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Reference Manual
00809-0100-4514, Rev BA
January 2008
3-3
Rosemount 1154
Figure 3-2. Zero and Span
Adjustment Screws.
Example (for Range Code 4)
Initial transmitter calibration: 25 to 125 inH
2
O
(100 inH
2
O span with zero suppressed 25 inH
2
O)
Desired transmitter calibration: –75 to –25 inH
2
O
(50 inH
2
O span with zero elevated 75 inH
2
O)
1. Adjust the zero to eliminate any existing zero elevation or
suppression. With 0 inH
2
O pressure applied to the transmitter, turn
the zero adjustment until the output reads 4 mA. The unit is now
calibrated for 0 to 100 inH
2
O.
2. Adjust the span to the desired new span. To reduce the span, turn the
span screw until the output, with 0 inH
2
O pressure input, equals:
3. Adjust the zero screw to bring the output, with 0 inH
2
O input, back to
4 mA. The transmitter calibration should now be very close to 0 to 50
inH
2
O.
4. Check full-scale output and fine tune the span and zero adjustment if
required. Remember zero adjustments do not affect span, but span
adjustments do affect zero predictably. Adjusting the span screw
affects the zero
1
/5 as much as it affects the span. To compensate for
this effect, simply overadjust by 25 percent.
For example, if, after completing step 3, the transmitter output reads
19.900 mA at 50 inH
2
O, turn the span potentiometer until the output
(at 50 inH
2
O) reads 20.025 mA.
19.900 + (20.000 – 19.900) ϫ 1.25 =
19.900 + 0.125 = 20.025
Since the span adjustment affects zero
1
/5 as much as the span, the
0.125 mA increase in span causes a 0.025 mA increase in zero.
Therefore, turn the zero adjustment (at 50 inH
2
O) until the output
reads 20.000 mA. The unit should now be calibrated for 0 to 50
inH
2
O.
5. Zero Elevation/Suppression. Elevate zero. Turn the screw until the
output reads 4 mA with –75 inH
2
O applied to the high side of the
transmitter (applying 75 inH
2
O to the low side of the transmitter will
give the same result). The output may stop changing before the
desired 4 mA reading is obtained. If this occurs, turn off power to the
unit and unplug the amplifier board (refer to “Electrical Housing
Disassembly” on page 5-6 and Table 5-1 on page 5-5). To elevate or
suppress zero a large amount, use the following procedure:
Zero
Span
4mA
Existing Span
Desired Span
------------------------------------× 4mA
100 inH
2
O
50 inH
2
O
------------------------------× 8mA==