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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). 1. 2. 3. 4. 5. 6. 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. 1 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. 2 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 3 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. 4 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. 2. 3. 4. 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. 5 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. 6 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. 7 1. Parts for the constant-current power supply (4 boards): • • • • • • • • • 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 • • • • 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 • • • 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] 8