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Sap-Happy
Sap flow probe construction and use
(version 0.2)
Overview
There are three parts to this document: Part 1 deals with the construction of sap flow sensors
that use the constant-heating method for measuring sap flow. Part 2 deals with the installation
of the probes. Part 3 deals with the measurement theory and data collection and analysis and
is under construction.
Materials (materials and some vendors are listed in more detail at the end of this document).
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Thermocouple wire, 36 AWG (0.005” or 0.127 mm) copper-constantan.
Constantan wire, 36 AWG.
Stainless steel hypodermic needles, 19 gauge, 1 ½" long.
Aluminum tubing, round, 3/32" (2.38 mm) OD, 0.014” (0.36 mm) wall thickness.
Components for the construction of a constant-current source for heating.
Shielded copper wire (24 AWG) for connecting thermocouples to datalogger and
unshielded copper wire for connecting constantan heating wire to current source.
7. Reflective/insulating wrap for putting over installed probes.
8. Car battery to supply heating current.
Tools to make your life easier:
1. Small, battery-powered drill.
2. Butane soldering iron.
3. Volt-amp-ohmmeter.
Part 1. Sap flow sensor construction
Overview of sap flow sensor construction
1. Two hypodermic needles are needed for each set of sap flow probes.
2. Notch each needle in the middle and then cut to either 1 or 2 cm.
3. Construct fine-gauge copper-constantan thermocouple junctions and insert into the
midpoint of each needle.
4. Wrap one needle with a single layer of constantan heating wire.
5. Determine the resistance of the heating wire wrapped around the sensor.
6. Cut aluminum sleeves.
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Construction of sensors
Thermocouples and heating wire
1. Cut two, approximately 15 cm long pieces of 36 AWG copper-constantan thermocouple
wire per set of sap flow probes to be constructed.
2. Strip off about 1 cm of both outer and inner insulation from one end of the wire using a
sharp razor blade or wire strippers to expose the copper and constantan wire. The wire is
very fine and this may take some practice.
3. Twist the exposed wires together and solder them, shaking off any excess solder, to make
a small thermocouple junction.
4. Trim the junction to about 2 mm in length.
5. Remove the outer, clear insulation covering the two inner insulated wires (otherwise the
wire is too big to fit into the needle along with the heating wire). To do this, start at the
unsoldered end of the wire and with a sharp razor blade, cut between the wires. Often you
can pull the wire through the razor blade, cutting right down the middle without nicking
either wire. Be careful at the soldered junction.
6. Strip about 1 cm of the inner insulation from both the copper and constantan ends opposite
the thermocouple junction.
7. Use an ohmmeter to check the resistance between the two leads of the thermocouple
junction. It should be greater than about 3 ohms and less than about 50 ohms. If the
resistance is greater than 50 ohms then the junction should be discarded.
8. Cut 50 cm of constantan heating wire (or about 30 cm for a 1 cm probe) and strip about 1
cm of insulation off of both ends.
Needles (instructions are for 2 cm long probes; for 1 cm probes, use half the distances)
1. Mark a pair of 19 gauge stainless steel hypodermic needles at 1 cm from the base using a
fine permanent marker. Using a small triangular file, gently file a notch at this mark. You
do not want to file too deeply because this will weaken the sensor. File until just a film of
metal is remaining in the wall of the needle. Poke out the hole with the sharp end of a
spare needle.
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2. Cut the needles at 2.1 cm (1.1 cm for 1 cm sensors) with a diagonal wire cutter. Gently
expand the closed crimped end of the needle with a pair of small pliers and then file off
the deformed end of the needle to make it flat. Using a stiff wire, ream out the end hole
and the entire needle to remove any burrs that will snag the insulation of the thermocouple
wire.
3. For one needle of the pair (the heated sensor), poke a hole in the plastic base of the needle
with the sharp tip of a spare needle. The hole in the plastic base of the needle is for the
heating wire to be eventually threaded through.
4. For the same needle with the hole in the base, make a small notch at the cut tip of the
needle with the file. This will help with starting the wrapping of heating wire.
Sensor assembly
1. In the needle without the hole in the plastic base, insert both lead wires from a
thermocouple through the cut end of the needle and gently push until they come out the
needle base. Pull the wires through the needle from the base until the junction is visible at
the hole in the center of the needle.
2. Apply a small drop of cryanoacrylate glue (Super Glue) at the hole. It will be sucked into
the hole and will fix the thermocouple in place.
