Table of Contents
Advertisement
If the system pressure is higher than 250 psig (1.7 MPa) the pressure induced error in the span reading
becomes significant. This error will approach 16% for nitrogen at 1000 psig. For accurate high
pressure measurements this error must be corrected.
The formula for predicting the nitrogen mean error expressed as a fraction of the reading are:
Where P is the pressure in psig and Error is the fraction of the reading in error.
The flow reading can be corrected as follows:
Where the Indication is the indicated flow and Error is the result of the previous formula (or read from
charts above).
2.6.4. Vertical Mounting
When operating at high pressure or with
high density gases, the increased density of
gas will cause natural convection to flow
through the sensor tube, if the instrument
is not mounted in a level position. This
natural convection flow will be
proportional to the square of the gas
density and therefore to system pressure.
This will be seen as a shift in the zero flow
output that changes as the system pressure
changes. Heavier gases will have more of
an effect than light gases.
If the instrument is measuring light gases or
low pressure, the change in the zero due to
natural convection will be minimal. Larger
shifts could be zeroed out with the zero pot
as long as the upstream pressure is stable.
It is not recommended to mount the
instrument vertically if high pressure or high density gas is being measured. A chart is shown here for
nitrogen/air versus pressure.
If the error exceeds 20-30% (of full scale), the zero circuit installed in the instruments, may no longer
be able to be adjusted to compensate for the error.
2.6.5. Blending of Gases
This section describes two methods by which to achieve a controlled blending of different gasses. Both
methods use the flow signal (Output) from one flow instrument as the Master to control the Command
signal (Input) to a second unit.
The first method requires that the two controllers use the same signal range (0 to 5 VDC or 4 to 20 mA)
and that they be sized and calibrated to provide the correct ratio of gasses. Then, by routing the
actual flow Output signal from the primary meter/controller through the secondary controller's
External Input pin (See Tables 2.1 & 2.2), the ratio of flows can be maintained over the entire range of
gas flows.
Manual: 151-032015 300-302 Series
Error
(
. 2
2915
*
10
26
Error
(
. 7
1066
*
10
17
Error
(
. 5
3091
*
10
14
Corrected
Indication
7
2
5
)
P
(
. 5
7198
*
10
)
8
2
5
)
P
(
. 5
2403
*
10
)
8
2
5
)
P
(
. 6
1278
*
10
)
Indication
*
P
,
(
. 0
026
"
Sensor
)
P
,
(
. 0
017
"
Sensor
)
P
,
(
. 0
014
"
Sensor
)
Error
Page 12 of 22
Advertisement
Table of Contents
