Martin Marietta's Space Simulation Laboratory (SSL) has a Thermal Environment Simulator (TES) with 56 individually controlled heater zones. The TES has a temperature range of approximately minus 129 C to plus 149 C. Because of the ability of TES to provide complex irradiance distributions, it is necessary to be able to measure a wide range of irradiance levels. SSL currently uses ambient temperature controlled radiometers with the capacity to measure sink irradiance levels of approximately 42.6 mw/sq cm, sink temperature equals 21 C and up. These radiometers could not be used to accurately measure the lower irradiance levels of the TES. Therefore, it was necessary to obtain a radiometer or develop techniques which could be used to measure lower irradiance levels

Oleary, Scott T.

English

NASA Technical Reports Server

N89-12619
COST EFFECTIVE ALTERNATIVE TO LOW IRRADIANCE MEASUREMENTS
Scott T. O'Leary
Martin Marietta Astronautics Group
Space Systems Company
ABSTRACT:
Martin Marietta's Space Simulation Laboratory (SSL) has a Thermal Environment
Simulator (TES) with 56 individually controlled heater zones. The TES has a
temperature range of approximately -129°C to +149°C (-200°F to +300°F). Because
of the ability of the TES to provide complex irradiance distributions, it is
necessary to be able to measure a wide range of irradiance levels. SSL currently
uses ambient temperature controlled radiometers with the capability to measure
sink irradiance levels of approximately 42.6 mw/cm2 (135 Btu's/ft2hr), sink
temperature =21°C (70°F), and up. These radiometers could not be used to
accurately measure the lower irradiance levels of the TES, therefore it was
necessary to obtain a radiometer or develop techniques which could be used to
measure lower irradiance levels.
INTRODUCTION:
There are a number of low irradiance measurement devices available, but they are
complicated and very expensive. An alternative was to use the existing radiometers
by cooling them to liquid nitrogen (LN 2) temperatures and establishing a new
output (mv) vs. irradiance mw/cm2 (Btu/ft2hr) curve. This was done by developing
a black body source which could be used in the calibration process and a method
for modeling the characteristics of the radiometer at LN 2 temperatures. A method
for obtaining an output (mv) vs. irradiance (Btu/ft2hr) calibration curve was
established as well as a model of the radiometerN response to temperature
fluctuations. The following information was obtained for each radiometer:
i.) An output vs. irradiance curve was generated with a maximum error
of ±l.26mw/cm 2 (±4 Btu/ft2hr) at the upper end of the irradiance
curve(69.4mw/cm 2 (220 Btu/ft2hr))
2.) A model of how each radiometer responds to temperature fluctuations
at three fixed sink temperatures; -129°C, -20°C, 60°C (-200°F,
-4°F, and 140°F).
The results demonstrate the ability to measure low irradiance levels with an
acceptable error, and at a fraction of the cost of other low irradiance
measurement devices.
TEST PLAN:
A test was developed to ascertain the ability of the radiometers to measure low
irradiance levels.
The test had four primary objectives:
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https://ntrs.nasa.gov/search.jsp?R=19890003248 2020-03-20T04:38:20+00:00Z
i.) Obtain the irradiance vs. output data for fourteen P-8400 and two
P-8410 radiometers, at liquid nitrogen temperatures. The black body
cells were cycled between -129°C and 60°C (-200°F and 140°F).
Temperatures of the black body source, the radiometer, and the
radiometer's mv signal data was collected to establish the new
calibration curve for the liquid nitrogen cooled radiometers.
2. Obtain data on the response of each radiometer to temperature
fluctuations. The radiometer temperature was varied between -193°C
and -162°C (145°R and 200°R) at three fixed black body cell
temperatures, -129°C, -20°C and 60°C (-200°F, -4°F and 140°F). Due
to the difficulty of maintaining the radiometer at a constant
temperature during an actual test, this information could be used
to adjust the data if necessary.
. Compare the black body calibrated radiometer to the vendor
calibrated ambient temperature radiometer data to evaluate the
quality of the black body source. It was necessary to control the
black body calibrated radiometer at ambient temperature while
cycling the black body source. The radiometer was maintained at
26.7°C i 3°C (80°F i 5°F) with GN2, while the sink temperature was
cycled between 24°C (75°F) and 93.5°C (200°F). The data obtained
from this test compared favorably to the vendor's calibration data.
. Three cycles were run to demonstrate repeatability and to establish
confidence in the test methods used to obtain the irradiance data.
The data from these cycles compared favorably.
BLACK BODY SOURCE FIXTURE DESCRIPTION:
