TITLE: TDT Sandwich: An Open Source Dry Heat System for Characterizing the Thermal Resistance of Microorganisms
AUTHORS: Soon Kiat Lau, Jeyamkondan Subbiah

[STEPS]
?. [Thermocouple assembly]
The materials needed for assembling one thermocouple assembly are listed below. For brevity, the components will be referred to by the designators defined in the bill of materials (https://osf.io/z79jy/) and annotated in the picture below. T1: 1 ea. PFA-insulated thermocouple, type T, 40" long, 40 AWG, stripped leadsT2a: 1 ea. Connector part of T2 (Miniature thermocouple connector, type T, male)T2b: 1 ea. Shell of T2 (Miniature thermocouple connector, type T, male)T2c: 1 ea. Clamp of T2 (Miniature thermocouple connector, type T, male)T3: 1 ea. Silicone wire grommetT4: 1 ea. Silicone clamp grommet
?. [Printed circuit board]
The printed circuit board (PCB) used in the TDT Sandwich can be manufactured either in-house or by a PCB manufacturer (e.g. JLCPCB or OSHPark). The Gerber files for manufacturing the PCBs can be downloaded from https://osf.io/wgyxp/. When ordering the PCB, it is recommended to order an accompanying PCB stencil in preparation for the next step.
?. [Final Assembly]
Once all the parts from the previous sections have been assembled, the TDT Sandwich can be put together. The materials needed for assembling one TDT Sandwich are listed below. For brevity, some components will be referred to by the designators defined in the bill of materials (https://osf.io/z79jy/) and annotated in the picture below. A2: 2 pcs. 1±0.2 cm x 1±0.2 cm Adhesive Transfer Tape, 4" X 20 yd, 2.30 mil ThickB2: 4 pcs. 2.5±0.5 cm x 2.5±0.2 cm Polyurethane Foam Mounting Tape, Open-Cell, 1/4" Thick, 1" Wide, 54' LongC6: 1 ea. USB-A to mini USB-B cable, 80 cmC7: 1 ea. Power cord, NEMA 5-15P to IEC 320-C15, 6 ftZ5: Instant bonding adhesive, 0.5 ozSandwich box: 1 ea. Sandwich box assembled in earlier stepsControl box: 1 ea. Control box assembled in earlier stepsHeating pads: 2 ea. Thermocouple assembly assembled in earlier steps
?. [Computer program]
The computer program for operating the TDT Sandwiches can be downloaded from https://osf.io/edpku/.No installation is required; simply run the file. If you have an antivirus program, it may ask for your permission to approve the program; approve the request. Instructions on how to use the computer program are provided in the white paper detailing the TDT Sandwich.
?. [Printed circuit board]
The electronic components to be soldered onto the PCB are listed under the "Printed circuit board" section of the bill of materials available at https://osf.io/z79jy/. The "Designators" are labels for the components that are printed onto the PCB beside the footprints of the associated component (except for S1 and S2, which are fasteners).The electronic components consist of through-hole and surface mount components. To assemble the surface-mount components, it is highly recommended to use a PCB stencil to ensure consistent solder paste distribution. The PCB stencil can usually be ordered along with the PCB. A tutorial on using PCB stencils is given by Sparkfun: https://www.sparkfun.com/tutorials/58.The remaining through-hole components can be soldered on using a soldering iron. A tutorial on using a soldering iron is given by Sparkfun: https://learn.sparkfun.com/tutorials/how-to-solder-through-hole-soldering/all. The through-hole components are:BZ1F1J1 to J8U1, U2, U3, U6, U7, U13
?. [Heating pads]
The materials needed for assembling one heating pad are listed below. For brevity, some components will be referred to by the designators defined in the bill of materials (https://osf.io/z79jy/) and annotated in the picture below. The polyimide etched-foil heater is a custom order part from a manufacturer suggested in the bills of material.H1: 1 ea. Polyimide etched-foil heater, 120 mm x 120 mm, 144 WH2: 19±0.5 cm Expandable Polyester Sleeving, Red, 1/8" ID, 100' LongH3: 2 pcs. 3±0.5 cm Heat-Shrink Tubing, Red, 25' Long, 0.19" ID Before ShrinkingH4: 2 ea. Plug contacts, Female 18-24 AWGH5: 1 ea. Plug, 01x02H6: 10±0.5 cm Polyimide tape, Silicone Adhesive, 4" Wide, 15 Feet Long, 0.0025" Overall ThicknessH7: 2 pcs.12±0.1 cm Silicone Foam Strip with Adhesive, 3/4" Wide, 1/16" Thick, 30' LongH7b: 2 pcs. 8±0.1 cm Silicone Foam Strip with Adhesive, 3/4" Wide, 1/16" Thick, 30' LongA2: 2 pcs. 1±0.2 cm x 1±0.2 cm Adhesive Transfer Tape, 4" X 20 yd, 2.30 mil ThickAluminum pads: 2 ea. Aluminum pads assembled in earlier stepsThermocouple assembly: 1 ea. Thermocouple assembly assembled in earlier steps
?. [Thermocouple assembly]
Trim the exposed lead wires of T1 to 3 mm in length.
