DESIGN OF A SIMPLE PLANT EXPOSURE CHAMBER
U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
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DESIGN OF A SIMPLE PLANT EXPOSURE CHAMBER
by
Walter W. Heck
John A. Dunning
and
Henry Johnson
U. S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
Bureau of Disease Prevention and Environmental Control
National Center for Air Pollution Control
Cincinnati, Ohio
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National Center for Air Pollution Control Publication APTD-68-6
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ABSTRACT
The chambers used in plant exposure studies at the National Center for Air
Pollution Control utilize a dynamic, negative-pressure, single-pass flow system
with uniformity of toxicant flow, mixing, and distribution in the chamber. The
simple design, described herein, permits easy installation of numerous chambers
in a single air-handling system while still permitting individual control of cham-
bers.
Key Words: Exposure, Chamber, Plants, Air Pollution
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DESIGN OF A SIMPLE PLANT EXPOSURE CHAMBER
PRINCIPLES OF CHAMBER DESIGN
The effects of phytotoxic air pollutants on vegetation have been studied in a
variety of exposure chambers. Many early chambers were either closed-system
designs or small greenhouses redesigned for exposure studies. Certain basic
design features of the chambers discussed in this paper have been reported.
Ideal chamber design permits maintenance of natural environmental con-
ditions during plant exposures. Good design can be achieved by closely following
design specifications that consider the plants to be studied and the purpose of the
research. The most desirable flow characteristics, the toxicants to be used,
and the degree of environmental control desired are other important consider-
ations. Versatility combined with simplicity should be the overriding considera-
tion.
Chambers should be fabricated from materials having low adsorption
characteristics to avoid possible reactions with the interior of the chamber,
since such side reactions could produce injury not directly related to the toxicant
being studied. The chambers should be either easily cleanable or inexpensive,
so that they may be discarded and replaced.
Dynamic air systems are superior to static air systems, and can be of a
negative- or positive-pressure type. The negative systems, however, eliminates
the potential hazard of toxicant release into the area in which the exposure cham-
ber is situated.
Toxicant and air must be well mixed before the exposure "atmosphere"
enters the exposure chamber. Air movement in the chamber may be laminar or
turbulent, but design for a laminar flow system is exacting, since the plants in
the chamber interrupt the laminar flow and cause a modified-turbulent flow. A
turbulent flow system (with essentially instantaneous mixing with air already in
the exposure chamber) can be constructed rather simply.
Air can be recycled on a percent basis or completely exhausted from the
system. Recycled air can cause corrosion or deposit particles on the air-
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handling equipment, and substances in the air may interact with certain chemical
components of the air-handling equipment to form products capable of producing
abnormal plant injury. A single-pass system eliminates most of the problems
encountered with a recycling system and makes monitoring and control of
chamber levels of specific toxicants easier. The single-pass system also adds
to the simplicity of total design.
Gases that are potentially phytotoxic should be added in a diluted form in
the intake duct. The duct should be designed to aid in the mixing of air and
toxicant and should be capable of accommodating several toxicants at a level at
which they would normally interact no more than under ambient conditions.
Lines carrying the toxic substances should be chemically inert and heat
resistant to allow the use of high-temperature liquids in the low-ppm vapor phase.
When toxicants readily adsorb on chamber materials, lines and chambers should
be well cleaned or changed before other toxicants are used.
The degree of environmental control depends on the basic purpose of the
experiment. Chambers should be usable outdoors, in greenhouses, or as in-
serts into special plant-growth chambers. Where chambers are used under
natural conditions, exposures should be considered only on days conductive to
producing injury, unless environmental variations are part of the experiment.
When comparisons are desired, most exposures require environmental
control. Although greenhouse conditions often suffice, lighting must be controlled
in most geographical locations for a planned program. Close environmental
control can be obtained by the use of a special plenum on the inlet of each ex-
posure chamber, by insertion of the chamber into a plant growth chamber, or
by inserting an exposure chamber equipped with the special plenum into a plant
growth chamber.
