a single sensor for air temperature, … · temperature and air velocity to evaluate thermal...
TRANSCRIPT
A Single Sensor for Air Temperature, Mean Radiant
Temperature and Air Velocity to Evaluate Thermal
Comfort Conditions
Gianni Pezzotti
Institute of Crystallography
CNR and Biosensor srl
Rome, Italy
Paolo Coppa
Department of Mechanical Engineering
University of Rome “Tor Vergata”
Rome, Italy
Abstract—This paper present the realization of a sensor for
measuring variables governing the thermal comfort. The
device has been studied, designed, realized, calibrated and
tested. The sensor of independent comfort to be used as an help
to the test driver, able to supply during the same test
independent information of three of the four quantities, which
have major influence on the comfort variable: air temperature,
mean radiant temperature and air velocity. Initially two single
elements were studied (Nickel wire and PT100 with an special
configuration), for design of the sensor, that consist in a sensor
of double nature (it is divided in two parts: a blackened and a
bright part), dimensions of a medallion, with a sensible element
is a deposit of conducting metal (gold), insulated from a
normal glass substrate, build in the Microsensors Laboratory
of the CNR, Research Area of “Tor Vergata”.
I.INTRODUCTION
The study in the field of the thermo hygrometric
comfort conditions arose in early 70s, with the theoretical
and empirical study of Fanger [1]. Successively thermal
comfort sensor have been developed, and commercially
distributed (e.g. Bruel & Kier Thermal Comfort Meter, type
1212).
According to Fanger’s treatise, thermal comfort depends on
6 variables, 2 individual (clothes and metabolism) and 4 due
to environmental conditions (air temperature, mean radiant
temperature, air velocity and relative humidity). Among
these last the first three,Ta, Tmr and ua, play a major role, at
least when ambient conditions are not far from comfort
conditions.
Besides, in special cases it is important to evaluate
comfort conditions in ambient where they are quite variable
both in time and space, as inside the car compartment [2], or
where comfort conditions must be accurately controlled, as
in hospitals. So a singe sensor able to measure ambient
comfort quantities and to give an independent evaluation,
could be highly useful [3] .
Clearly single sensors exist to measure the three
variables independently, as thermal sensors (thermocouples,
PTRs, thermal diodes), radiations sensors (thermopiles,
bolometers, quantum sensors, eg. in [4] and [5]) and air
velocity meters (vane meters, hot wires, etc), but it could be
useful to have a single sensor able to supply with a single
test all the desired results.
In this optic if has been realized the preliminary test for
search the principal characteristics of the sensor at
developer. The first consist in a single metal wire (sensor
based on metal wires; ie Fig. 1), heated by an electrical
current and cooled by the incoming air flow, can supply two
quantities: air temperature though wire electric resistance,
when the electric current is so low to avoid self heating, and
air velocity when heating is much higher, and air flow
results in wire cooling (in the same way as in the widely
used hot wire anemometry). Preliminary tests have been
carried out with a nickel wire, 0.1 mm in diameter,
electrically insulated and connected as a four wire resistance
(Kelvin bridge).
The impossibility of recognizing Tmr , from the
difference in temperature behaviours between the bright and
black wire (painted with colloidal graphite aquadag, with
emissivity higher than 0.95), is due the very high convection
coefficient of a highly curved surface (150÷700 W/m2K)
when compared t a flat one (6÷10 W/m2K). I.e. surface
curvature increases very much hc , while radiation heat
transfer coefficient doesn’t change. This is the reason why a
flat surface is better used as a sensor for Tmr. Consequently a
flat temperature sensor is more suited to measure Tmr beside
Ta and ua. A first test has been carried on with a commercial
flat PT100 (platinum resistance thermometer with flat
windings laying on a ceramic base, Fig. 1, 7mm x 3 mm
1-4244-2581-5/08/$20.00 ©2008 IEEE 375 IEEE SENSORS 2008 Conference
wide). Two identical sensor were mounted on a base, one as
supplied and another coated and blackened in order to
increase emissivity. Even if the emissivity of the bare sensor
was unknown, a difference in temperature behaviour was
detected, showing a higher influence of radiation on the
total heat transfer coefficient.
Figure 1. Preliminary test :Left - Nickel wire; Right - PT100
Thus in the present work, a single sensor able to
determine air temperature, mean radiant temperature and air
velocity has been studied, designed, built, calibrated and
tested [6]; all the desired quantities are measured through
the evaluation of the heat transfer coefficient h .