?. [Thermocouple assembly]
From the exposed lead wire end, insert one T4, followed by one T3.
?. [Thermocouple assembly]
Loosen one of the screws on T2a. This would cause a metal plate underneath that screw to lift up. Insert one of the exposed lead wire of T1 under the plate, making sure that the colors match (e.g. the brown/copper wire goes to the brown/copper terminal). Tighten the screw to secure the lead wire. Repeat this with the other exposed lead wire.Note: If the thermocouple wires are not matched to the appropriate terminal, the thermocouple may give abnormal temperature readings.
?. [Thermocouple assembly]
Move the T3 strung onto T1 earlier onto the groove on T2a.
?. [Thermocouple assembly]
Align the T4 strung onto T1 earlier to the end of T2a.
?. [Thermocouple assembly]
Gently wind T1 around the T2 assembly. This step is optional but recommended as it helps to reduce clutter and prevents T1 from tangling up. Leave a 17 cm gap between T2 and the end of T1. Use a piece of wire/twist-tie to secure T1 onto T2.
?. [Thermocouple assembly]
Place T2b on top of T2a and tighten the screws.
?. [Thermocouple assembly]
Place T2c on top of T4 and tighten the screws. T1 should now be secured to the T2 assembly.
?. [Heating pads]
Route H2 onto both the lead wires of H1, then route both H3 over H2 such that both the ends of H2 are covered by H3. Apply heat onto both H3 pieces using a hot air gun or hair dryer until they shrink and grip the wires tightly.
?. [Heating pads]
Position one H4 onto one of the exposed lead wires of H1 such that its "long arm" is on the wire insulation while the "short arm" is on the exposed lead wires, as shown in the picture below. Subsequently, crimp the arms onto the lead wire. Repeat for the other lead wire.
?. [Heating pads]
Align both crimped wires to H5 as shown below before pushing them in. A "click" sound indicates that the crimped wires are secured within H5.
?. [Heating pads]
Clean the heating surface of H1 with a cleaning solvent such as ethanol or propanol to remove any residual oils on the surface. Using a permanent marker, create markings on H1 based on the measurements in the picture below. A few things to note:Note the orientation of the heating pad based on the lead wire location at the top right of the picture. These markings must be made on the flat side of the heating pad; the other side would have a bump at one of the corners due to the lead wires.The marking at the center must be on the heating element. The heating element is the light-colored strip that zigzags over the entire heating element.
?. [Heating pads]
Place the tip of the thermocouple assembly at the center marking and straighten it so that the entire length of the thermocouple is parallel to the heating element. Place the two squares of A2 onto the thermocouple, one over the thermocouple tip and the other just above the upper marking. Apply pressure on the A2 squares and carefully remove the paper backing.NOTE: It is extremely critical to ensure that the thermocouple tip is on the heating strip. Preferably, the thermocouple wire should also be within and parallel to the heating strip as long as it's in contact with the heating pad. The system relies on the thermocouple measuring the temperature of the heating strip. Incorrect placement of the thermocouple on the darker areas (i.e. no heating strip) delays temperature measurement and may cause the system to operate outside of specifications.
?. [Heating pads]
Remove the paper backing of the adhesive on one of the aluminum pads and align the aluminum pad to the four markings on H1, such that the four edges of the aluminum pads are flush with the markings. Once aligned, press the aluminum pad onto H1. Apply even pressure on the aluminum pad by placing the assembly on a flat surface, turning the pad over, and rolling a cylindrical object such as a pipe.
?. [Heating pads]
Apply H6 on the aluminum pad, making sure that all sides of the aluminum pad are covered with H6. The application must be done carefully to minimize air bubbles and creases formed during the application process. It is recommended to start applying H6 from a corner and slowly pressing it down using fingers or with a hard, flat object.
?. [Heating pads]
Turn over H1 and apply the other aluminum pad. No markings are necessary; simply align the aluminum pad to the outline of the one on the other side. Apply even pressure with fingers or with a roller.
?. [Heating pads]
Affix H7a and H7b on the perimeter of the aluminum pad. It is recommended to affix H7a first, then cover the remaining areas with H7b.
?. [Heating pads]
Repeat the steps to produce a total of two heating pads per TDT sandwich.
