Overview

1. Soldering iron and solder
2. Single-edge razor blades
3. Small triangular metal file
4. Wire cutters and needle-nose pliers
5. Portable volt-amp-ohmmeter (multimeter)
6. Small, battery-powered drill for the field.
7. Butane soldering iron for the field.

Part 1. Sap flow sensor construction

Materials

1. Thermocouple wire, 36 AWG (0.005 ichn or 0.127 mm) copper-constantan.
   
2. Constantan wire, 36 AWG.
   
3. Stainless steel hypodermic needles, 19 gauge, 1½ inch (38 mm) long.
4. Aluminum tubing, round, 3/32 inch (2.38 mm) OD, 0.014 inch (0.36 mm) wall thickness.
5. Components for the construction of a constant-current source for heating.
6. Shielded, two-conductor copper wire (24 AWG) cables for connecting thermocouples to datalogger and unshielded two-conductor copper wire cables for connecting constantan heating wire to current source. We have also used all-in-one shielded, four-conductor cables as well.
7. Reflective/insulating wrap for putting over installed probes.
8. Car battery to supply heating current.

Overview of sap flow sensor construction

1. Two hypodermic needles are needed for each set of sap flow probes.
2. Cut each to either 1 or 2 cm and notch each needle in the middle.
3. Construct fine-gauge copper-constantan thermocouple junctions and insert one each into the midpoint of each needle.
4. Insert and 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.

Construction of sensors

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 55 cm of constantan heating wire (or about 35 cm for a 1 cm probe) and strip about 1 cm of insulation off of both ends.

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.
   
   
   
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. Please note: some have found notching the end difficult and instead have secured the heater wire (in step 5, below) with a drop of super glue at the tip, which is allowed to dry before wrapping.

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 sure that the lead wire of the constantan heating wire is about the same length as the thermocouple leads. Please note: some have found that putting in the thermocouple first then the constantan heating wire works best.
   
   
   
   
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.

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

1. Choose sensors with similar heating-wire resistances.
2. Drill appropriately-sized holes in the branches for probes.
3. Insert the aluminum sleeves and sensors into the wood.
4. 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

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.

Connecting the sensor wires and the cables to the sensor 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-conductor shielded cable (or both the 2-conductor shielded cable and the 2-conductor unshielded 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 does not directly pull on the sensor connections. Run this cable to the datalogger and power supply.
3. Connect each of the two copper wires (blue insulation) from the thermocouples to each of the two conductors in the shielded cable by twisting and soldering.
4. Connect the two heating wires from the upper sensor to the two wires of the unshielded cable (or to the other wires in your 4-conductor 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.

Connecting the other end of the cables to the datalogger and the power supply

1. Connect the wires from the shielded cable (attached to the sensor thermocouples) to a differential channel of the datalogger. For example, on a Campbell datalogger one copper wire would go into the "H" of channel 1 and the other would go into the "L" of channel 1. You may also add a jumper wire from the "L" to the "ground" terminal (immediately to the right of the "L").
   
   Note: the polarity of the voltage from the thermocouples will depend on which wire goes into which terminal (voltage will go up or go down with sap flow with different connections). This does not actually make any difference and data can be corrected after the fact to make every sap flow sensor respond in the same "direction". However, for uniformity, after the system is set up and the heater is on, you can review the data and switch the wire pairs that have negative voltages to make them all positive.
   
   
2. Connect the wires from the unshielded cable (attached to the constantan heater wires) to the power circuit. The polarity of this connection makes no difference.

Setting the power

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. Adjust the variable resister on the constant-current power supply (schematic below, click for PDF; 31kb) to correctly set the voltage across the connecting wires to the heaters and start heating the sensors.

Part 3. Parts and Supplies

1. Parts for the constant-current power supply (4 boards):
   
   
   • 4, fixed resistors: ¼ W each: 12 ohm, 47 ohm, 180 ohm
   • 4, variable resistors 1K ohm 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 inch x 8in, .1 inch x .1 inch spacing, 0.042 inch hole perfboard


2. Sap Flow Sensors:
   
   
   • Thermocouple wire: #TT-T-36 gauge copper-constantan thermocouple wire and #TFCC-005-100 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 suggestion:s
   
   
   • 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. 1-800-843-7422. 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

Acknowledgements