Construction of a Low-cost Mobile Incubator for Field and Laboratory Use
LEHRERVORBEREITUNG
KONZEPTE
SCHÜLERPROTOKOLL
1. Heating Unit
- Gather the following components (Figure 1):
Support plate (280 x 250 mm) with required anchorage holes
Axial fan (60 x 60 x 25 mm); 2x
Spacer (length 20 mm, internal diameter 4.25 mm (M4)); 4x
Luster terminal with three pins
Screw nut (M4); 4x and (M3); 1x
Washer (M4); 8x and (M3); 1x
Screw (M4); 4x and (M3); 1x
Figure 1: Individual components of the heating unit. Support plate, axial fans, spacers, luster terminal, screw nuts, washers and screws. Please click here to view a larger version of this figure.
- Drill the required holes (Figure 2) into the support plate to secure the axial fans as well as the luster terminal (Figure 1).
Figure 2: Schematic diagram of support plate. Indications to drill the anchorage holes into the support plate to fix the axial fans as well as the luster terminal. Distances are given in millimeters. Please click here to view a larger version of this figure.
- Anchor the axial fans in the center on the support plate as shown in Figure 3 with two M4 screws, screw nuts, and washers , , per fan. Use the spacers to leave a distance between the fans and the support plate (Figure 3).
- Anchor the luster terminal to the support plate using M3 screw, screw nut, and washer. Secure the cable fans. (Figure 3).
- Connect the fan cables with the luster terminal. Connect the positive cables of each fan together and the negative cables of each fan together (Figure 3). The speed sensor is not required.
Figure 3: Axial fans fixed on support plate. Please click here to view a larger version of this figure.
NOTE: The cable colors mentioned correspond to the ones used in the figures. The cable colors might change depending on the material used.
2. Control Unit (Power Supply)
- Gather the following components:
Universal enclosure (here 200 x 120 x 60 mm, but dimensions will depend on the size of the DC/DC converter and the PID temperature controller)
On/off-Switch
DC/DC converter, input voltage range 9 – 36V, output voltage 12V
PID temperature controller, 12 – 35 V/DC operating voltage
Cable gland, M12 x 15 mm, clamping range 2 – 7.5 mm (or according to the cable used)
Temperature sensor Pt100
AC power supply
NOTE: The incubator can be connected to the mains power supply or to a battery. In the case of the mains operation, the AC power supply is required and if the unit is exclusively connected to mains, the DC/DC converter is not mandatory. In the case of battery operation, the DC/DC converter is highly recommended, and a two-wire cable is required instead of the AC power supply. This protocol presents the version with the DC/DC converter and the AC power supply. An electrical diagram of the incubator electrical core is detailed in the supplementary material (Figure S1). - Mill the openings for the PID temperature controller, on/off switch, and cable glands into the enclosure with a drill and jigsaw, or an equivalent tool (Figure 4).
Figure 4: Schematic diagram of the universal enclosure. (a) Indications to place the temperature controller 
, on/off switch 
and the cable glands 
into the universal enclosure; distances are given in millimeters. (b) 3D view of the universal enclosure. Please click here to view a larger version of this figure.
- Connect the DC/DC Converter to the on/off switch: connect the positive cable of the AC power adapter to the on/off switch and the negative cable of the AC power adapter to the "– Vin" of the DC/DC converter (Figure 5).
- Use a cable to connect the on/off switch to the "+ Vin" of the DC/DC converter (Figure 5).
Figure 5: Mounted control unit. Universal enclosure with DC/DC converter connected to PID temperature controller and on/off switch . Please click here to view a larger version of this figure.
- Connect the cables from the heating unit to the PID temperature controller as follows (Figure 6):
- Connect the terminal "1" of the PID temperature controller to the “DC –“ wire from the heating unit connection and to the "– Vout" terminal of the DC/DC converter.
- Connect the “DC +” wire going to the heating unit to the terminal "4" of the PID temperature controller as well as to the terminal "2" of the PID temperature controller (see point 3.2).
- Connect the terminal "2" of the PID temperature controller to the "+ Vout" terminal of the DC/DC converter.
