Campbell Scientific CR10X Measurement and Control System - Manual (Page 164)

SECTION 10. PROCESSING INSTRUCTIONS
10-6
*** 57 VAPOR PRESSURE FROM ***
WET-/DRY-BULB TEMPERATURES
FUNCTION
Calculate vapor pressure in kilopascals from
wet and dry-bulb temperatures in
°C. This
algorithm type is used by the National Weather
Service:
VP = VPW - A(1 + B*TW)(TA - TW) P
VP = ambient vapor pressure in kilopascals
VPW = saturation vapor pressure at the wet-
bulb temperature in kilopascals
TW = wet-bulb temperature,
°C
TA = ambient air temperature,
°C
P = air pressure in kilopascals
A = 0.000660
B = 0.00115
Although the algorithm requires an air pressure
entry, the daily fluctuations are small enough
that for most applications a fixed entry of the
standard pressure at the site elevation will
suffice. If a pressure sensor is employed, the
current pressure can be used.
PARAM. DATA
NUMBER TYPE DESCRIPTION
01: 4 Input location of
atmospheric pressure
in kilopascals
[PRESSURE]
02: 4 Input location of dry-
bulb temp. [DB TEMP.]
03: 4 Input location of wet-
bulb temp.
[WB TEMP.]
04: 4 Destination input
location for vapor
pressure
[VP or Z]
Input locations altered: 1
*** 58 LOW PASS FILTER ***
FUNCTION
Apply a numerical approximation to an analog
resistor capacitor (RC) low pass (LP) filter using
the following algorithm.
F(Xi) = W*Xi + F(Xi-1) * (1-W)
Where X = input sample, W = user entered
weighting function (O< W <1). If W=O,
F(Xi)=X
1
; if W=1, F(Xi)=X and F(Xi-1) = output
calculated for previous sample
The equivalent RC time constant is given by
T/W, where T is the sampling time in seconds.
For values of W less than 0.25, the analogous
"cut off" frequency (the frequency where the
ratio of output to input is .707) is accurately
represented by W/(2
πT). For larger values of
W, this "analog" estimate of the cutoff frequency
becomes less representative.
On the first execution after compiling, F(X) is
set equal to X.
PARAM. DATA
NUMBER TYPE DESCRIPTION
01: 2 Repetitions [REPS]
02: 4 First input location for
input data
[X]
03: 4 Dest. input location for
first filtered result[F(X)
or Z]
04: FP Weighting function, W
[W]
Input locations altered: 1 per repetition
Intermediate locations required: 1 per repetition
*** 59 BRIDGE TRANSFORM ***
FUNCTION
This instruction is used to aid in the conversion
of a ratiometric Bridge measurement by
obtaining the value for R
s
which is equivalent to
R
f
[X/(1-X)], where X is the value derived by the
standard CR10X Bridge Measurement
Instructions (with appropriate multiplier and
offset, Section 13.5) and R
f
represents the
MULTIPLIER value. The result of Instruction 59
is stored in the same location that X was.
PARAM. DATA
NUMBER TYPE DESCRIPTION
01: 2 Repetitions [REPS]
02: 4 Starting input location
& result destination [X]
03: FP Multiplier (Rf) [MULT]
Input locations altered: 1 per repetition