(Drawing #1 - 14KB jpeg file)
The unit will consist of a thermal grid (approximately 8x10 cm), made of
narrow strips (approximately 3 per cm) that can each be set at different temperatures.
(Drawing #2 - 65KB jpeg file)
Initially, only two temperatures will be available, but it will be possible to
assign either of the two temperatures to each element in order to produce any of the thermal patterns
First Prototype (feasibility study): (Two small Peltier per element).
(Drawing #3 - 32KB jpeg file)
The unit would be made of 24 or 27 pieces of brass stock, 3/32" square, 10cm long. The temperature of each
element would be controlled by two small thermoelectric (Peltier) devices set in a pattern as seen in the
One element with 2 small Peltier devices
(Photo #1 - 12KB jpeg file)
was assembled and tested. The results were very disappointing: the temperature gradient along
the element was too great and this design was immediately abandoned.
Second Prototype (feasibility study): (Each element made of a 5cm long 3/32" square brass stock).
This unit is based on a 5cm x 5cm Peltier device with a flat 5cm x 5cm brass water tank as a heat sink on one side, and 15 elements made of
3/32" square brass stock on the other. Each element (Photo #2 - 179KB jpeg file)
has a thermocouple soldered through a small hole at its middle and a 10 cm long Nichrome heater
wire is threaded through its whole length in both directions. The tube is filled with thermal epoxy
(Delta Bound 152-K-A by Wakefield Engineering).
Initially, only two temperatures will be available: a lower temperature that will be determined by
the Peltier device and a higher temperature delivered by those elements that have their heaters turned on.
(Drawing #4 - 16KB BMP file)
Four elements as seen in photo #2
were epoxied (using thermal epoxy) to the Peltier device leaving a 1/32" air gap between the elements.
The Peltier temperature was controlled by means of a prototype unit designed in 2003 for the "Oral Hydration" project. This device
connects to a PC through a USB port.
A preliminary Visual Basic program was written to control the Peltier
(Software Panel #1 - 42KB jpeg file)
One of the element thermocouples and a special thermocouple, epoxied to the heat sink side of the Peltier, were used for Peltier
The heaters in the two elements adjacent to the Peltier control element were connected to a variable power supply and the
thermocouple of one of them was connected to a thermometer for manual heat control.
May 2, 2007
- With this setup
we could only reach a temperature differential of 10°C, at which point the current through the nichrome wires was 1.2Amp and the insulation on the
wires started to melt. However the design should work after some modifications:
using 3/32"channels rather than square tubing would make it easier to build the elements and may permit doubling the length of the heater
wires and more evenly packing the assembly with epoxy. The heater wires should also be welded to conductor wires before they exit the
epoxy thus keeping them at a lower temperature.
Element Pattern: (Drawing #4 - 16KB BMP file)
#1 - No Heat
#2 - Heat - Secondary Temperature
#3 - No Heat - Peltier Temperature = Primary Temperature
#4 - Heat
May 10, 2007
- The assembly was reworked using the same elements but after the Nichrome wires had been
shortened, soldered to conductor wires and epoxied to the side of the assembly.
The test was repeated using the same "element pattern" as above. Although a much higher current could be used without any problem
(2.2 vs 1.2 Amp) only a differential of close to 14°C could be reached as seen on the (improved) software window
(Software Panel #2 - 40KB jpeg file).
The current through the Peltier was then 2.8Amp, the maximum attainable with the current Peltier controller.
The test was repeated using a different element pattern:
(Drawing #5 - 16KB BMP file)
#1 - Heat - Secondary Temperature
#2 - Heat
#3 - No Heat
#4 - No Heat - Peltier Temperature = Primary Temperature
A temperature differential of 30°C could then be reached without any
problem. The heater current was just below 2.0Amp and the Peltier
current was only 1.1Amp as seen on the software window
(Software Panel #3 - 40KB jpeg file)
. Notice also that, in this case,
the Peltier temperature was equal to the command temperature.
1 - The water bath temperature was around 8°C.
2 - The "Peltier Temperature" on the "Software Panel" is actually the
temperature read by a thermocouple connected to an element with a
disconnected heater: not the actual temperature of the Peltier.
