculate the new average. Once the previous data is
replaced by new data, or the average value falls within the new range, the
out-of-range condition will disappear. To avoid this, reduce the number
of samples to average entry. For more information, refer to
Averaging
page 51.
De-Selecting the AUTO Operation
To de-select the AUTO operation, press either the [
AUTO
] key or a range selection key
( [
X
1], [
X
0.1], or [
X
0.01] ). The 670 instrument either maintains to the last range used
while in the AUTO mode (if you press the [
AUTO
] key), or changes to the new range (if you
press another range selection key). The light on the appropriate range selection key illuminates
to indicate the active range.
For more information on range selection, refer to
How To Select the Range Setting Remotely,
page 79.
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