A six
inch piece of two-conductor cord was cut, stripped, and tinned. Small gauge
wire was used which was approximately 26AWG. Small pieces of shrink tubing were
placed over two of the tinned ends. Those ends were soldered to the terminals
of a 3.5mm phono socket. The shrink tubing was heated and snugged down onto the
phono terminals and wires. The tinned ends with no shrink tubing were inserted
into the NTC screw terminals of the temperature controller. No concern was
given to polarity. The 3.5mm socket was glued in place with hot glue. The wires
of the temperature controller were divided so the low-voltage DC wires of the
thermometer did not mix with the 120VAC wires.
Short wire with tinned ends and 3.5mm socket
Heat shrink on 3.5mm terminals
Shrinking heat tube on a 3.5mm socket
Attaching the 3.5mm socket to the NTC terminals of the controller
3.5mm socket glued in place
The high voltage wires are separated from the low voltage wires
The
wire lead ends of the temperature probe were stripped, tinned, and soldered to
the terminals of a 3.5mm jack. Shrink tubing was used on each wire. The crimp
supports on the jack were tightened onto the wires then the jack was sealed up.
The temperature sensor construction was complete.
Shrink tube on a 3.5mm jack
Crimped 3.5mm jack
Completed temperature probe
Testing
showed that the controller would display a higher temperature than read by any
other thermometer. This is hazardous since water controlled by this would be
cooler than necessary and may not cook food thoroughly since it would not reach
a safe temperature. This was tested by a digital IR thermometer and a kitchen
dial thermometer. At higher temperatures the discrepancy between the IR, dial,
and controller widened due to the logarithmic change of resistance.
A digital thermometer disagrees with the controller
A dial thermometer disagrees with the controller
At higher temperatures the controller still reads low
At even higher temperatures the discrepancy increases
The function of the temperature controller was tested by reaching a set point, 130ºFahrenheit, then adding ice to cool the water. The purpose was to see how finicky the controller was. The result was that the controller would not reapply heat until the temperature dropped more than two degrees below the set point. This is good because it means the controller will not cause the relay to chatter excessively.
Testing the controller's operation by cooling the water
To
test against a known
successful recipe using this controller two eggs were put in while the
controller reported it was at 167º Fahrenheit. The first egg was cooked for
fifteen minutes according to the chart from Sous Vide Supreme. An egg timer was
also put into the oven which displays the doneness of eggs by changing color as
heat permeates the timer and eggs. After fifteen minutes one egg was removed
and the egg timer displayed it had reached the soft doneness point. The egg
white was liquid and lumpy but the yolk was the correct consistency and flavor.
Egg timers and 167ºFahrenheit water
Eggs were put in and the temperature dropped
The egg timer shows soft cooked eggs
One egg cooked at 167ºFahrenheit for fifteen minutes
Liquid and lumpy egg white
Well cooked yolk
The
second egg was removed after 23 minutes when the egg timer showed it to be
fully heated. In this egg the white was still liquid and lumpy but the yolk had
partially hardened. These symptoms suggest that the water was not sufficiently
heated. To fix this problem two solutions were apparent at first. The first
option is to make a chart showing the desired water temperature and the
temperature to set on the controller. The second option is to insert a
potentiometer or rheostat to change the temperature reading so that a
calibration is possible.
The eggs timer shows a fully cooked egg
One egg cooked at 167ºFahrenheit for 23 minutes
Video of opening a cooked egg
To
test the properties of the NTC an ohmmeter was attached while the temperature
was changed. Since the resistance falls as the temperature rises it will be
necessary to increase the resistance of the probe. This should be able to be
accomplished by putting a variable resistor in series with the probe.
If
people reproduced this project and did not encounter a discrepancy with the
thermometer they would be done with the build.
To
do:
- Insert calibration resistor. ≤1KΩ
- Draft controller template for cut-out
- Sketch circuit with optional calibrating potentiometer
- Retest
- Document
Journal page 1
Journal page 2
Journal page 3
Journal page 4
A list showing of all the final posts of COMPLETED projects.
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