II. GOLD DEPOSITED SENSOR
The principal characteristics of the realized new flat
sensor are: the double nature (it is divided in two parts: a
blackened and a bright part), and the dimensions of a
medallion in order to be hung to the suit of a test driver. The
total sensible area was chosen 250mm2, each part 25 mm x
50 mm; It has been decided to realize the sensors depositing
a conducting metal (gold) on an isolating glass substrate.
Figure 2. Double sensor with gold deposition
The Electric resistances must be enough low (5-10 Ω) to
allow good voltage drop measurements with usual data
acquisition systems, but enough high to maximize signal to
noise ratio.
Two metal layers were deposited in vacuum, the first,
chromium, 300 Ǻ thick, for grabbing, and the second, gold,
2500 Ǻ thick, as main conductor. Standard lift off procedure
for gold deposition has been adopted, the sensor is
represented in the Fig. 2
In the Fig. 3 shows the final look of the double sensor,
after gluing the glass slab on an thermal insulation
(polystyrene foam, 20 mm thick), and blackening of one of
the two circuits, and completing connections.
Figure 3. Flat gold sensor deposited on glass substrate (left ), Final look of
the double sensor (right)
III.SENSITIVITY TO AIR TEMPERATURE
The calibration process consists in changing the sensor
temperature (when it is completely immersed in an ambient
at constant temperature as a thermostat) and measuring the
electric resistance. Calibration was performed according to
ITS90 [7], the set up developed is a data acquisition system
(Multimeter Keithley 2700) records the 4 wire resistance
data of both the sensor elements (bright and blackened) and
the standard PT470, while the thermostat maintained the
temperature constant (within 0.1 °C), in the range –
15°C÷80°C. Temperature and resistance data are plotted in
Fig. 4.
Figure 4. Calibration behaviours for the two elements of the sensor a.
Blackened b. Bright
Lest square regression gives the best estimate of the
parameters of the equation R=f(T) , reported in Fig.. Table 1
reports the sensitivity (∂T/∂R) of the two elements and the
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calibration uncertainty, as obtained by the least square
algorithm, both in Ω and in °C
TABLE I.
TYPE R0.01°C (Ω) ∂T/∂R (°C/Ω) sy/x (Ω) sx/y (°C)
Blackened 7.531 0.0235 8.1 10-3 0.43
Bright 7.467 0.0238 1.0 10-2 0.35
Figure 5. a. Results of flat sensor calibration
IV.SENSITIVITY TO AIR VELOCITY
Air velocity is changed by means of a fan (DC
Brushless fan Model FBM-06A12HF DC12V/0.2A) fed by
a DC power supply. Air velocity is measured by a vane
anemometer, LT Lutron model Vane Probe AM-4201. In
Fig. 5 is reported the experimental set up.
Figure 6. Calibration set-up: sensor for measurement of the air velocities
Figure 7. Characteristic curves of the air velocity vs coefficient convection
of the gold sensor .
The characteristic curves of convection coefficient h , as
obtained by the above reported least square regression,
versus air velocity are reported in Fig. 5 for the two
elements. It is recognizable an h increasing when air
velocities are higher than 1.0 m/s. With lower air velocity
free convection is considered predominant. As it can be
expected, sensitivity is about the same for the two elements,
the bright and the blackened one.
V. SENSITIVITY TO MEAN RADIANT TEMPERATURE
Mean radiant temperature Tmr of an ambient can be changed
when any wall temperature changes. Tmr is either measured
with a suited sensor (thermopile) or calculated from the
formula:
44
1i
n
iismr TFT ∑
=
−= (1)
Being Ti the temperature of the i-th wall, and Fs-i the
radiation configuration factors between the sensor s and the
i-th wall. In order to get a meaningful variation of Tmr , a hot
circular surface has been locate in front of the sensor, and
the two mean radiant temperatures (hot surface and other
ambient walls) are measured by a thermopile aiming the hot
surface and other walls; the experimental setup is
represented in the Fig. 8. The thermopile (Melexis
MLX90601B) had been previously calibrated with a black
body whose temperature was measured and controlled by a
calibrated type J thermocouple.
Figure 8. Calibration set-up: sensor for measurement of the air velocities
The radiation configuration factors between the sensor
and the hot surface has been calculated by the formulas of
[10], p. 120-124. Results of tests are shown in Fig. 9. A
meaningful difference between the two elements is clearly
recognizable: the blackened element shows higher values of
h and an higher slope of the behaviour versus Tmr.