- Connect the terminal “5” of the PID temperature controller to the “command” wire going to the heating unit. (see point 3.2).
- Connect the temperature sensor to the terminals "10", "11" and "12".
NOTE: The red cable of the temperature sensor must be connected to terminal “11” of the PID temperature controller.
- Anchor the DC/DC converter with Velcro tape at the bottom of the enclosure, and close the universal enclosure.
Figure 6: Cable connection of DC/DC converter with PID temperature controller. DC/DC converter , PID temperature controller , connection to Incubator (cable A) and connection to temperature sensor (cable B). Please click here to view a larger version of this figure.
NOTE: The functions of the PID temperature controller terminals used are given in Table 2.
| PID temperature controller terminal | Function |
| Terminal "1" | Supply input + |
| Terminal "2" | Supply input – |
| Terminal "4" | Control Output Common Contact |
| Terminal "5" | Control Output Normally Open Contact |
Table 2: Functions corresponding to the PID temperature controller terminals.
3. Assembly of the Incubator Electrical Core
- Gather the following components:
Heating unit from section 1
Control unit from section 2
Heating foils, self-adhesive, 100 x 200 mm, 12 V/20 W, 2x - Link the connection cables from the control unit to the heating unit as follows (Figure 7):
- Connect the “DC-” wire from the control unit with one conductor of each of the heating foils and the negative wire of each fan.
- Connect the “DC+” wire coming from the control unit with the positive cable of each fan.
- Connect the “command” wire from the control unit to the remaining two conductors of the heating foils.
Figure 7: Cable connection of heating foils with PID temperature controller. Please click here to view a larger version of this figure.
NOTE: The completed field incubator electrical core of the incubator is shown in Figure 8.
Figure 8: Complete field incubator electrical core. Heating unit , control unit and temperature probe . Please click here to view a larger version of this figure.
4. Assembly of the Incubator
- Gather the following components:
Incubator electrical core
Incubator shell (here a polystyrene foam box, but can be any type of box made of insulating material)
Support rack (here a metal rack, but can be another material) - Place the incubator components together as follows (Figure 9):
- Place the incubator shell on its side, so that the opening of the incubator (door) is located on a side.
- Place the support plate with the heating unit at the bottom of the incubator shell.
- Place the support rack on top of the heating unit, leaving a space of a minimum of 10 cm between the heating unit and the support rack.
- Place the temperature probe on the support rack and secure it in the incubator.
- Drill holes in the door of the incubator to allow for the entry of the cables (Figure 9).
- Connect the incubator to the power source.
- Turn the incubator on and adjust the settings of the PID temperature controller (see Table S1 in the supplementary material for detailed settings).
Figure 9: Complete field incubator. Open (left) and closed (right). Heating unit , support rack , temperature probe , control unit 
, incubator shell 
and holes for cables in the incubator shell (circled area). Please click here to view a larger version of this figure.
NOTE: The shell of the incubator can be a box of any type of material. It is recommended to use an insulating material, and that the box closes tightly to avoid dissipation of the heat. The support rack should contain large holes to avoid the accumulation of heat in the rack, and the material can be metal or other (e.g. plastic).
Construction of a Low-cost Mobile Incubator for Field and Laboratory Use
Learning Objectives
The reliability of a robust field incubator lies in its ability to reach and maintain a set temperature under various conditions. To monitor the performance of the various incubator set-ups, the following measurements were made: time needed to reach the set temperature, effect of opening the door for one minute, power consumption over 24 hours of operation, inner temperature stability over 24 hours of operation, and observation of E. coli growth. The temperature inside the incubator was measured every minute with 4 temperature logging devices placed in different positions in the structure (support rack, wall, top, inside a growth plate). The set temperature was considered to be attained when all measurements were within plus or minus 2 °C, which is the acceptable range for the incubation of E. coli.13
The electronic core was tested with three types of shells, using materials that are typically found in many countries: a polystyrene foam box (78 liters), a hard plastic cooler box (30 liters), and a cardboard box covered with a survival blanket (46 liters) (Figure 10). To cover a range of ambient conditions that can be experienced in the field, these incubator set-ups were tested at three ambient temperatures: ambient (about 27 °C), cold (about 3.5 °C and 7.5 °C) and hot (about 39 °C). Performance measures were tested setting the inner temperature at 37 °C and 44.5 °C.