?. [Aluminum pads]
The materials needed for assembling one aluminum pad are listed below. For brevity, the components will be referred to by the designators defined in the bill of materials (https://osf.io/z79jy/) and annotated in the picture below. A1: Aluminum 3003 Sheet, 0.032" Thick, 4" x 10" (pack of 6)A2: Adhesive Transfer Tape, 4" X 20 yd, 2.30 mil ThickZ3: Paper trimmerZ4: Bastard Cut Mill File
?. [Aluminum pads]
The goal is to trim A1 into squares with edge lengths of 8±0.1 cm. Due to the small tolerance and possibly large number of output, it is recommended to use Z3 due to its in-built measuring ruler and an easy-to-use cutting blade. Although Z3 is meant for cutting paper, it can be used on A1 due to the softness and thinness of the aluminum. Specialized metal cutting devices can be used if available.To use Z3, simply align A1 to the measuring lines of Z3. As shown in the picture below, an 8 cm (~3.15 in) section is about to be trimmed off from a piece of A1. Upon cutting off this rectangular section, it would be rotated and the cutting process repeated to form an 8 cm x 8 cm square piece.
?. [Aluminum pads]
The sharp corners of the A1 square must be deburred to prevent it from damaging other parts of the system. Simply rub Z4 repeatedly on the corners until they are rounded off.
?. [Aluminum pads]
Carefully apply A2 on one side of the A1 square, making sure to prevent air bubbles or creases during the application. Trim off any excess A2 on the edges.
?. [Thermocouple assembly]
Repeat the steps to create a total of two thermocouple assemblies.
?. [Aluminum pads]
Repeat the steps to produce a total of four aluminum pads.
?. [Final Assembly]
Apply small amounts of Z5 on the brown strips located on the underside of the heating pads. Z5 should be applied in thin, zigzagging streaks instead of giant droplets to improve attachment and reduce drying time.
?. [Final Assembly]
Open up the sandwich box and route the wires and thermocouple of the heating pad (heating pad 1) through the holes in the lower half of the box as shown in the picture below. Place the heating pad, glue side down, on the approximate center of the lower half foam sheet with the side.The wires and thermocouple can be labelled with a labelling tape as '1' for easier identification.Place two squares of A2 on the sides of the aluminum pad on heating pad 1 and remove their paper backing.
?. [Final Assembly]
Route the wires and thermocouple of the other heating pad (heating pad 2) through the other set of holes. Place this heating pad, thermocouple side down, on top of heating pad 1. Ensure that both heating pads are aligned with each other. Apply small amounts of Z5 on the brown strips on heating pad 2. Close the sandwich box and lock in the front latch. This presses the foam sheets against the heating pads and ensure proper attachment of the glue. Allow at least 20 min for the glue to completely cure.The wires and thermocouple can be labelled with a labelling tape as '2' for easier identification.
?. [Final Assembly]
Affix the B2 squares near the corners of the underside of the control box where the screws are located. Remove the paper backing of the B2 squares and attach the control box onto center of the top of the sandwich box.
?. [Final Assembly]
Attach CB4, CB5, and the wires and thermocouples of both heating pads to the back of the control box as shown in the picture below. Labelling the wires and thermocouples of the heating pads in the earlier steps will make this process easier.NOTE: The heating pads and thermocouples must be connected in the order shown below. The printed circuit board inside the control box also has labels for the connectors. Incorrect connections for the heater wires and thermocouples may result in unexpected system behavior.
?. [Sandwich box]
The materials needed for assembling one sandwich box are listed below. For brevity, the components will be referred to by the designators defined in the bill of materials (https://osf.io/z79jy/) and annotated in the picture below. B1: 1 ea. Polypropylene Box, 6" x 6" x 2"B2a: 4 pcs. 7±0.5 cm x 2.5±0.2 cm Polyurethane Foam Mounting Tape, Open-Cell, 1/4" Thick, 1" Wide, 54' LongB2b: 8 pcs. 2.5±0.5 cm x 2.5±0.2 cm Polyurethane Foam Mounting Tape, Open-Cell, 1/4" Thick, 1" Wide, 54' LongB3: 2 ea. 15.5±0.5 cm x 15.5±0.5 cm Buna-N/PVC Foam Insulation Sheet, 4' x 36" x 1"
?. [Sandwich box]
Cut holes on the back and front of B1 using a utility knife according to the picture below. Eye protection is recommended. To make the process easier, B1 can be placed in an oven set to 60°C for 20 min to soften the plastic, thus making it easier to cut.
?. [Final Assembly]
After waiting at least 20 min for the glue on the heating pads to dry, gently nudge a flathead screwdriver between the heating pads and use it as a lever to separate the heating pads from each other. Remove the residue of A2 on the heating pads by rubbing it off using a finger.
?. [Sandwich box]
Place B2a and B2b on the inside of B1 according to the picture below. It is not necessary to place them at exact locations; B2b is placed on the corners of the "lines" on the lid and base of B1 while B2a are placed side by side in the approximate center. Remove the paper backing once all the B2a and B2b pieces have been placed.