1 - A larger gap and insulating material between elements will most
likely be necessary, as suggested by Dr. Mark
Hollins. But there's hope.
2 - Keeping the heating elements farther from the Peltier device would
also most probably minimize the influence of each element on its
Third Prototype (feasibility study): (Each element made of a 5cm long 3/32" brass channel on top of a 3/32" brass square stock).
(46KB JPEG file)
The difference between this unit and the previous (second) one is in the construction of the elements.
1 - Instead of feeding the thermocouple and heater wires through a square brass stock, a brass
channel was used ( HTML file
). This gave better control
over the positioning of the heater wire inside the conduit by holding the wires in the epoxy, against the
channel ( Construction Photo
2 - The assembled channel was then epoxied on top of a piece of square brass stock, thus doubling its height, to keep
the heater farther from the Peltier. The thermocouple was threaded through a hole in the piece of square stock.
3 - The joints between the heater and conductor wires were encapsulated within a short piece of 1/8" channel to minimize heat exchange between
4 - Ceramic tape by Cotronics Corp
was used as an insulation between elements.
June 28, 2007 -
The same test pattern shown in drawing #5
used May 10, was repeated with the new elements (Drawing #6
). There were 4
elements when the tests were started, but when increasing the heater current to some value between 1.5 & 2.0 Amp., a short developed in
both heated elements. One element was removed.
1 - The difference between the two temperatures (~10°C) changes little when changing the Peltier temperature without changing the heater
current (set @ 1.0Amp): Panel 1
and Panel 2
2 - Increasing the heater current to 1.5Amp.increased the difference in
temperature to 25°C: Panel 3
3 - Increasing the heater current some more increased the
difference in temperature but caused a heater element fault before it could be recorded.
One faulty element was removed for inspection (see Meltdown Analysis below), and some more
testing was performed using only 3 elements. The center one was at low temperature (Peltier temperature) the other two on either side were
heated (Element Temperature).
4 - Heater current set at 1.6 Amp.gave a 25°C difference in temperature
5 - Heater current set at 1.8 Amp.gave nearly 32°C difference in
temperature Panel 5
Notice that the Peltier temperature was lowered in order to avoid any more meltdown.
Photos of heater element after meltdown:
1 - Heater element in its entirety. (Meltdown #1
2 - Connection with conductor wire that had signs of excessive heat damage (after removal of melted insulation ).(
3 - The other connection also with obvious heat damage.(Meltdown #3
Photos of heat tests of various insulations :
1 - Tefzel
melts at 1.3 Amp. The green jacket (made of Tefzel
a type of Teflon
) that, so far, has been used throughout the
experiment for heater wire insulation (from Vector 28 gage wire).
2 - Omega black Teflon
jacket possible replacement for the Tefzel melts at 1.5 Amp.(Omega PTFE
3 - A blue TFE Teflon
wire insulation, also a possible replacement for the Tefzel did quite
well at 2.0Amp. This jacket was stripped from a piece of Belden 83001
4 - A piece of black shrink
next to a piece of transparent shrink tubing (that turned
dark from the heat) they both also did well at 2.0 Amp. (Black shrink tubing by 3M
1 - Blue Teflon insulation
2 - Black shrink tubing over the solder joints
3 - Blue Teflon conductor wires (instead of the white ones).
4 - Double the length of the heater wire if there is enough room inside
the brass channel.
Fourth Prototype (feasibility study): (Heating elements made of coiled Chromium wire from K type thermocouple).
The K type thermocouple wires that have been used throughout this investigation have one Alumel conductor and
the other is Chromel an alloy of Chromium and Nickel just like the
Nichrome (Nickel Chromium) heater wire we use. Since the thermocouple
conductors come with a Teflon insulation and their overall outside
diameter is significantly smaller than any we could make, we decided to
give it a try and use it as heater wire. Because of its small OD
(Outside Diameter) we were able to wind it onto a 1/64" piano cord thus
forming a tiny coil that could fit inside our brass channel stock. The
effective length of the wire was 4 times the length of the grill
element thus twice the length of the heater wires we previously
used. Another advantage of using a coil is that the heater wire
does not crisscross itself thus avoiding hot spots that are potential
cause for over heating and shorts. This truly sounded promising.
