Fluid composition sensor using reflected or refracted light monitoring

A gas density sensor having a prism in contact with a fluid whose density is determined. A light source shines light into the prism. The light is reflected off prism surfaces in contact with the fluid. As the fluid density changes, the amount of light reflecting off these surfaces changes depending upon fluid density. A detector placed to receive light reflecting off the surfaces determines density from sensed light.

FIELD OF THE INVENTION 
The present invention concerns method and apparatus for measuring fluid 
properties such as gas density, gas or liquid chemical composition, 
physical state, temperature, or pressure. 
BACKGROUND ART 
This invention concerns a method of directly measuring a physical property 
of a fluid such as gas density and/or using the measured physical property 
to determine another property such as gas pressure. Conventionally, gas 
density can be determined by weighing a gas container, measuring its 
volume and subtracting the container's empty weight. The weight difference 
is then divided by the container's volume to obtain the density. A major 
difficulty resides in trying to measure the filled container's weight, 
especially when the container is mounted for use. Gas density can also be 
determined by applying gas law data to a measured pressure. This requires 
an accurate pressure and temperature measurement, and an accurate 
determination of the gas law for the measured gas. The gas law in turn 
will vary from gas to gas and at different combinations of pressure, 
density and temperature. 
Gas pressure measurement also presents challenges. At the present time, one 
of the most common methods of gas pressure measurement is the Bourdon 
Tube. This simply consists of a bent or coiled tube closed at one end with 
the other end open and mechanically fixed to the system whose gas pressure 
is to be measured. The outside of the tube is kept at a reference pressure 
(usually atmosphere). When the pressure inside the tube exceeds the 
pressure outside the tube, the tube will began to straighten. When the 
pressure inside falls, the tube will began to return to its bent form. 
Typically, a device such as a hinged meter needle, is attached to the 
closed end of the Bourdon tube. As the tube moves the needle then moves 
along the meter scale. By appropriate choice of materials and tube 
dimensions, it is possible to achieve a meter indication that is 
approximately linear or at least smoothly varying with respect to pressure 
variation. The disadvantages of the Bourdon tube reside in the difficulty 
of repeatably reproducing the same mechanical behavior from tube to tube. 
This results in a need to either mechanically calibrate each tube or 
accept a large variation in pressure response from tube to tube. The 
motion of the tube is also susceptible to temperature variation. As the 
tube heats its thermal expansion will cause extension and its other 
mechanical properties will also vary. Additionally, a mechanical linkage 
is required to whatever is used as a meter. Mechanical linkages 
necessarily exhibit hysteresis, wear and `play`. 
Another common measuring technique is to place a strain gauge on a 
diaphragm. Strain gauges can be composed of materials such resistive films 
or piezoelectric elements, among others. The diaphragm in turn is fixed at 
the edges and is placed between the pressure to be measured and a 
reference pressure. As the pressure difference between the measured and 
reference sides of the diaphragm varies, the diaphragm will flex towards 
or away from the reference pressure. This flexing results in strain 
applied to the strain gauge which in turn provides an electrically 
measurable indication of the degree of strain. The degree of strain is 
then hopefully proportional to the pressure difference. Strain gauge 
pressure measurement has the advantage of providing a direct electrical 
signal that can be readily monitored by automatic equipment such as 
computers or controllers. Strain gauge meters can also be made much 
smaller than Bourdon tube devices and can even be micromachined into 
integrated circuit wafers. Disadvantages include temperature induced 
variation in response, poor unit to unit repeatability and high cost. 
Optek Technology, Inc. of West Crosby Road, Carrollton, Tex. 75006 has a 
published data book (copyright 1989, 1990) that indicates its OPB model 
XXX series sensors can be used to sense the absence or presence of a 
liquid. FIG. 5 of Optek Application Bulletin 204 (July 1989) notes that 
light signal variations due to reflection at an interface between a liquid 
and a transparent material can be used to detect the presence of a liquid. 
In laboratory settings it is possible to indirectly determine gas density 
or pressure by monitoring variations in the speed of sound or speed of 
light in the gas. However, until now this has not translated into a low 
cost, practical and readily employable device outside the lab. 