An ideal black body has a very simple relationship between temperature and
radiated energy, l=e(Sig)T 4. The value of the emissivity (e) is i, Sig is
Stefan-Boltzmann's constant 0.57x10-8W/m2.K(l.714x10 -7 (btu/ft2hr)/°R4), and
T is the temperature of the black body source.
The black body source was built using a cylindrical body with a large length to
diameter ratio. This inherently gives a high value for the emissivity. Individual
cells were constructed of 0.635 cm (1/4") wall aluminum tube, 4.76 cm (I 7/8")
inner diameter by 30.48 cm (12") long, (see figure #i for sketch of black body
cell). This gave the black body source a length to diameter ratio of approximately
6.4. The thickness of the tube allowed the heat being applied to the external
surface of the cell to be distributed evenly on the internal radiating surface. The
inside of the cells were painted with Cat-A-Lack-Black which has a published
emissivity of 0.92.
This geometry inherently contibutes to a good temperature uniformity on the
internal radiating surface. The walls in the cavity radiate energy to each other
and therfore have a tendency to wash out any local temperature gradients on the
inside of the cell.
467
The black body cells were mounted on a temperature controlled base plate. The base
plate was used to add or remove heat from the bottom portion of the cell to reduce
the temperature gradient from top to bottom of the cell. The resulting black body
fixture consisted of 16 individually controlled black body cells and a temperature
controlled base plate. The external fixture was painted black for effective heat
radiation. Cells were individually controlled, using a Variac power supply,
manually controlled. This was done to eliminate the use of contact relays which
could introduce noise in the radiometer output signal.
Heaters were spiraled up the external portion of the cell with the heater tape laid
edge to edge in such a way as to prevent overlapping (hot spots) or gapping
(cold spots) of the heating element.
To minimize the error in the reading due to the gradient, the temperature sensors
were imbedded in the body of the cell as close as possible to the internal
radiating surface.
RADIOMETER INSTALLATION AND INSTRUMENTATION CALIBRATION:
A number of actions were taken to ensure proper radiometer installation:
i.) The radiometers had a view of 180 ° . Therefore, care was taken to
ensure the receiver surface was perpendicular to the cell so that it
would view only the cell. Since the receiver surface was placed
0.635 cm (1/4") into the opening of the cell, care was taken to
ensure that the radiometer did not come in contact with the sides.
See figure #2.
2.) An indium sheet was used as an interface between the radiometer and
the tubing to provide good conduction between the radiometer and
the liquid nitrogen cooled line.
3.) Each radiometer and all liquid nitrogen lines were wrapped with
multi-layer insulation (MLI) to limit losses.
4.) The thermocouples used to monitor the black body fixture and
radiometer temperatures were calibrated end-to-end to reduce
temperature measurement error. The calibration showed that the
thermocouples were within ±0.2°C for the majority of the temperature
ranges in which the black body cells were cycled.
5.) A thermocouple was imbedded approximately 0.635 cm (1/4") into the
surface of the radiometer, and the wire wrapped twice around the
body of the radiometer and down the LN 2 plumbing to reduce the
thermal gradients which might exist along the wire.
RESULTS:
Results are best explained graphically:
468
PLOT #i: Calibration Curve: This curve plots the radiometer's output against
a calculated irradiance level. The irradiance was calculated using the equation
l=e(Sig)T 4. The data was compiled and a Least Squares Fit calculated to obtain the
best fit line. One can see from the high correlation coefficient and the low value
for three standard deviations, that the data is linear and has a small degree of
error.
PLOT #2: Calibration Curve With Error Boundaries: This curve plots the calibration
curve as well as three standard deviations of the data and the largest additional
error due to the recorders. The curves indicate that the maximum error expected is
approximately ±1.26 mw/cm 2 (±4 Btu/ft2hr). This is a fairly small error for the
higher irradiance ranges, but leads to a large percentage error for the lower
irradiance ranges. The error in the lower ranges is exaggerated because the three
standard deviation calculations take into account the entire temperature range
tested.
PLOT #3, #4 and #5: Mill±volt Signal Fluctuations at Fixed Sink Temperatures
-129°C, -20°C and 60°C (262°R, 455°R and 600°R): These curves plot the radiomater's
output against its temperature, in degrees Rankine, for a fixed sink. The sink
temperature was held constant while the radiometers were cycled between -195°C and