Simplified construction and appropriate air-handling allow the use of a
number of chambers in parallel without reducing the utility of the specific cham-
ber design.
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DESIGN AND CONSTRUCTION DETAILS
The type of chamber used in experiments for the past 3 years utilizes a
dynamic, negative-pressure, single-pass flow system. Flow, toxicant mixing,
and air are uniform, and several toxicants can be injected at once. Simplicity of
design permits easy installation of numerous chambers in a single air-movement
system and individual control of chambers. With minor modifications, these
exposure chambers have been used in plant growth chambers to study effects of
pollution on vegetation under rigid control of environmental conditions. Cost
breakdown for these chambers is detailed in the appendix.
GENERAL CONSTRUCTION
A bank of eight chambers (Figures 1 and 2) was constructed with a single
air-handling system. Construction details for the two chamber sizes used
(Figure 3) are identical, and flow characteristics and performance data are
similar. The large chambers are 30 by 36 by 30 inches high. For the sake of
brevity, only the smaller chambers, which are 24 by 24 by 30 inches in size, are
discussed here.
Details of construction are shown in Figures 3 and 4. The chamber frame
is made of 3/4-inch-thick plywood. Plywood ties at all corners join the front
and back. The base is made of 1/2-inch-thick plywood. All joints are nailed
and glued, and wood surfaces are sanded and finished with several coats of
white gloss enamel. A false floor of 1/4-inch pegboard that has been painted
on both sides is placed 6 inches from the bottom of the chamber.
The frame is covered with J-mil Mylar* film attached with heavy cloth
tape. The Mylar is wrapped around the sides and back, and a separate piece
placed on the top. The plywood-frame door is also covered with Mylar. Four
wood strips are used to position the door on the front frames, and a clamp is
centered on each strip. The door gasket is fashioned from 3/8-inch-OD Tygon
plastic tubing with heat-fused ends. This gasket has good resiliency and has
lasted for several years.
*Mention of product or company name does not constitute endorsement by the
Public Health Service or the Department of Health, Education, and Welfare.
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ALUMINUM
INLET
PIPE
HIGH-PRESSURE
BLOWER
S.P., 335 Off
IrlLET HEADER
(21 18 by 18 by 1-
CHARCOAL FILTER
\\AIR PLENUM
18 by 18 by 42
Figure 1. Schematic showing general orientation and construction of exposure chambers
in greenhouse. (All dimensions shown are in inches.)
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Figure 2. Exposure chamber set up in greenhouse.
The basic chamber is fitted for specific attachments. A 1/2-inch opening
is made in the upper front right-hand corner for the air inlet; a 3/4-inch opening
on the right side is available for sampling probes; a 1/4-inch opening at the
lower right rear provides for temperature-sensing elements; a 7/8-inch opening
in the rear on the lower right-hand side accommodates the air outlet; and just
below the latter, a 1/4-inch opening is provided for filling the wet-bulb pan.
AIR-HANDLING SYSTEM
The air-handling system for chambers used in greenhouses is detailed in
Figure 1. A high-pressure blower on the exhaust side of the chambers maintains
4-inch negative static pressure in the exhaust header. All headers are galvanized
downspout material, and all joints are soldered and taped to reduce air leakage.
The system has a 3/4-inch-diameter exhaust duct with a gate valve and a calibra-
ted orifice plate, which has pressure taps to control and measure airflow through
the chamber. This design maintains a negative pressure of 0. 1 to 0.2 inch of
water in the exposure chambers at an airflow of 5 cfm (one change every 2 min-
ute s).
Air enters the chamber through a 1-1/4-inch-ID aluminum duct from an
inlet leader at a linear rate of approximately 600 feet per minute. This high-
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8-J/4
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Figure 3. Details of exposure chamber design. (All dimensions shown are in inches.)