DESCRIPTION OF THE PRESENT INVENTION 
The present invention utilizes a variation in index of refraction (ratio of 
light speeds) with respect to a chemical or physical property of a fluid. 
This variation is exploited by placing a refracting and/or reflective 
device such as a prism in a light path so that the light reflects from an 
interface between the reflective device and a fluid whose chemical or 
physical property is to be measured. The amount of light refracted or 
reflected varies as the index of refraction of the fluid in contact with 
the device varies. This variation in light can then be measured by devices 
such as photodiodes or photocells and used to determine the chemical or 
physical property. 
Apparatus constructed in accordance with a preferred embodiment of the 
invention monitors a density of a gas contained within a vessel. The 
vessel includes a gas port for putting gas into the vessel and at least 
one optics port for allowing radiation to enter the vessel. A multi-sided 
radiation transmissive prism is supported within the vessel to intercept 
radiation entering the vessel and includes first and second reflecting 
surfaces at an interface between the gas and the radiation transmissive 
prism. A radiation source directs a beam of radiation into the vessel 
through the optics port to the radiation transmissive prism along a path 
that causes the radiation to strike a first reflecting surface at a 
controlled angle and then reflect off the first reflecting surface and 
pass through the prism to the second reflecting surface. A detector 
monitors radiation intensity after the radiation reflects off the second 
reflecting surface and provides an indication of gas density based upon an 
output from the detector. 
Challenges include locating cost effective and temperature stable light 
sources and light measuring devices as well as designing and building 
optical components that are sensitive to small variations in index of 
refraction. These and other challenges are overcome in the preferred 
embodiment of the present invention which is described in greater detail 
below in conjunction with the accompanying drawings.

BEST MODE FOR PRACTICING THE INVENTION 
The physical principle behind the invention comes from the Clausius Mossiti 
Relation. 
##EQU1## 
For instance, in Argon the standard temperature and pressure density is 1. 
7832.times.10.sup.-6 g/cm.sup.3 and the STP permitivity is 1.000517. This 
gives a value of 0.097 for alpha. Equation 1 implies that the permitivity 
of a gas arises primarily from the individual dipole moments of the 
constituent molecules. As a result, the greater the density of the gas, 
the greater the number of dipoles (molecules) in a given volume of gas 
which increases the value of the permitivity. In the appropriate units the 
permitivity is given by equation 2. 
##EQU2## 
Equation 3 for electric susceptibility as a function of density is then 
obtained from equations 1 and 2. 
##EQU3## 
If the susceptibility (X) is small, one can apply the binomial expansion to 
equation 3 expand the denominator. If the susceptibility is less than 0.01 
one can truncate to only linear terms and obtain the following: 
##EQU4## 
Keeping only the first order term one has equation 5. 
##EQU5## 
Substituting the definition of susceptibility from equation 2 the relation 
becomes 
##EQU6## 
This implies that for gasses with a permitivity no greater than 1.01, the 
permitivity (.epsilon.) will vary linearly with density to 99% or better. 
For most if not all gasses the relationship between susceptibility and the 
index of refraction n for the gas is: 
##EQU7## 
Substituting for the susceptibility and using the binomial expansion (first 
3 terms) the following is obtained: 
##EQU8## 
For small susceptibilities the non linear term can be dropped to obtain: 
##EQU9## 
This shows that the index of refraction n will be linear to better than 98% 
for susceptibilities &lt;0.01. Snell's Law states that for two media with 
indexes of refraction of nl and n2, respectively: 
##EQU10## 
In this relation, the angles are measured relative to the normal to the 
interface between the two media and indicate the trajectory a light ray 
will follow in going from one media to the next. For n1&gt;n2, it is possible 
to set the angle .crclbar.2 to 90 degrees to obtain the critical angle: 
##EQU11## 
Substituting for n2 from equation 10 the variation in critical angle with 
respect to gas density is shown as: 
##EQU12## 
When the density term is sufficiently small, the critical angle will vary 
approximately linearly with density. 
At angles less than the critical angle, partial reflection will occur. The 
vector component of light polarized perpendicular to the plane of the 
incident, reflected and refracted rays, can be determined as can the 
vector component of light polarized coplanar with the incident, reflected 
and refracted rays. With n.sub.g being the index of refraction of the gas, 
the fraction of co-planar polarization light is given by equation 13: 
##EQU13## 
Where n is the index of refraction on the incident media. 