The time to reach the set temperature in the incubators was influenced by the ambient temperature and the material of the incubator shell. At an ambient temperature of about 27 °C, the three incubators set-ups reached the set temperatures (37 °C and 44.5 °C) in a similar time (Figure 11a and Figure 12a) and comparable with the performance of a standard incubator (Table 3). In cold environments (3.5 °C and 7.5 °C), the incubators with thicker shells, i.e., the polystyrene foam and cooler box, reached the target set temperatures (37 °C and 44.5 °C) in a similar time; about four times longer than under an ambient temperature of 27 °C. With its lower insulation, the cardboard box with survival blanket never fully reached the set temperatures under cold ambient temperature conditions (Figure 11b and Figure 12b). In a warm environment (39 °C), the three incubator set-ups reached the target temperature of 44.5 °C in under 10 minutes (Figure 12c). However, when the set temperature was of 37 °C, i.e., lower than the ambient temperature, none of the incubators could lower the temperature, resulting in overheating for all three incubator set ups (Figure 11c).
The ambient temperature and the type of incubator shell influenced the impact of opening the door of the incubator for one minute. The heat loss was greater in the cold environment, and the time to recover the inner set temperatures was longer, with the exception of the cardboard box incubator where the set temperatures were never reached (Figure 13b and Figure 14b). In the warmer environments, the heat loss was limited and the set temperatures were recovered in under 10 minutes (Figure 13a,c and Figure 14a,c). In an ambient temperature of 39 °C and set temperature of 37 °C, opening the door did not cause nor reduce overheating of the incubators (Figure 13c).
The power consumption increased with cold environments and with an increase in the set temperature. Better insulating incubator shells (polystyrene foam and cooler box) showed a reduced power consumption as compared to the cardboard box incubator. In similar environments (ambient temperature of about 27 °C), the three incubator set-ups consumed 0.22 to 0.52 kWh/24h less energy than the standard incubators tested (Table 3).
The temperature in the incubator remained stable over 24 hours with all type of incubator shells and ambient temperature tested (Figure 13 and Figure 14). Slight variations of the measured temperature compared to the set temperature were observed according to the position of the temperature logging device in the incubator. With the exception of the tests with the ambient temperature (39 °C) warmer than the set temperature (37 °C) (Figure 13c), the variations in temperature were all within the 2 °C acceptable range for E. coli incubation.
All tests were performed in the presence of E. coli and total coliform measurement materials (membrane filter placed on growth plate). Replicates of a sample were placed in each incubator set-up and in a standard incubator for comparison. In all set-ups and conditions, the growth of E. coli and total coliform was successful and comparable to the growth observed in the standard incubator. A summary of the incubator configurations and ambient temperature conditions tested with results are shown in Table 3.
