Campbell Scientific CR23X Micrologger - Manual (Page 279)

F-1
APPENDIX F. DYNAGAGE SAP-FLOW (P67)
F1. FUNCTION
The Dynagage Processing Instruction, P67, is
used in conjunction with a Dynamax, Inc.
Dynagage stem flow gage. Instruction 67
calculates the sap flow rate within a plant stem
by using an energy balance equation and inputs
from a Dynagage sensor.
Briefly, the energy input into the stem is a
known quantity (Q
h
), conductive radial and
vertical energy losses are calculated (Q
r
and Q
v
,
respectively). The residual energy is assumed
lost through sap flow (Q
f
) and is given as:
Q
f
= Q
h
- (Q
r
+ Q
v
).
By measuring the change in temperature (dT) of
the stem by heating and assuming 4.186 Joules
of energy are required to heat each gram of sap
1
°
C, the flow rate can be calculated.
Accumulated flow can be calculated using
datalogger Instruction 72 (Totalize).
F2. INSTRUCTION DETAILS
Dynagage Sap-Flow (P67)
1:
0000 Begin Input Location
[ _________ ]
2: 0.0 K
sh
(W/mV)
3: 0.0 Heater Resistance(Ohm)
4: 0.0 Stem Area (sq cm)
5: 0.0 Thermal Conductivity
(W/m
∗
K)
6: 0.0 Thermocouple Gap (cm)
7: 0.0 Low-Flow Cutoff
Temperature (C)
8: 0.0 High-Flow Cutoff Velocity
(cm/s)
9: 00 Output
10: 0000 Destination Input Location
[ _________ ]
11: 0.0 Mult
12: 0.0 Offset
F2.1 INPUT VOLTAGES
PARAMETER 1
Four voltages must be stored sequentially
beginning with the Input Loc option stated in
Parameter 1.
INPUT LOCATION
Loc 1: Ch (Thermopile (mV))
Loc 2: Ah (Upper TC (mV))
Loc 3: Bh (Lower TC (mV))
Loc 4: Voltage Input
These voltages are measured using Instruction
2. Voltages for locations 1-3 are measured on
the 2.5 mV slow range. The voltage for location
4 is measured on the 2500 mV slow range.
F2.2 CONSTANTS
Parameters 2 through 6 are constants that must
be entered for each Dynagage.
PARAMETER 2
K
sh
(W/mV), a constant required to relate the
thermopile output proportionally to the radial
heat transfer.
PARAMETER 3
The heater resistance (Ohms), measured with
an Ohm meter.
PARAMETER 4
The cross sectional area of the stem (sq cm).
PARAMETER 5
Thermal conductivity (W/m
∗
K). This number is
generally 0.54 W/m
∗
K for herbaceous plants or
0.42 W/m
∗
K for woody plants.
PARAMETER 6
Thermocouple gap (cm) is the distance
between the thermocouples in the Dynagage,
measured in cm. This value is determined by
Dynamax and is found in the Dynagage
documentation.
F2.3 FILTERS
Two predictable conditions may occur when the
output from the Dynagage is not valid. In these
cases, the output must be "filtered" so only
reasonable flow rates are processed. These
conditions are very low flow rates and high flow
rates, both of which cause dT to approach zero.
The values generally placed in Parameters 7