For a non-polarized light source, an approximately equal mixture of 
co-planar and perpendicular components can be expected, this results in an 
average reflected to incident power ratio as given by equation 14: 
##EQU14## 
Note that the index of retraction n.sub.g will be relatively independent of 
temperature as long as the dipole moments of the individual molecules in 
the gas remain constant. For instance, monoatomic molecules such as in 
gaseous Argon could be expected to have relatively stable dipole moments 
over temperature. More complicated molecules such as H.sub.2 O might be 
expected to have vibrational modes that would affect the dipole moment 
with increasing temperature. 
FIG. 1 schematically depicts a sensor 10 that monitors gas density changes 
by monitoring changes in the index of refraction of the gas. A pressure 
vessel 11 includes two glass optical ports 12, 14 and a prism 16. The 
pressure vessel defines a vessel interior 18 that contains a pressurized 
gas mixture of 95% Ar and 5% He. 
The prism 16 is composed of crown glass with an index of refraction of 
1.523. The prism angles are as shown in FIG. 1. The prism angles are 
chosen to produce controlled optical paths for light entering the prism 
when the gas density inside the vessel 10 is that found at 20.degree. C. 
and 3,100 psi. A principle ray 26 passing through the port 12 enters the 
prism 16 at an angle normal to a prism base 28 and strikes a surface 30 at 
the critical angle of 43.7.degree., as determined by equation 12 (with 
assumptions concerning the gas mixture and gas density at 20.degree. C. 
and 3,100 psi) and is totally reflected. A reflected principle ray 32 
passes through the prism 16 and strikes a prism surface 34 at the critical 
angel (approximately 43.7.degree.) and is again totally reflected along a 
path 36. The reflected principle ray 36 then exits the prism through the 
base 28 and leaves the vessel 11 by the exit port 14. 
A light ray 42, 44, 46 strikes the prism surface 30 at an angle less than 
the critical angle and is partially reflected at the surface 30 according 
to equation 14 and will be totally reflected at the surface 34. 
A light ray 52, 54, 56 that strikes the prism surface 30 at an angle 
greater than the critical angle will be totally reflected at the prism 
surface 30 and will be partially reflected at the surface 34. 
As the gas density rises above the 20.degree. C., 3100 psi density the 
index of refraction n.sub.g of gas within the vessel interior 18 will 
increase as will the critical angle (via equation 12) and all light rays 
entering the prism 16 through the base 28 will be partially reflected at 
one or both of the prism surfaces 30, 34. When the gas density falls below 
the 20.degree. C., 3100 psi density, the index of refraction and critical 
angle will decrease resulting in the principle ray 26, 32, 36 being 
totally reflected, at both prism surfaces 30, 34. For rays that are 
partially reflected, the power fraction of light reflected will be given 
by equation 14. As the gas density continues to fall, rays with angles 
close to those of rays 26, 32 and 36 will progressively begin to go into 
complete internal reflection at both faces of the prism. As a result, the 
amount of light making reflections will increase with decreasing gas 
density. 
In the preferred embodiment of the present invention, an infrared LED 60, 
(Hamamatsu part L3080) is placed in front of the port 12 to shine light 
through the port and into the prism 16. An infrared photodiode 62, 
(Hamamatsu part S2506) is placed in front of the port 14 as shown and a 
second `reference` infrared photodiode 64, (Hamamatsu part S2506) is 
placed near the first LED 60 as shown. The LED 60 has an intensity vs. 
emission angle relationship such as is shown in FIG. 2. 
The dimensions and separation of the ports 12, 14 limit the emission 
pattern that reaches the prism surfaces 30, 34 and passes through the exit 
port 14. Ideally, the emission pattern for the selected LED 60 would be 
sharply peaked at the center line to a region offset by 5 degree on either 
side of the centerline. However, in practice such tight emission patterns 
involve higher LED costs and require tighter assemble tolerance. By using 
a comparatively broad emission pattern such as shown in FIG. 2, the prism 
16 remains well illuminated even if the LED 60 is installed with 5.degree. 
of angular misalignment. As shown by the equations described above, the 
fraction of light beam reflected through the prism 16 back to the detector 
62 is a function of the dielectric density of the gas under test, which is 
in turn strongly dependent on the gas density. However, the absolute 
strength of the light beam and as a result, the total amount of light 
reaching the photo detector 62, will depend directly on the amount of 
light emitted from the LED 60. 
