Emerson Bristol 2808 Series, Instruction Manual

Page: 59 / 140

2808B Calibration / 3-5
Coarse Span URL Range = P
max
- P
min
------------ X 100%
URL
Once span has been calculated the desired switch position can be determined from Table 3-
A or Table 3-B.
Coarse Zero : Coarse zero adjustments are provided by switches S1:1-8. When these switches
are all set to ON, the maximum zero suppression (600%) is provided; when all are set to
OFF, the maximum zero elevation (600%) is provided.
The coarse zero is set for the region of full scale that the user desires to be the zero
reference. Consequently the possible zero positions are listed in %URL. This is not to be
confused with the %URL from the span. Unlike the span this does not represent an actual
range, instead it describes a pressure level that the user desires to set as a zero. The
following equation shows how to calculate the zero level in %URL:
Coarse Zero URL Range = P
min
------ X 100%
URL
Once the zero levels are calculated, the respective coarse zero switch positions can be
determined
from Table 3-A.
Fine Adjustments : At full-scale pressure adjust A1 so that the output is either 5V or 20mA.
Then, decrease the unit to zero pressure and adjust A2 so that the output is 1V or 4mA. The
adjustment of A1 should be such that the setting is barely 5V or 20mA (i.e. A small rotation
in the opposite direction should result in an immediate decrease in the output voltage.). If
this is not the case the screw may be over rotated. Repeat this procedure until the output
yields the appropriate values for high and low pressure. Once calibrated the output stage is
set and should need no further attention besides periodic tweaking.
3.5 TYPES OF RANGE CALIBRATION
When selecting a range, one of three types of calibration schemes will be encountered. Each
of these three methods refers to the manner in which a 0 psi input signal is referenced to
the 1-5V output of the transmitter. The three methods are defined as follows:
Zero Based Calibration :
0 psi = 1V (4mA) output
Sample Ranges:
0 to 50 psi
0 to 100 psi
Elevated Zero Calibration :
0 psi > 1V (4mA) output [0 psi results in an output greater than 1V (4mA)]
Sample Ranges:
-10 (vacuum) to +20 psi
-30 to 0 inHg