2808B Calibration / 3-9
2. Calculate the desired output reading for 0 psi. For this example, 0 psi = 25% of the span
from -10 to 30 psi. The output should therefore be 25% of the way from 1-5V, or
2V(8mA). Set the Dip and Rotary switches in accordance with Table 3-A. Elevated zero
values are expressed as positive percentages of calibrated span. Zero percent represents
zero elevation. If the desired percent elevation is in between values listed in the table,
try the next closest setting.
3. Apply a 100% pressure input to the transmitter equivalent to the upper range value
(URV). For this example, URV = 30 psi.
4. If transmitter output is not 5.000V (20.00mA), adjust A1 to make a minor correction.
5. Vent the input pressure. Adjust A2 to set the output to the 0 psi reading calculated
above.
6. Recheck the transmitter output with the input pressure vented, and 100% inputs. The
DMM should provide respective readings of the output calculated above, and 5V (20mA)
(± 0.15% full scale).
7. If necessary, repeat the A1 and A2 adjustment procedures.
8. If problems persist, recheck the DIP switch settings. Try setting the switches to the
lower percentage. For instance, if the desired percent elevation is 15%, set the switches
for an elevation of 10%.
3.7.1 Zero Elevation Example (see Table 3-A)
A DP Sensor is mounted across an orifice plate to measure gas flow. Full-scale differential
pressure is 40” H
2
O in either direction (±40” H
2
O). The span is +40” - (-40”) = 80” H
2
O. If a
100” Transmitter is used, the Coarse Span Rotary Switch (SW1) setting is found by dividing
the span (80” H
2
O) by the Transmitter’s range; in this case 100” H
2
O, i.e., 80/100 = 80%.
Therefore, the SW1 should be set to position 2 (64% - 84%).
Set the elevation to 50% of the calibrated range (00100010). This will set up the 2808 for
calibration of 1.000V (4.00mA) at -40” and 5.000V (20.00mA) at +40”. At 0 psid the output
will be ½ scale, i.e., 3V (12mA). Apply +40” H
2
O to the High Side of the Transmitter and
use Fine Adjustment Screw A1 to set the output to 5.000V (20.00mA). Vent the unit and
use Fine Adjustment Screw A2 to adjust the Transmitter’s output to 3V (12mA). Apply +40”
H
2
O to the Low Side of the Transmitter and observe the output. Maximum error is the
deviation from 1.000V (4.00mA).
Note: Transmitter factory calibration and compensation is for positive DP pres-
sure only. Negative pressure indication is possible with the above-
described method of reduced accuracy. Full-scale negative indication can-
not be achieved unless the DP range is at least 2 times the negative range.
3.8 SUPPRESSED ZERO CALIBRATION
1. Calculate the total span required for the suppressed zero range. For example, if the
desired suppressed zero range is +15 to +80 psi, the total span will be: 80 - 15 = 65 psi.
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