Emissions from an LED can vary with time (often decreasing), temperature 
and the amount of current drive. For instance, over a temperature range of 
-40.degree. C. to +85.degree. C., a LED's light emission can vary by 50%. 
Over time, a LED's light emission can decrease to 80% or less of its 
initial level. Also, to a large extent, an LED's light emission will vary 
proportionally with drive current. 
In contrast, the light current from a photodiode will be comparatively 
stable over time and temperature at a constant level of illumination. 
Additionally, light current from a photodiode will vary linearly with 
respect to the amount of illumination it receives over a broad range of 
illumination levels (assuming the illumination is in the photodiode's 
spectral range of sensitivity and does not vary in its spectral 
distribution). This comparative stability of photodiodes relative to LEDs 
is exploited by the use of the reference photodiode 64. 
The reference photodiode 64 is placed so it is illuminated by light coming 
directly from the LED 60 along a light path 66. Light passing through the 
port 12 to the prism 16 and out the port 14 that reaches photodiode 62 
will depend on the gas density (via equations 1-14) and on the amount of 
light emitted from the LED 60. Light reaching the photodiode 64 will 
depend only on the amount of light emitted from the LED 60. As a result, 
the ratio of the light currents from the photodiodes 62, 64 cancels out 
dependence on the amount of LED emission and varies primarily with gas 
density, i.e., if the LED 60 light emission increases by 30%, the light 
currents from the photodiodes 62, 64 will both increase by 30%. 
In the preferred embodiment, the light current from each photodiode 62, 64 
is amplified to produce a voltage that varies directly with light current 
and amplifier gain as shown in the simplified schematic in FIG. 3. When 
the sensor 10 is assembled, the gains of two amplifiers 70, 72 are 
adjusted to produce a preselected voltage `Vref` from the reference 
photodiode and a preselected voltage `Vsig` from the signal photodiode at 
a preselected calibration gas density `Deal` at a calibration temperature 
`Teal`. 
The LED 60 is driven in one of two ways (see FIG. 4). In method A, the 
voltage across a resistor 76 in series with the LED 60 is kept constant. 
This results in a constant current drive mode for the LED 60. In method B, 
a Vref signal 78 from the reference photodiode 64 is fed back into an LED 
drive amplifier 80 which in turn alters Vdrive to keep Vref equal to a 
control voltage Vset 82. This effectively keeps the light output from the 
LED 60 constant despite variations in its emission or emission efficiency 
with respect to changes due to an otherwise variable drive level, 
temperature or aging. Method B is the preferred mode if the output Vsig 84 
is going to be used as an analog indication of gas density. Method A is 
useful when all that is required is a `greater than` comparison between 
Vsig 84 and Vref 78 for switchpoint gas density detection. It is also 
useful in situations where overall current usage must be controlled. 
Given an angular acceptance of about 5.degree. and the arrangement of FIG. 
1, the fraction of the LED beam that can be expected to reach the photo 
detector 62 is shown as a function of gas density in FIG. 5 (curve is for 
pure Argon). Note the curve is normalized relative to the fraction 
predicted at a density of 0.0017832 g/cm.sup.3 (20.degree. C. and 760 mmHg 
density). As can be seen, there is an approximately linear relationship 
between the beam fraction and density up to the critical density of 0.35 
g/cm3. Thereafter the beam fraction shows a slow exponential decrease. 