3. In the needle with the hole in the plastic base, push the two lead wires from another
thermocouple plus the single strand of constantan heating wire into the tip of the needle.
Pull all three wires from the base until the thermocouple is lined up with the hole. Make
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sure that the lead wire of the constantan heating wire is about the same length as the
thermocouple leads.
4. Apply a small drop of glue to fix the thermocouple and heating wire in place.
5. Start wrapping the constantan heating wire that sticks out of the needle back down and
around the needle starting at the tip. The small notch should keep it from slipping or you
can apply a small drop of glue at the tip. Continue wrapping for the entire length of the
needle. Keep a steady tension on the heating wire as you wrap and make sure the wraps
are as close together as possible. Gripping the started coil between your fingers and
rotating the base of the needle to coil the wire produces a nice wrap.
6. When you come to the end of the needle place a small amount of glue where the wire
contacts the plastic base to keep it in place. After the glue dries, pull the heating wire
through the hole in the plastic base and place another small drop of glue in the hole.
7. You can fill the plastic base of the needle with epoxy or cement to stabilize the sensor.
Heated sensor resistance value
1. Use an ohmmeter to measure the resistance across the heating wires. This should be less
than 20 ohms. Make a note of this value as R-total.
2. Measure the length of both leads of the heating wire from where they emerge from the
needle shaft. Multiply the total length of the leads by 0.4 ohms/cm to calculate the value
for R-leads.
3. R-coil = R-total – R-leads. Write the value of the resistance of the heating coil, R-coil, on
the plastic base of the needle with a fine permanent marker.
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Cut aluminum sleeves
1. Score the aluminum tubing with a sharp razor blade at 21 mm for 20 mm probes and 12
mm for 10 mm probes by rolling the tubing back and forth under the razor blade.
2. The tubing should snap off easily. Cut one section of tube for each set of sensors.
3. Remove any burrs and sharp points with a blunt, pick-like tool.
Part 2. Installation of the sap flow sensors
Overview of installation
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Choose sensors with similar heating-wire resistances.
Drill appropriately-sized holes in the branches for probes.
Insert the aluminum sleeves and sensors into the wood.
Connect the sensors together and to the extension wires leading to the datalogger and to
the constant-current power supply.
5. Determine the correct voltage to apply to the heated sensors.
Inserting the sensors
Because the constant-current power supply described in this document (see end of Part 2) can
supply power to three sets of sap flow probes in series, it is important that the resistances of
the three heated probes be similar. The heat generated by each probe must be close to the 0.2
Watts (or 0.1 W for 1 cm probes) required for the sap flow calculations to be accurate.
1. Choose three heated probes with similar R-coil resistances and three non-heated probes to
make three sets of sap flow probes to be installed.
2. For each set of probes, punch out two small disks of bark about 2 cm in diameter from the
tree, one above the other and separated by about 10-15 cm. The higher up in the tree, the
less variable is the sap flow. The north sides of trees (in the Northern Hemisphere)
receive less sun and so will have fewer problems with thermal gradients.
3. Drill a 2.5 mm diameter hole (slightly larger than the diameter of the aluminum sleeve)
into the center of the upper hole and into the exposed xylem, going in about 22 mm for 20
mm sensors and 12 mm for 10 mm sensors. Drill a 1.1 mm hole (slightly larger than the
diameter of the unheated probe) likewise into the lower exposed xylem.
4. Position the aluminum sleeve on a round and pointy tool that will not deform the tube
when pressure is exerted (an unheated probe works if the bark is thin, as does the round
end of the triangular file) and firmly insert the sleeve into the top drilled hole, making sure
to be very straight. Push until the end of the sleeve is level with the exposed xylem.
5. Dip a heated sensor in heat conducting paste, then insert very carefully into the sleeve all
the way, making sure to go straight and slow. Rotating the sensor while inserting helps.
Wipe off any excess heat conducting paste.
6. Insert a reference sensor into the smaller, lower hole.
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Connecting the wires
1. Connect the two constantan wires (red insulation) from the thermocouples together by
twisting and then soldering. Do not confuse the constantan heating wire with the
constantan wire of the
thermocouple.
2. Support a 4-wire shielded
cable to the tree with either
duct tape, bungee cords, or
string, with some extra loops
so that if someone trips over
the cable it doesn’t directly
pull on the sensor
connections. Run this cable
to the datalogger and power
supply.