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Figure A. Greenhouse exposure chamber
detaiIs.
speed linear flow produces violent turbulence in the chamber and causes essen-
tially "instantaneous" mixing. The air passes into the bottom of the chamber
through the pegboard separator. When all the chambers are running, a single
chamber can be opened without upsetting the airflow in the remaining chambers
by closing a ball valve in the individual chamber inlet duct. Air passes into the
inlet header through a cleaning plenum equipped with an initial dust filter and a
charcoal filter. After the air is filtered, it is ozonated to remove reactive
substances; and then it passes through a charcoal filter to remove the added
ozone.
The air-handling system for controlled exposure has a 2-inch exhaust duct
with a damper and a calibrated orifice plate, which has pressure taps to control
and measure airflow through the chamber. The exposure chamber design and
air movement are the same as those for the greenhouse system (Figures 5 and 6).
The inlet system is modified so that each chamber has an individual inlet. The
inlet duct connects to a special plenum, which is designed for close control of
temperature and humidity (Figures 7 and 8). Air enters the plenum through a
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FRONT SECTION VIEW
SIDE SECTION VIEW
Figure 5. Schematic showing two exposure chambers with conditioning plenums inside a plant growth
chamber, front and side views. (All dimensions shown are in inches.)
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Figure 6. Exposure chamber inserts in
plant growth chamber.
charcoal filter (to which an ozonating device may be attached), then passes over
a set of heaters, through a steam humidifying unit, over the temperature-sensing
device that controls the heaters, and into the inlet duct. The control plenum and
inlet duct are well insulated with one-inch fiberglas insulating material.
TOXICANT ADDITIONS
Toxicants are added through ports in the inlet duct from various toxicant
dispensing systems. The dispensing systems have an initial dilution system, so
that toxicants enter the inlet duct in concentrations of about 100 to 1. A fluted
piece of aluminum foil within the duct above the point of toxicant addition gives
the air-toxicant mix a circular motion before it enters the exposure chamber.
Present inlet duct construction limits the number of toxicants that can be added,
but a design change will allow up to 10 different toxicants to be added and mixed
before entering the chamber.
ENVIRONMENTAL CONTROL
Greenhouse exposure chambers are normally maintained at greenhouse
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-HEATERS
SENSOR
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Figure 7. Details of conditioning plenum for exposure chambers
located within plant growth chambers. (All dimensions
shown are in inches.)
conditions. On a cloudy day these conditions include low light, low temperature,
and high humidity. On a sunny day a chamber receives about 80 percent of the
light intensity of the greenhouse, is 4° to 6°F warmer, and has about the same
humidity as the greenhouse. All of the experiments with ozone and sulfur dioxide
exposure at the National Center for Air Pollution Control have shown that supple-
mental light is essential to obtain statistically significant results when parameters
10
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Figure 8. Plenum attachment to exposure chamber
inserts in plant growth chamber.
other than light are considered. An eight-lamp bank of 8-foot-long, 235-degree
reflector, VHO lamps mounted on a 2-inch aluminum angle frame with no top is
used. Ballasts are mounted on the frame. This bank covers three chambers and
gives 1800 to 2200 foot-candles of light in the center of each chamber (Figures 9
and 10). Airflow can be varied from 2 to 10 cfm through the small chambers and
from 5 to 20 cfm through the large chambers.
The controlled-exposure chambers are mounted as inserts inside standard
plant growth chambers. Inside dimensions of growth chambers currently in use
are 30 by 30 by 66 inches. Each chamber permits the use of two exposure cham-
ber inserts. Lights can be controlled to give intensities up to about 6, 000 foot-
candles. Both incandescent and fluorescent lighting can be used; however, the
incandescent generally has not been used because of the higher heat load. Light
intensity in the two inserts in a given growth chamber can be varied by shading
one chamber with cheese cloth or another material. Temperature, humidity,
and toxicant are controlled independently in the two exposure chamber inserts.