The linear region, 0.0018932 to 0.35 g/cm.sup.3, corresponds to the density 
range where some fraction of the central .+-.5.degree. of LED beam is 
totally reflected at prism surfaces 30, 34. As shown in equations 1-12 for 
a small electric susceptibility X, the relationship between critical angle 
and gas density is approximately linear. The portion of the .+-.5.degree. 
beam emission that is totally reflected through the prism can then be 
expected to be approximately linear in gas density. Since the totally 
reflected beam component will have a much higher energy than the partially 
reflected component (see equation 14), an approximately linear 
relationship between the total amount of light reflected through the prism 
16 and gas density is seen. Above the 0.347 g/cm3 gas density, the only 
beam components that make it through the prism are those that are 
partially reflected from prism surfaces 30, 34. Referring to equations 10, 
12 and 15, one sees that little linearity in partial reflections vs. 
density are expected. 
At a constant temperature the gas density would be expected to vary 
approximately linearly with pressure. In FIG. 6, normalized beam fraction 
reaching the photo detector 62 vs. gas density data is shown for pure 
Argon gas and a prism 16 and pressure vessel 11 constructed according to 
FIG. 1. As can be seen, the relationship is approximately linear up to the 
highest pressure. A strong and monotonically decreasing relationship 
between beam fraction and density is shown. This is in general agreement 
with the type of behavior anticipated by the preceding analysis. Note that 
in FIG. 5, the predicted beam fractions are approximately 0.8, 0.65 and 
0.4 at gas densities of 0.12, 0.2 and 0.27 respectively. In FIG. 6, 
experimental data show that the actual beam fractions at the same 
densities are 0.81, 0.6 and 0.4 respectively. This shows a good agreement 
between theory and practice for the sensor 10. 
A major advantage of the invention is the fact that the light signal 
follows the unchanging path of the optics without physically contacting 
the gas under test. As long as the prism surface is "wetted" by the gas 
within the vessel 11, the light signal should be relatively immune to dust 
contamination or variations in gas color and light absorption 
characteristics. Additionally, the reflection effect in the prism is 
affected by the gas density, not the gas temperature or pressure. As a 
result, the sensing effect is highly stable against changes in temperature 
and pressure at the same gas density. On the other hand, knowledge of 
temperature and gas law for a given gas allows the density measurement to 
also be used in determining pressure. 
FIGS. 7, 7A and 8 depict a sensor 110 constructed in accordance with a 
preferred embodiment of the present invention. The sensor 110 is coupled 
to a vessel 112 having a necked down end 114 that engages the sensor 110. 
A sensor body 116 defines a fill hole 117 which extends through a 
cylindrical post 118 of the body 116 and opens into an interior 120 of the 
vessel. The fill hole 117 allows fluid whose density is under 
determination to be injected into the vessel 112. 
The sensor body 116 is formed from metal and includes a cavity 122 into 
which an optical subassembly 130 and printed circuit board 131 (FIG. 13) 
are inserted. During fabrication of the sensor 110, a prism 132 is fixed 
to the sensor body 116 in a position that covers two glass-filled optical 
ports 134, 136 in the sensor body 116. The optical ports 134, 136 transmit 
light from a light-emitting diode 60 mounted to the optical subassembly 
130 into the prism. The prism has surfaces 132a, 132b in direct physical 
contact with a gas within the vessel interior 120. 
A cavity or recess 140 is machined in the sensor body 120 so that after the 
prism 132 is attached to the sensor body the prism is protected from 
damage. During shipment of the sensor 110, a removable protective boot 
(not shown) also covers the prism 116. A metal clip 141 (FIGS. 14, 15) is 
also attached to the sensor body 120 to prevent movement of the prism 132 
if the attachment between the prism 132 and sensor body 116 should loosen. 
The optical subassembly 130 is depicted in greater detail in FIGS. 9 and 10 
of the drawings. A well 142 in the sensor body extends beneath a base 144 
of the cavity 122. The well 142 is shaped to receive the optical 
subassembly 130 as the sensor is being built. When inserted into the well, 
the subassembly 130 positions the LED 60 in a region 146 so that light 
emitted from the LED 60 passes through the optical port 134 and into the 
prism. When mounted to the subassembly 130 the LED 60 abuts an exit 
aperture 152 (FIG. 10) that opens into a passageway 154 leading to the 
optical port 134. 
A carrier member 150 defines two annular bosses 160, 162 that extend 
outwardly from a bottom surface 164 of the carrier 150. These bosses 160, 
162 seat within counterbores 166, 168 in the sensor body and help position 
the carrier 150 as the sensor 110 is assembled. 