3. Connect the copper wires
(blue insulation) from the
thermocouples to a shielded
cable by twisting and
soldering.
4. Connect the two heating
wires from the upper sensor
to two wires of the shielded
cable.
5. Insulate all exposed
connections with electrical
tape.
6. Wrap a reflective insulating
covering over the sensors to
reduce external thermal
gradients. If rain is
expected, seal the upper edge with silicon sealant (acetic acid-free if possible to reduce
damage to the plant) to prevent water entry. If working on branches, pipe insulation
clamped down with plastic cable ties works very well.
Setting the power
To determine the current required to power the heating coils, use the sensor with the lowest
coil resistance of the three to be hooked up in series (but all should be very close in resistance
values). This will supply slightly more power to the other two if they are well-matched, but
this is preferred to providing less than 0.2 W per coil, which can lead to larger measurement
errors if sap flow reduces the temperature difference significantly between the sensors.
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1. Solve for current in the equation (a variation of Power = Voltage × Current):
Power = Current2 × Resistance
where for 2 cm sensors, Power = 0.2 Watts and for 1 cm sensors Power = 0.1 Watts, and
Resistance is the lowest coil resistance (R-coil) of the three sensors. This is the current
that you will need to send to all three sensors in series.
2. With all the cables strung and wires connected, measure the total resistance of the heating
circuit (connecting wires, heater lead wires, and the three coils in the circuit). Record this
value as R-circuit.
3. Solve for Voltage in the equation for Ohm’s Law:
Voltage = Current × Resistance
where Current is the current you determined in Step 1 and Resistance is R-circuit. This is
the voltage to provide across the circuit to the heating coils and which is adjustable on the
constant-current power supply (see the heating circuit schematic).
4. Adjusting the variable resister on the constant-current power supply to correctly set the
voltage across the connecting wires to the heaters and start heating the sensors.
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1. Parts for the constant-current power supply (4 boards):
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4, fixed resistors: ¼ W each: 12? , 47? , 180?
4, variable resistors 1K? trimpot
4, 220µF polarized capacitor (radial)
4, LM317T Voltage regulators
4, LM317T heat sinks
8, 1N4001 Garden variety diodes – many others work
8, 3 position terminal blocks
4, 1A, 250V fuses – or 0.5 A fuse works fine – and fuse clips
1, 6”x 8”, .1”x .1” spacing, 0.042” hole perfboard
2. Sap Flow Sensors
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Thermocouple wire: #TT-T-36 gauge copper-constantan thermocouple wire and #TFCC005-100 constantan heating wire. OMEGA Engineering, INC., One Omega Drive,
Stamford, Connecticut 06907-0047, P.O. Box 4047, (800)-848-4286 or (203)-359-1660.
Fax: (203)-359-7700, www.omega.com
Hypodermic needles: Becton-Dickenson 19G, 1 1/2 stainless steel hypodermic needles –
order through your institutes purchasing dept. or else you will be suspected as a heroin
junkie. Box of 100 yields 50 sensor pairs. www.bd.com
Aluminum tubing: #TTRA-2-36. Small Parts Inc., P.O. Box 4650, Miami Lakes, FL
33014-0650, 1-800-220-4242, fax 1-800-423-9009. www.smallparts.com
Glue: ‘Extra runny’ superglue or Krazy Glue. www.krazyglue.com
3. Tool suggestions
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Cordless drill: Makita 6043DWK - 4.8V 3/8" Cordless Drill Kit (Reversible). Makita
U.S.A., 14930 Northam St., La Mirada, CA 90638. Phone: (714) 522-8088.
www.makita.com
Butane soldering iron: UT-40SI Ultratorch Master-Appliance. Jensen Tools, 7815 South
46th Street, Phoenix, Arizona 85044-5399. Phone: (800) 426-1194, Fax: (800) 366-9662.
www.jensentools.com
Volt-amp-ohmmeter: 29-Range Digital Multimeter, RadioShack Catalog #: 22-813. 1800-843-7422. www.radioshack.com
4. Other supplies
Reflective insulation: Small project roll of Double Bubble Foil/Foil. Reflectix, P.O. Box 108,
Markleville, IN 46056. Phone: (800) 879-3645, Fax: (765) 533-2327. www.reflectixinc.com
Please direct questions or comments for improvement of this document to: Eric Graham,
Center for Embedded Networked Sensing, UCLA, 3563 Boelter Hall, Los Angeles, CA
90095-1596, USA. (310) 825-8777. [email protected]
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