| Test 1: Time to set temperature |
Test 2: Side door opening one minute |
Test 3: Power consumption over 24-hour period |
Test 4: Temperature variation over 24-hour period |
Test 5: E. coli growth observed |
|||
| Ambient temperature | Set temperature | (min) | Maximal loss of temperature (°C); time to recover set temperature (min) | (kWh/24 h) | Absolute maximal temperature (°C); absolute minimal temperature (°C) * | (Yes / No) | |
| Polystyrene foam box | 3.5 °C | 37 °C | 45 | 10 °C; 17 min | 0.78 | 37; 35.5 | Yes |
| 7.5 °C | 44.5 °C | 74 | 16.5 °C; 31 min | 0.89 | 44.5; 42.5 | ND† | |
| 27 °C | 37 °C | 12 | 2.5 °C; 3 min | 0.28 | 37.5; 36.5 | Yes | |
| 44.5 °C | 20 | 4.5 °C; 7 min | 0.43 | 44.5; 43.5 | ND† | ||
| 39 °C | 37 °C | 0 (overheat) | 2 °C; 0 min (overheat) | 0.11 | 42.5; 42 | Yes | |
| 44.5 °C | 7 | 3.5 °C; 5 min | 0.17 | 45; 43.5 | ND† | ||
| Hard cooler box | 3.5 °C | 37 °C | 54 | 8 °C; 10 min | 0.86 | 37.5; 36 | Yes |
| 7.5 °C | 44.5 °C | 96 | 12 °C; 30 min | 1.05 | 45; 43 | ND† | |
| 27 °C | 37 °C | 13 | 1.5 °C; 0 min | 0.27 | 37.5; 36.5 | Yes | |
| 44.5 °C | 25 | 2 °C; 4 min | 0.50 | 45; 43.5 | ND† | ||
| 39 °C | 37 °C | 0 (overheat) | 1 °C; 0 min (overheat) | 0.11 | 43; 42.5 | Yes | |
| 44.5 °C | 9 | 4 °C; 3 min | 0.19 | 45.5; 44.5 | ND† | ||
| Cardboard box with survival blanket | 3.5 °C | 37 °C | Never reached (stable temperature after 109 min) | 6.5 °C; stable temperature after 30 min | 1.24 | 33.5; 30.5 | Yes |
| 7.5 °C | 44.5 °C | Never reached (stable temperature after 120 min) | 8 °C; stable temperature after 20 min | 1.28 | 36.5; 32 | ND† | |
| 27 °C | 37 °C | 15 | 2.5 °C; 6 min | 0.42 | 36.5; 35.5 | Yes | |
| 44.5 °C | 24 | 3 °C; 8 min | 0.70 | 44.5; 42.5 | ND† | ||
| 39 °C | 37 °C | 0 (overheat) | 1.5 °C; 0 min (overheat) | 0.11 | 41.5; 40 | Yes | |
| 44.5 °C | 9 | 2 °C; 0 min | 0.20 | 45; 43.5 | ND† | ||
| Standard incubator | 27 °C | 37 °C | 18 | 1 °C; 0 min (overheat) | 0.64 | 38.5; 36 | ND† |
| 44.5 °C | 23 (overheat) | 2.5 °C; 0 min | 0.95 | 47.5; 43.5 | ND† |
Table 3: Results summary for the incubator configurations and ambient temperature conditions tested. *Test 4: Absolute maximal and minimal temperatures recorded during the stable periods, i.e., from 10 minutes after the end of a disruptive event (time to reach set temperature, opening the door). †ND: No data, test not run.
Figure 10: Incubator shells tested. Open (upper row) and closed (bottom row). Polystyrene foam box (left), thickness of 3.5 cm, outer dimensions 39 x 56 x 36 cm; hard plastic cooler box (middle), thickness of 2.5 cm, outer dimensions 32 x 41 x 47 cm; cardboard box (right) covered with a standard survival blanket of a thickness of 12 µm folded twice, outer dimensions 30 x 42 x 37 cm. Please click here to view a larger version of this figure.
Figure 11: Time to reach set temperature (37 °C) of the incubator set-ups under different ambient temperature conditions. Performances of incubators with a shell made of a polystyrene foam box, a hard cooler box, and a cardboard box covered with a survival blanket. At room ambient temperature (a), cold ambient temperature (b), and warm ambient temperature (c). Temperatures recorded on the support rack of the incubators. Please click here to view a larger version of this figure.
Figure 12: Time to reach set temperature (44.5 °C) of the incubator set-ups under different ambient temperature conditions. Performances of incubators with a shell made of a polystyrene foam box, a hard cooler box, and a cardboard box covered with a survival blanket. At room ambient temperature (a), cold ambient temperature (b), and warm ambient temperature (c). Temperatures recorded on the support rack of the incubators. Please click here to view a larger version of this figure.
Figure 13: Temperature variations over 24-hour period and effect of door opening under different ambient temperature conditions. Set temperature of 37 °C. Performances of incubators with a shell made of a polystyrene foam box, a hard cooler box, and a cardboard box covered with a survival blanket. At room ambient temperature (a), cold ambient temperature (b), and warm ambient temperature (c). Circled areas show the temperature variations due to the door opening for one minute. Temperatures recorded on the support rack of the incubators. Please click here to view a larger version of this figure.