The optical subassembly 130 also supports two photodiodes 62, 64 for 
monitoring light emissions from the light-emitting diode 60. A reference 
photodiode 64 is positioned within a cavity 170 so that a small percentage 
of the light generated by the light-emitting diode 60 passing through a 
passageway 172 reaches the photodiode 64. The second photodiode 62 is 
positioned in a cavity 174 to monitor light passing through the optical 
port 136 that has been twice reflected at the two surfaces 132a, 132b 
between the prism and gas within the vessel interior 120. By monitoring a 
ratio of the outputs of these two photodiodes 62, 64, a determination of 
the fluid density widen the vessel can be made. 
As seen :most clearly in FIGS. 14 and 15, the clip 141 is a stamped flat 
metal piece bent along its length and including a mounting opening 176 to 
allow a connector 177 (FIGS. 7, 7A) to connect the clip 141 to the sensor 
body. A second opening 178 overlies a prism apex and assures the prism 
will not separate from the sensor body. 
Circuitry 180 (FIGS. 20A, 20B) for activating the light-emitting diode 60 
and monitoring output signals from the two photodiodes 62, 64 is supported 
on the printed circuit board 131. An opening 182 in the circuit board 131 
fits over the column 118 in the sensor body 116 to allow the board to be 
placed into the recess 122 in the sensor body 116. 
Two capacitors 183a, 183b and two diodes 184a, 184b provide input noise 
filtering and overvoltage protection to the circuit 180. The capacitor 
183a provides a low impedance path across the input terminals 185a, 185b 
to shunt high frequency noise around the circuit 180 and protect against 
electrostatic damage. The diode 184a prevents reverse polarity current 
flow. The capacitor 183b acts as a DC noise filter and the diode 184b 
provides overvoltage protection by clamping the voltage across inputs 
185a, 185b to the circuit to a maximum value. 
A transistor 186 and biasing resistors 187a, 187b form a solid state switch 
188 for providing power to the circuit 180. The resistor 187a limits 
current to the base of the transistor 186 and the resistor 187b provides 
pull-up voltage to the base when the transistor 186 turns off. 
The light-emitting diode 60 is coupled to the collector of a transistor 
190. When the transistor 190 conducts, the diode 60 conducts and emits an 
infrared output. A precision regulator 192 (Texas Instrument's Part No. 
TL431IPK) maintains a constant current through two resistors 194, 195 by 
controlling the base current through the transistor 190. Current at the 
base of the transistor 190 creates collector-emitter current flow through 
the transistor 190 to develop a voltage drop across the resistors 194, 
195. The voltage generated across the resistors is used by the precision 
regulator 192 to control base current of the transistor 190. The collector 
to emitter current through the transistor 190 is the same current that 
flows through the infrared emitting diode 60. A resistor 196 acts as a 
current limit and series voltage drop for the regulator 192. 
Signals developed by the two photodiodes 62, 64, in response to an infrared 
light output from the light-emitting diode 60, are applied to two 
precision operational amplifiers 198, 199. Light striking the detector 62 
generates current flow from the cathode to the anode of the detector. The 
output voltage developed by the operational amplifier 198 increases until 
the current through a pair of factory settable based upon circuit 
performance feedback resistors 200a, 200b is equal to the current through 
the diode 62. A capacitor 201 filters high-frequency noise from the gain 
loop of the operational amplifier 198. 
In a similar fashion, the current through the diode 64 generates current 
flow from its cathode to anode. The voltage developed at an output from 
the operational amplifier 199 increases until the current through the 
series feedback resistors 202a, 202b equals the current through the diode 
64. A capacitor 203 filter high-frequency noise from the gain of the 
operational amplifier 199. 
The output from the operational amplifier 199 is a signal related to the 
strength of the output from the light-emitting diode 60 without the affect 
of gas density. This output is coupled to two comparator amplifiers 204, 
206 (FIG. 20B), having outputs coupled to two latch circuits 208, 210. An 
output from the operational amplifier 198 is also coupled to the 
comparator amplifier 204. The comparator 204 compares the voltage signal 
output from the reference photodiode 64 with the signal output from the 
receiver photodiode 62. Two resistors 211a, 21lb at the inputs to the 
comparator 204 prevent overloading of those inputs. A feedback resistor 
212 provides positive feedback and prevents oscillation of the output from 
the comparator amplifier 204. 
