Thermal measuring and testing – Temperature measurement – In spaced noncontact relationship to specimen
Reexamination Certificate
2001-08-22
2003-10-28
Fulton, Christopher W. (Department: 2859)
Thermal measuring and testing
Temperature measurement
In spaced noncontact relationship to specimen
C374S130000
Reexamination Certificate
active
06637931
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a probe assembly for use in an infrared ear thermometer, and particularly to a probe assembly which fast adapts to a dynamic enviroment where temperature may vary greatly.
2. Description of Related Art
Infrared ear thermometers have been extensively used to measure human body temperatures. When using the infrared ear thermometer, the probe is positioned into the ear canal; the thermal radiation emitted by the eardrum and nearby tissues is directed to an infrared sensor through a waveguide; a contact temperature sensor tracks the temperature of the infrared sensor; an electronic circuit processes the signals of the infrared detector and the contact temperature sensor according to Stefan Boltzman's law; and then a calculated human body temperature is displayed on a liquid crystal display.
Nowadays, most infrared ear thermometers employ an infrared detector, which has a thermopile chip and a thermistor packed in a same metal can and mounted on the same base plate. The purpose of this kind of arrangement is to keep the thermistor tracking the cold junction temperature of the thermopile chip. Inasmuch as the thermal time constat of the thermistor is different from that of the thermopile chip, it is easy to introduce measurement erros if the thermometer is in a dynamic environment where temperature may vary greatly along with time.
U.S. Pat. No. 4,895,164 discloses a probe assembly with a waveguide and an infrared detector being held in an isothermal state at ambient temperature by a heat conducting block. The drawback of this approch is that a substantially large heat conducting block is required. In addition, an isothermal condition is not necessary in the design of a probe assembly. Since the surfaces of most waveguides are coated with a layer of gold, which has very high reflectivity and very low emissivity, even though a temperature difference exists between the infrared detector and the waveguide, the infrared detector assembly receives little thermal radiation emitted from the inner wall of the waveguide.
U.S. Pat. No. 6,152,595 discloses a probe assembly comprising a radiation sensor (thermopile), a waveguide and a thermal coupling arrangement. As we can understand according to Stefan-Boltzman's law, a probe assembly is impossible to be made without a contact temperature sensor (thermistor) detecting the cold juction temperature of the thermopile chip. Without considering the difference of the thermal time constants between the thermopile sensor and the thermistor, measurement erros will inevitably occur when the thermometer is in a dynamic environment. Furthermore, the thermal coupling arrangement comprising fives parts will complicate the assembly process.
FIG. 1
shows prior art probe
10
of an infrared thermometer, which comprises waveguide
11
, heat conducting block
12
and infrared (IR) detector assembly
13
.
FIG. 2
shows the schematic diagram of IR detector assembly
13
, which comprises metal can
21
, IR filter
22
, thermopile chip
23
, thermistor
24
, base plate
25
and leads
26
. The thermal radiation directed by and through waveguide
11
first passes through IR filter
22
and is then detected by thermopile chip
23
, and the temperature of the cold junction of thermopile chip
23
is represented by the temperature of thermistor
24
, wherein thermistor
24
and thermopile chip
23
are placed on base plate
25
.