Figure 14: Temperature variations over 24-hour period and effect of door opening under different ambient temperature conditions. Set temperature of 44.5 °C. Performances of incubators with a shell made of a polystyrene foam box, a hard cooler box, and a cardboard box covered with a survival blanket. At room ambient temperature (a), cold ambient temperature (b), and warm ambient temperature (c). Circled areas show the temperature variations due to the door opening for one minute. Temperatures recorded on the support rack of the incubators. Please click here to view a larger version of this figure.
Figure S1: Electrical diagram of incubator electrical core wiring. Alternatives for mains operation and battery operation are indicated. Please click here to view a larger version of this figure.
| Parameter | Selected value | |||||
| 1 | Type of control output | Control Q1 / Alarm Q2 | ||||
| 2 | Type of connected sensor | Pt100 (-200 to 140°C) | ||||
| 3 | Lower limit selectable for setpoint value | 0 | ||||
| 4 | Upper limit selectable for setpoint value | 50 | ||||
| 5 | Type of control | Heating | ||||
| 6 | ON/OFF hysteresis or dead band for P.I.D. control | 0 | ||||
| 7 | Proportional bandwidth of the process expressed as units (°C if temperature) | 1 | ||||
| 8 | Integral time. Intertia of the process expressed as seconds | 80.0 | ||||
| 9 | Derivative time for P.I.D. | 20.0 | ||||
| 10 | Cycle time for time-proportioning output | 10 | ||||
| 11 | Allow/deny modification of setpoint values by frontal keyboard | Allow modification of all setpoints | ||||
| 12 | Software filter. Number of readings to calculate the comparison value PV-SPV | 10 | ||||
| 13 | Type of degree | °C | ||||
| 14 | Type of cooling liquid | Air | ||||
Table S1: PID temperature controller settings. Display of set values; other parameters not necessary to run the incubator were left to default values.
List of Materials
| Heating foil | Thermo | 2115337 | Self-adhesive 10×20 cm; Operating voltage 12V; Power 20W |
| Axial fan | Yen Sun Technology Corp. | FD126025MB | 6x6x2.5 cm; Operating voltage 12VDC; Power 1.44W; Max. current consumption 60mA |
| PID Temperature Controller | Wachendorff Automation GmbH & Co. KG | UR3274S | PID controller 32×74 mm; Universal input for process signals, thermocouples, Pt100; Operating voltage 24 VDC; Outputs (thermostats) 10 A relay, 5 A relay, SSR, RS 485 |
| Temperature sensor Pt100 | Conrad | 198466 | Temperature range -100°C to 200°C; Sensor Pt100, Type FS-400P |
| Universal enclosure | OKW Gehäuse System | C2012201 | Dimensions 200 x 120 x 60 mm |
| ON/OFF Switch | SHIN CHIN INDUSTRIAL CO. | R13-70A-01 | Connection Type C CEE 7/16 plug 6.3 mm; Contact resistance Max 50 mΩ; Switching voltage 24 VDC; Switching current (mx.) 10A; Insulation resistance Min 100 MΩ/500 Vdc |
| DC/DC converter | Traco Power | TMDC 60-2412 | Nominal voltage 24 VDC; Input voltage 9-36 VDC; Output voltage 12 VDC; Max. output current 5 A; Power 60W |
| AC power adapter | Bicker Elektronik | BET-0612 | Output voltage 12 VDC; Max. output current 5 A; Input voltage 115-230 VAC |
| Spacer | Schäfer Elektromechanik | 20/4 | Without thread; Thread size M4; Polystyrene; Distance 20 mm |
| Cable gland | WISKA | 10066410 | M12 x 1.5 cm; clamping range 3 – 7 mm |
| Luster terminal | Adels Contact | 125312 | Nominal current 25 A; Nominal Voltage 500V |
| Screw M4 x 50 | Bossard | 1579010 | M4 x 50 mm |
| Screw nut M4 | Bossard | 1241478 | M4 |
| Washer M4 | Bossard | 1887505 | M4 |
| Screw M3 x 25 | Bossard | 1211099 | M3 x 25 mm |
| Screw nut M3 | Bossard | 1241443 | M3 |
| Washer M3 | Bossard | 1887483 | M3 |
| Support plate | - | - | Insulating material (plastic or other); 28 x 25 cm |
Lab Prep