Temperature and humidity are initially controlled in the growth chamber, where
temperatures are maintained at levels at least 10°F below the lowest insert
temperature desired and humidity is kept as low as possible. Air is conditioned
in each insert plenum for close control of temperature and humidity within the
exposure chambers. Airflow can be varied from 3 to 40 cfm through each insert.
Wet- and dry-bulb thermistors are situated in front of the exhaust duct in
each chamber to monitor temperature and humidity.
11
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ALUMINUM
ANGLE FRAME
2 by 2 by 3/1
> SASH CHAIN
! HANGER
SECTION "A-A"
"A"
BALLAST
VHO-235-1 LAMP
VARIABLE DISTANCE J "A11— 1
t
EXPOSURE
CHAMBER
10
EXPOSURE
CHAMBER
- 10
EXPOSURE
CHAMBER
LAMP
'HOLDER
INLET HEADER ,
EXHAUST HEADER-
T-TO
CHAMBER
-FROM FILTER
-TO EXHAUST FAN
N— WORK PLATFORM
' 5 IN FRONT OF CHAMBERS
Figure 9. Details of supplemental lighting system for greenhouse exposure.
(All dimensions shown are in inches.)
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Figure 10. Details of lamp bank over greenhouse
exposure chambers.
13
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PERFORMANCE DATA
The value of any instrument is measured by its performance under actual
operating conditions. The principal concern with the exposure chambers dis-
cussed here is maintaining uniform toxicant and environmental conditions through-
out the chamber, and the ability to maintain consistent values over a given period
of time. The former is an indirect indication of chamber flow characteristics
and the latter is a measure of the stability of the toxicant dispensing and en-
vironmental control systems.
To determine the operating performance of the exposure chambers dis-
cussed, ozone was used as a test toxicant. The chambers were checked empty,
with a plant load, and at several ozone concentrations. Ozone was chosen for
several reasons, but the prime consideration was its reactive nature. If ozone
mixing is at an acceptable level, other toxicants should show an equal or better
distribution pattern.
Preliminary measurement of toxicant uniformity was made shortly after
the small chamber system was completed. Variations within the chamber were
less than ±4 percent both empty and with a plant load. Chamber concentrations
were compared with inlet levels and expressed as percentage values. Empty
chamber values were close to inlet values; loaded chamber values ranged from
as low as 50 percent to as high as nearly 100 percent of inlet values. The per-
centage values of loaded chambers seemed to be related to plant sensitivity. On
days of high sensitivity, a probe held against the lower leaf surface of tobacco
or pinto bean plants within the chamber showed a value of as low as 20 to 30 per-
cent of the inlet value. Much of the ozone reduction is possibly due to absorption
into leaf tissues. This is currently being investigated.
Environmental conditions similar to those found in greenhouses exist within
greenhouse exposure chambers. Under bright sunlight, the chamber temperatures
run from 4° to 6°F above greenhouse temperatures at an airflow rate of one
change every 2 minutes. Measurements at several points within the chamber
have been markedly consistent.
15
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Environmental control is particularly important within exposure chamber
inserts to plant growth chambers. A preliminary design similar to the one here-
in discussed, was set up to determine what was necessary for close temperature
and humidity control. At an airflow rate of 20 cfm with two changes per minute,
there was less than ±0. 5°F temperature fluctuation at a given level in the cham-
ber. Difference in temperature between top and bottom of chambers was approxi-
mately 2°F at a chamber temperature of 70°F. This was a consistent variation.
At higher temperatures the difference was less. At any given location no tempera-
ture or humidity variations could be picked up by using a wet-bulb, dry-bulb
thermistor sensing device.