The infrared thermometer calculates the temperature Ts of a target object according to Stefan Boltzman's Law, as shown in equation (1):
Ts
(
t
)=(
Q
(
t
)/
KC+Ttp
4
(
t
))
1/4
(1)
wherein T
tp
represents the cold junction temperature of thermopile chip
23
, Q represents the response of thermopile chip
23
to a thermal radiation source, KC represents a calibration coefficient. However, the cold junction temfperature T
tp
of thermopile chip
23
is obtained by monitoring the temperatue of thermistor
24
. Therefore, the temperature Ts′ of the target object calculated by the infrared thermometer is shown in equation (2),
Ts
′(
t
)=((
Q
(
t
)+&Dgr;
Q
(
t
))/
KC+Tth
4
(
t
))
1/4
(2)
wherein T
th
represents the temperature measured by thermistor
24
, Q represents the response of thermopile chip
23
to a thermal radiation source, &Dgr;Q represents the response of the thermopile chip
23
to waveguide
11
and &Dgr;Q can be expressed by equation (3),
&Dgr;
Q
=(
Twg
4
(
t
)−
Ttp
4
(
t
))×∈
W
×C
W
(3)
wherein ∈
W
represents the emissivity of waveguide
11
, C
w
represents a coupling factor. Measurement error &Dgr;Ts is shown in equation (4),
&Dgr;
Ts=Ts
′(
t
)−
Ts
(
t
) (4)
=((
Q
(
t
)+&Dgr;
Q
(
t
))/
KC+Tth
4
(
t
))
1/4
−(
Q
(
t
)/
KC+Ttp
4
(
t
))
1/4
=((
Q
(
t
)+(
Twg
4
(
t
)−
Ttp
4
(
t
))×∈
W
×C
W
)/
KC+Tth
4
(
t
))
1/4
−(
Q
(
t
)/
KC+Ttp
4
(
t
))
1/4
In other words, in a static environment where the ambient temperature is stable, Tth(t)=Ttp(t)=Twg(t), &Dgr;Q=0, and the measurement error &Dgr;Ts is equal to zero. On the contrary, in a dynamic environment where the ambient temperature varies Tth(t)≠Ttp(t)≠Twg(t), &Dgr;Q≠0, and a measurement error will occur.
FIG. 3
shows an equivalent model of the probe shown in
FIG. 1
responding to temperature, wherein T
amb
(t) represents the ambient temperature of the environment, R
hs
represents the thermal resistance of the heat-conducting block, C
hs
represents the thermal capacitance of the heat-conducting block, T
wg
(t) represents the temperature of the waveguide, R
th
represents the thermal resistance of the thermistor, C
th
represents the thermal capacitance of the thermistor, T
th
(t) represents the temperature of the thermistor, R
tp
represents the thermal resistance of the thermopile chip's cold junction, C
tp
represents the thermal capacitance of the thermopile chip's cold junction, and T
tp
(t) represents the cold junction temperature of the thermopile chip.
FIG. 4
shows the temperature measurement error of the probe shown in
FIG. 1
, wherein curve
41
represents the temperature variation T
amb
(t) of an environment and appears as a step function; curve
42
represents the temperature variation T
th
(t) of the thermistor, and curve
43
represents the temperature variation T
tp
(t) of the cold junction of the thermopile chip. Since the thermal resistance of the thermistor is less than that of the thermopile chip, the thermal time constant &tgr;
th
(&tgr;
th
=R
th
×C
th
) of the thermistor is less than the thermal time constant &tgr;
tp
(&tgr;
tp
=R
tp
×C
tp
) of the cold junction of thermopile chip, and the response of the thermistor to temperature variation is faster than that of the thermopile chip to temperature variation. In other words, before the prior art system becomes stable, the thermometer will introduce measurement errors.
Therefore, how to make a thermistor acurrately track the cold junction temperature of a thermopile chip without time lag is an important issue.
SUMMARY OF THE INVENTION
The main object of the present invention is to provide a probe for use in an infrared thermometer, which can accurately measure the temperature of a thermal radiation source in a dynamic environment.
The second object of the present invention is to provide a probe for use in an infrared thermometer, which is easy to manufacture and has a smaller volume than the prior arts.
The third object of the present invention is to provide a probe for use in an infrared thermometer, which could shorten the unstable time period in a dynamic environment.
For obtaining the above objects, the present invention discloses a probe for use in an infrared thermometer. The probe takes advantage of the heat transfer theorem and makes the thermistor accurately and fast respond to the actual cold junction temperature of
Chang Ryan
Lee Alich
Lee Burt
Wu Cruise
Fulton Christopher W.
Jagan Mirellys
Ladas & Parry
Oriental System Technology Inc.
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