Incubators are essential for a range of culture-based microbial methods, such as membrane filtration followed by cultivation for assessing drinking water quality. However, commercially available incubators are often costly, difficult to transport, not flexible in terms of volume, and/or poorly adapted to local field conditions where access to electricity is unreliable. The purpose of this study was to develop an adaptable, low-cost and transportable incubator that can be constructed using readily available components. The electronic core of the incubator was first developed. These components were then tested under a range of ambient temperature conditions (3.5 °C – 39 °C) using three types of incubator shells (polystyrene foam box, hard cooler box, and cardboard box covered with a survival blanket). The electronic core showed comparable performance to a standard laboratory incubator in terms of the time required to reach the set temperature, inner temperature stability and spatial dispersion, power consumption, and microbial growth. The incubator set-ups were also effective at moderate and low ambient temperatures (between 3.5 °C and 27 °C), and at high temperatures (39 °C) when the incubator set temperature was higher. This incubator prototype is low-cost (< 300 USD) and adaptable to a variety of materials and volumes. Its demountable structure makes it easy to transport. It can be used in both established laboratories with grid power or in remote settings powered by solar energy or a car battery. It is particularly useful as an equipment option for field laboratories in areas with limited access to resources for water quality monitoring.
Incubators are essential for a range of culture-based microbial methods, such as membrane filtration followed by cultivation for assessing drinking water quality. However, commercially available incubators are often costly, difficult to transport, not flexible in terms of volume, and/or poorly adapted to local field conditions where access to electricity is unreliable. The purpose of this study was to develop an adaptable, low-cost and transportable incubator that can be constructed using readily available components. The electronic core of the incubator was first developed. These components were then tested under a range of ambient temperature conditions (3.5 °C – 39 °C) using three types of incubator shells (polystyrene foam box, hard cooler box, and cardboard box covered with a survival blanket). The electronic core showed comparable performance to a standard laboratory incubator in terms of the time required to reach the set temperature, inner temperature stability and spatial dispersion, power consumption, and microbial growth. The incubator set-ups were also effective at moderate and low ambient temperatures (between 3.5 °C and 27 °C), and at high temperatures (39 °C) when the incubator set temperature was higher. This incubator prototype is low-cost (< 300 USD) and adaptable to a variety of materials and volumes. Its demountable structure makes it easy to transport. It can be used in both established laboratories with grid power or in remote settings powered by solar energy or a car battery. It is particularly useful as an equipment option for field laboratories in areas with limited access to resources for water quality monitoring.
Verfahren
Incubators are essential for a range of culture-based microbial methods, such as membrane filtration followed by cultivation for assessing drinking water quality. However, commercially available incubators are often costly, difficult to transport, not flexible in terms of volume, and/or poorly adapted to local field conditions where access to electricity is unreliable. The purpose of this study was to develop an adaptable, low-cost and transportable incubator that can be constructed using readily available components. The electronic core of the incubator was first developed. These components were then tested under a range of ambient temperature conditions (3.5 °C – 39 °C) using three types of incubator shells (polystyrene foam box, hard cooler box, and cardboard box covered with a survival blanket). The electronic core showed comparable performance to a standard laboratory incubator in terms of the time required to reach the set temperature, inner temperature stability and spatial dispersion, power consumption, and microbial growth. The incubator set-ups were also effective at moderate and low ambient temperatures (between 3.5 °C and 27 °C), and at high temperatures (39 °C) when the incubator set temperature was higher. This incubator prototype is low-cost (< 300 USD) and adaptable to a variety of materials and volumes. Its demountable structure makes it easy to transport. It can be used in both established laboratories with grid power or in remote settings powered by solar energy or a car battery. It is particularly useful as an equipment option for field laboratories in areas with limited access to resources for water quality monitoring.