?. [Sandwich box]
Place both B3 sheets on top of the B2a and B2b pieces. Close the box and clamp the latch down to compress the B3 sheets onto the B2a and B2b pieces and improve adhesion.
?. [Control box]
The materials needed for assembling one control box are listed below. For brevity, some components will be referred to by the designators defined in the bill of materials (https://osf.io/z79jy/) and annotated in the picture below. CB1a: 1 ea. enclosure from Enclosure, ABS, gray, 5.12"L X 3.94"WCB1b: 1 ea. short screws from Enclosure, ABS, gray, 5.12"L X 3.94"WCB1c: 2 ea. long screws from Enclosure, ABS, gray, 5.12"L X 3.94"WCB1d: 2 ea. panels from Enclosure, ABS, gray, 5.12"L X 3.94"WCB2: 1 ea. Arduino NanoCB3: 1 ea. Fuse, 250 V, 2.5 A, fast-blowAssembled printed circuit board: 1 ea. Printed circuit board assembled in earlier stepsCB1d is not needed and can be discarded.
?. [Control box]
Attach CB3 to the fuse holder on the printed circuit board as shown below. The orientation does not matter.
?. [Control box]
Place the printed circuit board onto the CB1a piece that has four screw holes. Align the four holes on the board to the four holes on the CB1a piece and screw in all four of CB1d.
?. [Control box]
Insert CB2 into the female headers on the printed circuit board as shown in the picture below. Note the orientation of CB2.NOTE: Ensure that CB2 is aligned correctly based on the picture below. Inserting CB2 in the opposite direction may cause permanent damage to the electronics upon powering the system.
?. [Control box]
Install the remaining half of CB1a, noting that each half has a lip/groove on its sides that prevents installing it in the wrong orientation. Screw in both CB1c into the two holes on the outside of CB1a.
?. [Printed circuit board]
The thermocouple jacks (J1, J2, and J3) are attached to the PCB with solder. However, since these jacks may experience a lot of mechanical abuse during use (i.e. plugging in/out thermocouples), it is highly recommended to provide additional mechanical support with a screw (S1) and nut (S2).
?. [Final Assembly]
A program/sketch needs to be uploaded to the Arduino microcontroller (C2) before the TDT Sandwich can be used. There are two ways to do this:Quick & easy (only for Windows computers)Download the compiled hex file (TDTSandwich.ino.eightanaloginputs.hex) from https://osf.io/fe62v/ and a hex file uploader program (XLoader) from http://www.hobbytronics.co.uk/download/XLoader.zip.Extract the XLoader zip file to an appropriate location. Connect the TDT Sandwich to the computer using C4.Open the XLoader.exe inside the folder extracted in step 2.Click the "..." button beside the "Hex file" field and select the compiled hex file downloaded earlier.Click the arrow button under "Device" and select "Duemilanove/Nano(ATmega328)."Check that the "Baud rate" has a value of 57600.Click the arrow button under "COM port" and select the port that corresponds to the TDT Sandwich. If you are unsure which is the correct port, some trial and error might be required here; simply choose one from the list and press the "Upload" button. Most of the time, the port is NOT COM 0 or COM 1. If an error occurs or the program doesn't seem to function after a while, repeat the upload process with another port until you find one that succeeds.The TDT Sandwich program is now uploaded.Advanced method (requires knowledge of Arduino IDE, but works in all operating systems)If you do not have the Arduino IDE, it can be downloaded from https://www.arduino.cc/en/main/softwareDownload the source code and libraries for the TDT Sandwich from https://osf.io/fe62v/. Install the libraries by following the "Manual installation" instructions at https://www.arduino.cc/en/guide/libraries#toc5.Connect the TDT Sandwich to the computer using C4.Upload the TDTSandwich.ino sketch, making sure to choose Arduino Nano for the board, ATmega328P for the processor, and selecting the correct port.The TDT Sandwich program is now uploaded.Advanced method (requires knowledge of Arduino IDE, but works in all operating systems)If you do not have the Arduino IDE, it can be downloaded from https://www.arduino.cc/en/main/softwareDownload the source code and libraries for the TDT Sandwich from https://osf.io/fe62v/. Install the libraries by following the "Manual installation" instructions at https://www.arduino.cc/en/guide/libraries#toc5.Connect the TDT Sandwich to the computer using C4.Upload the TDTSandwich.ino sketch, making sure to choose Arduino Nano for the board, ATmega328P for the processor, and selecting the correct port.The TDT Sandwich program is now uploaded.
?. [Final Assembly]
The TDT Sandwich is now complete. It is recommended to "burn-in" all newly-made TDT Sandwiches before performing any experiments by running it at 100°C for at least 10 min. Some of the adhesives (e.g. A2) perform better after receiving heat.Please refer to the manuscript of the TDT Sandwich for usage instructions.