Emerson Bristol 2808 Series - Elevated Zero Calibration; Zero-Based Calibration

3-8 / Calibration 2808B
Suppressed Zero Calibration:
0 psi < 1V (4-20mA) output [0 psi results in an output less than 1V (4mA)]
Sample Ranges:
1 to 10 psi
10 to 60 psi
50 to 100 psi
The above procedures are described in topics 3.6, 3.7, and 3.8. Select the procedure you
require.
3.6 ZERO-BASED CALIBRATION
This procedure describes the process by which the zero and span settings are obtained for
the 1-5V and 4-20mA output stages of the 2808 DP and PT transmitters.
The following parameters relate directly to the calibration process:
URL: Upper Range Limit of sensing element. The URL is the maximum input
pressure that can be applied without over-pressuring the sensing
element.
Span: The algebraic difference between the limits of the range (P
max
– P
min
).
The desired span must always be less than or equal to the upper range
limit (URL) of the sensing element. Span is expressed as a percentage of
the upper range limit of the sensing element.
Zero: The point within the available pressure range the user defines as a zero
pressure reference.
P
max
: Maximum input pressure of a desired range, not necessarily the
maximum range of the sensing unit.
P
min
: Minimum input pressure of a desired range.
Theory of Operation: The output stage can potentially take a user defined portion of the 0-
100% URL pressure input, and display it as a corresponding 1-5V or 4-20mA output. The
range, desired by the user, is obtained by setting the appropriate coarse and fine zero and
span settings. Each coarse setting is described in Tables 3-A and 3-B, whereas the fine
settings are adjusted manually until the desired output is achieved. Fine adjustments are
made with two external adjustment screws. The output stage is designed to yield a Turn
Down of better than 6, while the fine adjustment screws should affect the output zero and
span by no more than 1.5%/Turn.
3.7 ELEVATED ZERO CALIBRATION
1. Calculate the total span required for the elevated zero range. For example, if the desired
elevated zero range is -10 to +30 psi, the total span will be 30-(-10) = 40 psi.