-162°C (140°R and 200°R). For a given sink temperature, the different runs
compared favorably.
PLOT #6: Comparison Curves: This curve compares the black body calibrated
radiometer against the vendor's calibration data. The lower curve represents the
calibration curve, supplied by the vendor, for an ambient temperature controlled
radiometer. The higher curve represents data obtained using the black body source
and a radiometer maintained at ambient temperature. These curves compare favorably.
If the black body source had a poor emissivity or poorly represented an ideal
black body source, the line woukd have fallen below the vendor's calibration curve.
Recently, a comparison test was run using a Kendall Cavity Radiometer (considered
a standard), and two of the P-8400 radiometers. In this comparison test, the same
type of curve shift was noted.
ERROR SOURCES :
There are a number of factors invloved in this test which impact the data.
When cycling the sink temperature, the radiometer temperature varied. At the lower
sink temperatures the radiometers had a tendency to drift cold. This may have
caused the data points in the lower irradiance ranges to be on the higher mv side
of the curve. For this test, the temperature of the radiometer varied approximately
5.5°C (10°R).
The mv output for a given sink temperature while ramping cold is slightly different
than the mv output at the same sink temperature while ramping hot. This is caused
by the radiometer's slow response to sink temperature changes. This introduces a
small error in the results. Also, the inability to sample mv and temperature data
at precisely the same moment introduced a small error into the results.
The mv signal introduced an error of ± 0.03% of the reading + ±0.012% of the range.
This is the published maximum error of the recorder.
469
CONCLUSION:
The test could be refined to reduce the error in the results, but the results of
this test were adequate for the needs of most program requirements. The maximum
error in the irradiance vs. output curves for the radiometers was II.26 mw/cm 2
(±4 Btu's/ft2hr).
An NBS traceable calibration is not available at this time. Martin Marietta plans
to obtain NBS approval using this calibration method.
LESSONS LEARNED:
For future radiometer calibration tests, three changes could be implemented
to improve results:
i.) A temperature sensor could be used which is more accurate in the
lower temperature ranges.
2.) A pressure controlled liquid nitrogen dewar could be used to
precisely control the temperature of the radiometer, thus limiting
temperature fluctuations during test.
3.) Increase data point collection by stabilizing the sink temperature
every 5.6°C (10°F) during temperature transitions.
470
(4.76cm)
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O.D.
2 3/8"
(6.03cm)
(0.102cm)
.o4o" DRILL
TO MOUNT TC IN
(0.102cm)
O.O4O" DRILL
TO MOUNT TC IN
4 2/3"
(11.85cm)
FACE
1' 2"
(35.56cm)
OPPOSITE FACE
FIGURE 1.
4 2/3"
(11.85cm)
!
BLACK BODY CELL
471
OFFSET X2
OFFSET Xl
PERFECTLY
PERPENDICULAR
OFFSET BY 5°
RECEIVER SURFACE
SET AT 3/16" BELOW
TOP SURFACE OF CELL
FOR RADIOMETER WITH NO WINDOW
AND A 180° VIEW ANGLE
OFFSET ANGLES OF MORE THAN 5° COULD DISTORT THE
DATA DUE TO THE RADIOMETER VIEWING THE COLD WALL.
FIGURE 2. RADIOMETER TILT AND ORIENTATION
472
ORIGP_AC PA'C_ IS
OF POOR QUALITYj
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PLOT #1
CAUBRATION CURVE
I I I I I I I I I I I I I
!Y(mv) = 0.00194X - 0.01723!
Correlation Coefficient = 0.9999
3 Standard Deviations = 0.0168
f
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IRRADIANCE, mw/cm 2
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PLOT #2
CAUBRATION CURVE
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IRRADIANCE, mw/cm 2
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145
OF POOR QUALITY
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PLOT #3
COLDCASE
RADIOMETER CYCLE
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'=- 456.5+1.5 °R SINK, CYCLE 3
•n- 454 + 1 °R SINK, CYCLE 4
150 155 160 165 170
PLOT #4
MID CASE
RADIOMETER CYCLE
175 180 185 190
RADIOMETERTEMPERATURE,°R
195 200
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474
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m
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1.38
ORIGINAI_ PAC_ IS
OF POOR QUALITY
1.36
1.34
150 155 160 165 170
PLOT #5
HOT_
RADIOMETERCYCLE
175 180 185 190
RADIOMETERTEMPERATURE,°R
195 200 205 210
3.0
2.5
O 2.0
U
T
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U 1.5
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m
v
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0.5
0.0
10 20
PLOT #6
COMPARISON CURVE
30 40
IRRADIANCE, mw/cm 2
50 60 70
475


Cost effective alternative to low irradiance measurements

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