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Figure 9. The total heat transfer coefficient versus the mean radiant
temperature of the flat sensor.
VI.UNCERTAINTY ANALYSIS
The least square algorithm gives an uncertainty
estimation of h. In as much it is computed from repeated
measurements, this uncertainty can be considered as type A,
according to the [8] and GUM [9]. Anyway this values
results generally quite low (about 0.2÷0.3W/m2K).
For air velocity, the accuracy indicated by the vane
anemometer spread sheet is about 0.2 m/s (type B
uncertainty according to GUM), and an analysis of fig. 12
indicates about the same value.
For mean radiant temperature, calibration accuracy of the
black body thermocouple is quite high, about 0.03 °C. But
mean radiant temperatures have been estimated from
literature equations, and obtained uncertainty is strongly
dependent on how much theoretical hypothesises are
satisfied, i.e. if the configuration factors are just the ones
computed from [10] and if temperature of all the walls
surrounding the sensor, beside the lamp, are the same. Again
in this case a more realistic datum is the one due to the
experimenter experience (type B uncertainty), and can be
evaluated as about 0.5 °C.
VII.CONCLUSIONS
The gold coated flat sensor has shown the ability of
measuring all the three desired comfort variables: air
temperature, air velocity and mean radiant temperature. The
first is measured through the initial value of the resistance,
when no heating of the metal deposition occurs. The second
and the third quantities through the convective heat transfer
coefficient h. From the difference between the temperature
behaviour of the blackened and the bright part of the sensor
the mean radiant temperature can be deduced. The tests are
easy to be conducted, only a stabilized power supply is
needed. Test duration is about 25 s, and temperature increase
lower than 30 °C (but there is no necessity for the sensor to
be touched). An improvement of the quality of the sensor,
and also of its endurance, is the protection of the gold
deposit with a thin layer (0.1 µm) of silicon oxide (vacuum
deposited), or magnesium oxide, for avoiding metal
contamination, abrasion, and facilitating cleaning.
ACKNOWLEDGMENT
Authors are in debt with Fiat Research Centre (Dr.
Carloandrea Malvicino), for providing fellowship for one of
them ( Dr. G. Pezzotti), and for the help in deciding the
whole research address. They are also grateful to Mr.
Petrocco and the entire technical staff of the National
Research Council, Electronic Laboratory of “Tor Vergata”
Research Area, for vacuum deposition of gold and
related operations.
The instrument presented have been developed in Heat
Transfer and Thermal Laboratory of the faculty of
Engineering of University of Rome; the good results are
demonstrated with the two patents[3][6].
REFERENCES
[1] P.O. Fanger, Thermal Comfort, Mc Graw Hill,(New York), 1973.
[2] P.Godts et al. “A New self calibrating Radiation Planar Microsensor.Application to Contactless Temperature Measurement in a Car”, Proc.IEEE Instrumentation and Measurement Conference, Brussel, 1996,p.778-781.
[3] Gianni Pezzotti “Multivariables Integrated Instrument for Evaluationof the Comfort Conditions”, right reserved of the author, Italian level,patent pending N° RM2008A000142, 14
th March 2008, Rome Italy.
[4] S. Mola et al. “Windshield Fogging Prevention by Means of MeanRadiant Temperature Sensor”, in Advanced Microsystems forAutomotive Applications 2004, ed. J. Valldorf and W Gessner,Springer (New York), 2004
[5] R. Tmusic et al. “Dynamic and Static Characteristics Investigation ofIntegrated Radiation Thermopile Sensor”, Proc. of the 21
th Intern.
Conf on Microelectronics (MIEL’97), Yugoslavia, 14-17 Sept. 1997,pp. 565-568.
[6] G. Pezzotti, P. Coppa, “Multisensoristic Instrument by Measurementof the thermo hygrometric Variables in the Determination of theComfort Conditions”, patent pending N° RM2006A000724 29
th
December 2006, Italy.
[7] H. Preston Thomas, “ITS’90”, Metrologia, 27, pp. 3-10, 1990.
[8] S. Brandt, Statistical and computational methods in data analysis,North-Holland pub. (Amsterdam), 1976.
[9] ISO, Guide to the Expression of Uncertainty in Measurement, (1995).
[10] J. R. Howell, A Catalogue of Radiations Configuration Factors;McGraw-Hill, New York, 1982
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