RPM4-AD™ OPERATION AND MAINTENANCE MANUAL
© 2005-2007 DH Instruments, a Fluke Company
Page 18
updating of the
P
offset,G
value corresponding to atmospheric pressure. Gauge pressure is the
measured absolute pressure,
P
u
, minus the atmospheric offset.
P
gauge
= P
u
- P
offset,G
However, atmospheric pressure may change between opportunities to run AutoZ and update
the value of
P
offset,G
, for example when running an extended test without venting. RPM4-AD
uses
dynamic compensation for atmospheric pressure
to correct for changes in
atmospheric pressure between opportunities to run AutoZ and update
P
offset,G
. When AutoZ
runs, and
P
offset,G
is determined, the reading of RPM4’s on board barometer,
P
atm,0
, is also
recorded. Later, when no longer vented, the change in atmospheric pressure,
Δ
P
atm
, since
P
offset,G
was updated, is the difference between the current barometer reading,
P
atm
, and the
barometer reading at the time of AutoZ execution,
P
atm,0
:
Δ
P
atm
= P
atm
- P
atm,0
Dynamic compensation for atmospheric pressure uses
Δ
P
atm
to correct the value of
P
offset,G
,
thus always compensating real time for changes in atmospheric pressure:
P
gauge
= P
u
- P
offset,G
-
Δ
P
atm
Gauge pressure measurement on an Axxx (absolute) Q-RPT allows instantaneous switching
between gauge and absolute measurements modes. The additional uncertainty in gauge
pressure mode due to the dynamic compensation for atmospheric pressure technique is a
function of the resolution and short term stability of the on-board barometer, not its absolute
measurement uncertainty. This additional uncertainty is ± 1 Pa (0.00015 psi).
In RPM4-AD A160K/A160K the two A160K Q-RPTs are used in parallel measurement
mode in gauge mode to reduce measurement uncertainty (see Section 3.2.5). Gauge
mode can be used to measure airspeed in parallel measurement mode. This provides
airspeed measurement but only at ground altitude corresponding to the current value of
atmospheric pressure (see Section 3.2.3.1).
3.2.3
ALTITUDE AND AIRSPEED MEASUREMENT
RPM4-AD is designed to support the measurement of altitude and airspeed with minimal
uncertainty. See Section 3.2.3.1 for information on altitude and airspeed measurement with
RPM4-AD A160K/A160K (rotary wing) and Section 3.2.3.2 for information on RPM4-AD
A350K/A160K (fixed wing).
3.2.3.1
RPM4-AD A160K/A160K (ROTARY WING)
RPM4-AD A160K/A160K has two 160 kPa absolute Q-RPTs, one in the HIGH
RPT (Pt) position and one in the LOW RPT (Ps) position.
Air data measurements can be made in three different conditions.
The two Q-RPTs measuring two absolute pressures (Ps and Pt) separately:
airspeed at varying altitude
In differential mode, the two Q-RPTs are separated internally. One is connected
to the
TEST (Ps)
port and the other is connected to the
TEST (Pt)
port (see
Section 3.2.6).
RPM4 measures the absolute pressures applied to the
TEST (Ps)
and
TEST (Pt)
ports. The absolute pressure measured on the
TEST (Ps)
port is converted to
altitude. The difference between the
TEST (Pt)
and
TEST (Ps)
pressure
(Pt - Ps) is converted to airspeed (see Section 7.2.3). If a true airspeed unit of
measure has been selected, altitude and entered temperature are also used in
the airspeed calculation.
