59BAdjusting the Zero
5BChapter Two: Installation
32
Adjusting the Zero
The DMA unit uses a digital zero in the calculation of the reported pressure value. Figure 4, page
37, illustrates where the digital zero is incorporated into the calculation of the pressure value.
The transducer’s calibrated, analog pressure signal is measured from pin 1 on the Type DE-9S
interface connector (Table 10, page 29).
Coarse Zero Adjustments
Digital and analog signals can be zeroed either by manually pressing the OneTouch Push Button
Zero or by issuing a DeviceNet service (Service ID 0x32, the Coarse Zero Adjustment service).
The range of this adjustment is limited to approximately –4 to +20% of full scale and the assumed
target is zero pressure or vacuum. The DMA must be evacuated to a pressure below its resolution
before the OneTouch Zero Push Button or Coarse Zero Adjust Service is executed. See Table 13.
Fine Zero Adjustments
For Fine Zero adjustments the Digital and analog signals can be zeroed either with a target of zero
(usual practice) or an offset. This is accomplished with a DeviceNet service (Service ID 0x4B) and
the target value can be either zero or a nonzero value. The range of this adjustment is limited to
±
2
% full scale. If an unsuccessful response to the Fine Zero and Offset adjustment is received,
either issue the Coarse Zero Adjustment service (0x32) with a target of zero or press the
OneTouch Coarse Zero Push Button.
For best results, the transducer should be under vacuum while it is warming up. Table 13 lists the
recommended pressure levels for proper zero adjustment.
Table 13: Highest Pressures Suggested for Proper Zero Adjustment
Full Scale Range
(Torr)
Highest Pressure for Proper Zero
Adjustment (Torr)
0.1
< 5 x 10
-7
1
< 5 x 10
-6
10
< 5 x 10
-5
100
< 5 x 10
-4
1000
< 5 x 10
-3
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