Inexpensive replacement elements for laboratory hot plates


Material Information

Inexpensive replacement elements for laboratory hot plates
Physical Description:
3 p. : ill. ; 27 cm.
Rusk, Harold W
United States -- Bureau of Entomology and Plant Quarantine
U.S. Dept. of Agriculture, Agricultural Research Administration, Bureau of Entomology and Plant Quarantine
Place of Publication:
Washington, D.C
Publication Date:


Subjects / Keywords:
Laboratories -- Equipment and supplies   ( lcsh )
Heating -- Equipment and supplies   ( lcsh )
federal government publication   ( marcgt )
non-fiction   ( marcgt )


General Note:
Caption title.
General Note:
General Note:
"March 1947."
Statement of Responsibility:
by Harold W. Rusk.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 030363955
oclc - 783769572
System ID:

Full Text
Yanh 1947 rT-2 35

STATE FLANI O1A,\fted States Department of Agriculture
Agricultural Research Administration
Bureau of Entomology and Plant Quarantine


By Harold W. Rusk
Division of Insecticide Investigations

The hot plates generally available from supply houses do not
fill the unusual needs of some laboratories, being limited either
by physical size or temperature range. Inexpensive replacement
elements, which are available in 600-and 1000-watt sizes, may be
used as versatile heating units for custom-designed hot plates.
These elements consist of two equal arms wound with ribbon re-
sistance wire over a mica core, enclosed in sheets of mica and
connected to form a waferlike unit with leads at the end of each
arm coil and at the common center. At low heat both arms are
connected in series, at medium heat only one arm is across the
line, the other arm being dead this causes uneven surface temper-
atures), and at high heat both arms are across the line in paral-

A hot plate that would accommodate six 500-ml. irlenmeyer
flasks, and produce a temperature sufficient to reflux 50- to 100-
ml. samples of benzene solutions was needed. Since no hot plate
of proper size and heat range could be found in the catalogs, it
was decided to construct a unit for this purpose.

The framework of the hot plate consisted of 20-gage sheet
metal for the front, back, and sides, 1-by 1-inch angle iron for
corner posts, and 3/4-by 3/4-innh angle iron for the heater sup-
port rails. The support rails were riveted on the inside of' the
frontane back pieces about 1/2 inch below the top. The heating
unit was constructed from a piece of cold-rolled steel 3/16 by 5
by 36 inches for the top, three 6-inch, s-heat replacement ele-
ments for the heat source, and a base or backing plate of Transite
(an asbestos-cement insulating board) of the same dimensions as
the top. Each element was separated .y cutting the mica separator
strip at the end having the two leads. The mica at the center-
tapped end was then cut and the lead cut off, with care not to
cut the wire connecting tche two arms. Each elemert was spread
out and the distance between the two arms adjusted to about 1 inch
by pull.n, the connecting wire slightly. After the three elements
had been treated as above, they were then placed in position on
the Lransite base, the leacs were pulled through their holes as
shown in figures I and 2, a:d all -Pacvs not occupiec ty the ele-
ments were filled with asbestos paper thick enough to keep the
elements in place und prevent bucxling. The resistance wire con-
necting the arms of each element was insulated with asbestos
paper and the entire heating unit placed in position on the support


rails and drawn up tightly with the stove bolts, as shown in
figure 2, after which the sides and corner posts were bolted on,
switches and terminal block secured in place, and the wiring done
according to the diagram in figure 3. rs wired the hot plate has
an output of 450 watts with a current consumption of 3. ) amperes.

The design of this hot plate permits using each element
individually through its respective switch. however, many vari-
ations are possible. If a higher plate temperature is desired,
a few turns may be removed from each arm of each element, or,
conversely, if a lower temperature is desirable, fixed resistors
may be placed in series witi, each element. A suitable variable
resistor could also be placed in the main lead to provide a wide
range of temperatures. By using 4-position rotary switches and
utilizing the center-tapped leads, the hot plate can be changed
to a 3-heat unit similar to the usual laboratory hot plate.
however, consideration must be given to the current consumption
of such an arrangement, since the three elements on high heat
tl800 watts) would draw a current of about 16 amperes.

Variation of the described design proved practical in the
construction of a 2-heat unit, 10 172 by 13 inches, made of
materials similar to those in the 40-watt unit. Two heating
elements, spread apart and placed one behind the other to give
uniform distribution of heat over the working surface, were
employed in this hot plate and controlled by a double-pole,
double-throw icnife switch to obtain two operating heats (300
and 1200 watts). In the first, or lowheat the two arms of each
element are in series and the two elements in parallel, while at
high heat each arm of each element is in parallel.

This hot plate has proved satisfactory for general laboratory
use such as reducing the volume of organic solvents or boiling
down aqueous solutions. Its total cost is about $10.



Heating unit:
Top-i piece 3/16 x 5 x 36 inches, steel plate, drilled and
00untersunk to take 3/16-inch stove bolts
Elements-3 replacement units for 6-inch, 3-heat stove, 600-watt
Base-i piece Transits, 3/16 x 5 x 36 inchesdrilled as in
figure 1

Sheet-iretal, 20-gage-2 pieces 6 x 36 inches (front and back)
2 pieces 6 x 6 inches (sides)
Angle iron, I x 1 inch--4 pieces 1 x 6 inches (corner posts)
3/4 x 3/ inoh-2 pieoe. 3/4 x 35 1/2 inches (heater
unit supports)

Switches-3 recessed snap switches
Appliance oord--8-fte asbestos-covered cord with receptacle plug
Terminal blook- piece Transite 2 x 3 inches
2 small metal angles

Bolts, nuts, washers, and rivets as needed




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