Rosemount 755, Instruction Manual

Page: 33 / 94

I
NSTALLATION
748183-H
2-11
Rosemount Analytical
November 1997
calibration on a range of higher sensitivity, i.e., more narrow span, and then move the
% RANGE switch to the desired operating range. For example, if the operating range
is to be 0 to 50% oxygen, upscale calibration may be performed on the 0 to 25% range
to permit use of air as the upscale standard gas.
Recommendations on calibration gases for various operating ranges are tabulated in
Table 2-1 and are explained in Sections 2.5.1 and 2.5.2.
Each standard gas should be supplied from a cylinder equipped with dual-stage metal-
diaphragm type pressure regulator, with output pressure adjustable from 0 to 50 psig
(0 to 34.5 kPa).
A. ZERO BASED RANGES
RANGE
% O
2
RECOMMENDED DOWNSCALE
STANDARD GAS
RECOMMENDED UPSCALE
STANDARD GAS
0 to 1 Nitrogen 0.9% O
2
, balance N
2
0 to 2.5 Nitrogen 2.3% O
2
, balance N
2
0 to 5 Nitrogen 4.5% O
2
, balance N
2
0 to 10 Nitrogen 9% O
2
, balance N
2
0 to 25 Nitrogen Air (20.93% O
2
)
0 to 50 Nitrogen 0.45% O
2
, balance N
2
0 to 100 Nitrogen 100% O
2
B. ZERO SUPPRESSED RANGES
RANGE
% O
2
RECOMMENDED DOWNSCALE
STANDARD GAS
RECOMMENDED UPSCALE
STANDARD GAS
90 to 100 91% 0.5% O
2
, balance N
2
High-purity O
2
80 to 100 82% 1% O
2
, balance N
2
100% O
2
60 to 100 62% 1% O
2
, balance N
2
100% O
2
50 to 100 52% 1% O
2
, balance N
2
100% O
2
Note
Each standard gas used should have a composition within the specified limits, and should
have a certified analysis provided by the supplier.
T
ABLE
2-1. C
ALIBRATION
R
ANGE FOR
V
ARIOUS
O
PERATING
R
ANGES
2.5.1 D
OWNSCALE
S
TANDARD
G
AS
In the preferred calibration method described in Section 3.2.1, a suitable downscale
standard gas is used to establish a calibration point at or near the lower range-limit.
Composition of the downscale standard depends on the type of range:
A zero based range normally uses an oxygen-free zero gas, typically nitrogen.
A zero-suppressed range uses a blend consisting of a suitable percentage of oxygen
contained in a background gas. typically nitrogen.
(An alternative calibration method, described in Section 3.2.2, uses an upscale
standard gas only and does not require a downscale standard gas.)