July 23, 2007
Unfortunately, the results of the test were the most disappointing.
With a 2 Ampere heater current we could only reach a 20°C
Apparently the coil was not a good idea. We assume that heating the
sides and bottom of the brass channel instead of concentrating the heat
at the top caused too much wasted heat for which the Peltier had to
compensate. We obviously have to give up the coil idea and go back to
our previous design.
1 - We will first see if the Chromel wire, that is
much easier to solder than the Nichrome wire, can satisfactorily work
when not coiled.
2 - Then, if the Chromel wire is not satisfactory, we will look at the
available pre-insulated heater wires, such as the "Formvar-Insulated
Nichrome Wire" available from "A-M
custom Thermo foil heaters with optional built-in temperature
sensors. Although this does not look so good (Designer
Kit PDF file
Fifth Prototype (feasibility study): (2 heating elements made of straight Chromel wire from K type thermocouple,
2 made of Tefzel insulated Nichrome wire).
This prototype is similar in construction
the Third Prototype
but only 2 of the heater
wires are made of Tefzel insulated Nichrome wire, the other 2 are made
of Chromel from Omega K type thermocouple wire. We want to compare the
Chromel and Nichrome as heating elements. We expect that the Nichrome
would require less current than the Chromel but the difference in
insulation may negate the difference in wire. The reason for
going back to Nichrome and third prototype construction is what we
uncovered when taking the previous heating elements apart. To take the
heating elements apart, we apply a current of 2 Amperes to the heater
wire for a few seconds, until the high heat softens the Delta Bond
epoxy. Despite the high heat, the Tefzel insulation was in
amazingly good condition, and we assume that, as long as the
connections are kept epoxied inside the brass channel, there
should be no melted insulation problem.
To make sure the heater wires were as close to the
surface of the brass channel as possible, pieces of common hookup wire
were used as stuffing material and the elements were squeezed by means
of Binder Clips while the epoxy cured.
View the entire test panel: 1.7 Amp
. test - 1.8 Amp
. test - 1.9 Amp
As shown in drawing #6, the "Element temperatures" indicated on the
test panels above were measured at the surface of the heated element
sandwiched between 2 colder (not heated) elements. The
temperature of the other element, the one that had one side at room
temperature (Drawing #6b
at 68 °C when a heater current was 1.8 Amp.as shown on this panel
The Chromel is obviously not the way to go.
The new technique for attaching the Nichrome and conductor wires
together and making sure the heater wire is as close to the brass as possible seem to make quite a difference.
We will now make a dozen more elements and see if the Peltier can handle them.
Then, later, we will need an electronic monster to drive all this.
First 5cmX5cm Model: (22 January 2008).
Finally resuming this project building 12 heating elements following the
instructions for the Grill Element #2
As was mentioned in the instructions for the Grill Element #2: "This
single procedure needs more accuracy in weld timing and wire flattening
and positioning, but, with practice, seems to be the answer." We had
little problem welding 5 heater wires on the first day, but on
the second day the "Weld Energy" of 3 was too high and caused the wire
to actually be projected across the floor. "Weld Energy" of 2 was still
insufficient to melt the copper wire. Adding a variable
transformer on the UNITEK power line and fine tuning the voltage to
somewhere between "Weld Energy" of 2 and 3 seems to have help: we were
able to resume welding the heater wires.
It appears that 90 VAC is working quite well leaving the power on
continuously. We will try this again Monday hoping we do not get any more bad surprise.
Well, we did encounter more difficulties with the welding, although we
manage to make the 13 heaters needed + 3 as backups. In the process we
lost some Nickel-Chromium wire and Tefzel insulation both
becoming too short to be used after several welding attempts.
A solution to this problem would be to weld the 1" piece of copper wire
to a very short piece of Nickel-Chromium, resulting in little waste
when the weld fails. Then, when we have a good weld, weld the heater
wire to the short piece of Nickel-Chromium, and cover the whole thing
up with the Belden jacket (see illustration
However, this procedure will present a problem if the extra piece of
Nickel-Chromium wire is not properly physicaly connected to the
Grill Thermode 5cm x 5cm Model
(April 7, 2008).
Started in May 2007
Last revision: 2008 April 7