A more detailed measurement of ozone uniformity in both the large and
small greenhouse exposure chambers was recently conducted (Table 1). Four
Table 1. UNIFORMITY OF OZONE DISTRIBUTION IN EXPOSURE CHAMBERS3
Chamber and conditions
Large chamber, empty, no per-
forated delivery tube
Large chamber, empty, with
perforated delivery tube
Large chamber, light plant load,
with perforated delivery tube
Small chamber, empty
Small chamber, light plant load
No. of
runs
9
4
5
3
2
Chamber positions"
A
100
97
100
98
100
B
97
99
97
100
98
C
92
95
96
96
98
D
93
100
96
97
100
E
93
97
89
97
100
aThe values were obtained as comparative values for a given run, and then all values
in a run were compared, the highest value being given the arbitrary value of 100.
This was done because of variations between runs.
Positions A-D were located 2 inches from the corners at the vertical center!ine of
the chamber. Position A at the inlet corner and the other positions occurring
counter clockwise around the chamber. Position E was centrally located at the
same height as the other positions.
probes were centered vertically in each chamber within 2 inches of the four
corners (A-D); and a fifth probe was placed in the center of each chamber (E).
Results for each probe location were originally calculated on the basis of the per-
centage of the inlet concentration. Results of ozone uniformity for the different
runs varied so much, because of environmental conditions and chamber loading,
that all values were corrected so that the highest value would read 100 (Table 1).
16
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The large chambers were checked empty both with and without a perforated
delivery tube installed. (The inlet duct was connected to a 1-1/4-inch Plexiglas
tube extending across the inside of the chamber. This tube had eight 1/4-inch
holes for more uniform dispersion of air across the chamber. ) Results show
more uniform dispersion with the tube added. The ozone uniformity level at the
center of the chamber was definitely below that at the four chamber corners when
the chamber was tested under plant load conditions. This is probably a plant
response. The perforated tube was not used in the small chambers, and no cham-
ber center effect was noted. Results show excellent uniformity of mixing within
chambers of both sizes.
Determination of the rate of chamber equilibration after starting or stopping
toxicant flow into the chambers is also of interest. The rates of equilibration for
ozone were determined for all five probe positions for both chamber sizes. Aver-
age values for the five probe positions are shown for the large chambers in Figure
11. Results for the small chambers were similar. The equilibration rate after
starting the dispensing system and the decay rate after stopping the dispensing
system are the same. Thus, the length of the exposure can be determined by
knowing the time of starting and stopping the dispensing system. The time period
for equilibration is so short that for runs of one or more hours, the lower con-
centrations during equilibration and decay should not have a. major effect on plant
response. The lower concentration during the equilibration and decay time
periods could have an effect on plant response for exposure periods under one
hour. The shorter the exposure period the greater effect these lag periods would
have.
SUMMARY
The construction details of a simple, flexible plant exposure chamber have
been described. The chamber utilizes a dynamic, negative-pressure, single-
pass flow system, which provides uniformity of flow, toxicant mixing, and cham-
ber distribution. Environmental control of exposure inserts into plant growth
chambers can be maintained with no apparent light, temperature, or humidity
fluctuations. Chamber uniformity of ozone concentration is excellent, and
uniformity of less labile toxicants should be greater.
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EQUILIBRATION OZONE ADDITION
EQUILIBRATION OZONE REMOVAL _
234567
TIME TO EQUILIBRIUM, minutes
10 11 12
Figure 11. Rate of equilibration and removal of ozone for large
chambers (average of 5 probe positions).
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ACKNOWLEDGEMENTS
We would like to thank Mr. A. R. Schwarberg of the Health Effects Research
Program, National Center for Air Pollution Control, for the schematic drawings
in Figures 1, 3, 5, 7 and 9; and, Dr. O. C. Taylor of the Air Pollution Research
Center, University of California, Riverside, California, for suggested changes
in design as the result of use of this system at Riverside.
REFERENCES
1. Heck, W. W. , E. G. Pires, and W. C. Hall. The effect of a low ethylene
concentration on the growth of cotton. JAPCA. 11:549-556. 1961.
2. Heck, W. W. and E. G. Pires. Growth of plants fumigated with saturated
and unsaturated hydrocarbon gases and their derivatives. Texas Agric.
Exptl. Station. MP-603. 1962.
3. Heck, W. W. , L. S. Bird, M. E. Bloodworth, W. J. Clark, D. R. Darling,
and M. B. Porter. Environmental pollution by missile propellents. MRL-
TDR-62-38. Office of Technical Services. Dept. of Commerce. 1962.
19
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APPENDIX
CONSTRUCTION COSTS FOR EXPOSURE FACILITIES
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APPENDIX
CONSTRUCTION COSTS FOR EXPOSURE FACILITIES
1. Construction of an eight-exposure chamber setup for use in the greenhouse.
(Figure 1)
1 gallon of white enamel paint $ 8. 00
2 charcoal panel filters, 18 by 18 by 1-inch @ $7. 00 14. 00
4 sheets of 3/4-inch plywood, @ $8. 00 (4- by 8-foot sheet) 32. 00
1 sheet of 1/4-inch pegboard (4- by 8-foot sheet) 5. 00
100 feet of 3/8-inch-OD Tygon* tubing for gasketing and
connections 10.00
32 toggle clamps. No. 205-U DeStaCo, @$1.80 58.00
1 Dayton high-pressure blower, 530 cfrn, 1-inch static 54.00
60 feet of 5-inch round downspout, $3. 12/10 feet 20. 00
8 calibrated orifice plates adapted to 3/4-inch copper
tubing, estimate $30.00 each 240.00
24 feet of 1-1/2-inch-OD aluminum tubing 14. 00
20 feet of 3/4-inch-OD copper tubing 4. 00
1 roll of 1-mil clear Mylar film 10. 00
1 roll Mystic tape 5. 00
8 ball valves, 1 1/4-inch @ $9.00 72.00
8 gate valves, 3/4-inch No. 607 Hammond, @$3.11 25.00
1 inclined manometer, 3-inch Meriam Inst. No. 40GE4 50. 00
8 wet-bulb containers, @ $5. 00 40.00
Labor for construction, covering, setup, and installation 300. 00
i
$961.00
2. Construction of a single exposure chamber without connections. (Figure 3)
Paint $ 1-0°
Plywood 4- °°
Pegboard •60
Tygon tubing • 40
Toggle clamps 7- 40
*Mention of a trade name or company is for identification only and does not imply
endorsement by the USDHEW.
23
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Mylar film $ 1. 00
Mystic tape . 60
Wet-bulb container 5. 00
Construction and covering 25. 00
45.00
3. Construction of a single 8-foot supplemental lighting system for greenhouse
exposure chambers. (Figure 9)
8 sockets, No. 492 and 493 Levitron, @ $2.40 $ 19.20
25 feet of 2- by 2- by 3/16-inch aluminum angle 25. 00
8 fluorescent tubes, FR96T12/CWVHO-235-1, g $5. 75 46.00
4 ballasts, GE No. 7G1201, @$27.75 111.00
Labor: construction, wiring, etc. 25. 00
$226.20
4. Construction of a single exposure chamber insert for use in a plant growth
chamber. (Figure 5)
Chamber construction (#2) $ 45. 00
2-inch round downspout 1. 00
Conditioning plenum (#5) 87.00
Calibrated orifice plates adapted to 2-inch copper tubing 30. 00
Insulation 4. 00
Installation 50.00
$217. 00
5. Construction of a single conditioning plenum. (Figure 7)
Sheet metal $ 5. 00
4-1/2-inch diameter charcoal canister filter 14.00
2 heaters, strip, @ $5.65 11.00
Thermostat for heaters, Chromalox AR-2534 22.00
Steam connection with needle valve 10. 00
Construction 25.00
$ 87.00
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