In a similar fashion, two input resistors 213a, 213b prevent overloading of 
the comparator 206. A feedback resistor 214 produces a positive feedback 
and prevents oscillation of the output from the comparator 206. 
The latch circuits 208, 210 control application of power to the 
light-emitting diode 60. When the inverted output 214 (not Q) from the 
latch 208 allows the base of the transistor 186 to go high, the 
light-emitting diode 60 is extinguished. When this occurs, the latch 210 
receives a clock input and prevents the latch 208 from again turning on 
the light-emitting diode 60. If the signal from the comparator 198 is less 
than or equal to the signal from the comparator 199, the output from the 
comparator 204 will be high. 
A power-ON reset function is supplied by a transistor 215 and a charging 
circuit that includes a capacitor 216. The transistor 215 turns on when 
power (VCC) is applied to the circuit 180. As the transistor 215 conducts 
in response to the power input, a voltage drop develops across a resistor 
217 coupled to the non-inverting (+) input to the operational amplifier 
198. This positive saturation of the operational amplifier 198 causes the 
comparator 204 to output a positive or high-level signal which sets the 
outputs of the two latch circuits 208, 210. When the latch 208 is set, its 
inverted output (not Q) goes low, causing the transistor 186 to conduct 
causing the light-emitting diode 60 to produce an output signal. 
After a 15-millisecond delay, the capacitor 216 has charged to the point 
that the transistor 215 is rendered non-conductive. This allows the 
operational amplifier 198 to produce a valid signal from the photo 
detector 62. Stated another way, for the first 15 milliseconds that power 
is applied to the circuit 180, no valid signal is generated. 
An operational amplifier 218 has a reference input (+) coupled to a 
resistance divider formed by the resistors 194, 195. This operational 
amplifier 218 inhibits operation of the delay circuit signal output from 
the operational amplifier 219. If the signal at the junction between the 
two resistors 194, 195 falls below a certain level, voltage applied to the 
circuit is too weak for continued operation and the latch delay circuit is 
inhibited. 
When power is applied to the circuit at the input 185a, an output from a 
comparator 219 goes low. At the same time, the transistor 215 turns on, 
making the output of the operational amplifier 198 go high. This high 
output is coupled to the operational amplifier 204 and causes the output 
from this operational amplifier to set the latch circuits 208, 210. This 
turns on the transistor 186 and supplies power to the light-emitting diode 
60. As the capacitor 216 charges, the transistor 215 turns off, allowing 
the operational amplifier 198 to monitor signals derived from the 
photodiode 62. 
An output from the operational amplifier 218 stays low as long as the 
signal strength from the reference photodiode operational amplifier 
remains higher than the voltage from the reference input voltage supplied 
by the voltage divider formed by the resistors 194, 195 of the 
light-emitting circuit. The output from the comparator 218 goes high, 
however, if the reference voltage from the resistors 194, 195 is not high 
enough. When the output from the comparator 218 goes high, it inhibits the 
output from the comparator 219. If the reference signal at the junction of 
the resistors 194, 195 is high enough, the output from the comparator 219 
goes low at applied power and switches to high approximately 100 
milliseconds after power is applied to the circuit. The transition of the 
output from the comparator 219 from low to high that latches the "good or 
bad" reading of the sensor once power is applied. If the output from the 
comparator 198 that is applied to the comparator 204 is higher than the 
reference input to the comparator 204, the comparator 204 outputs a high 
signal which overrides the inputs to the latches and keeps their outputs 
in their set state. This output goes high if the output from the signal 
receiver is low, indicating a fault in the circuit 180. 
The circuit 180 is tied to a monitoring circuit (not shown) which 
determines the density of the gas by monitoring current passing through 
the transistor 186 and LED 60. If the sensed density is below a threshold 
value, the light-emitting diode 60 will remain on and the monitoring 
circuit will sense a current draw of approximately 50 mA by the circuit 
180. So long as the gas density stays above a specified level, indicating 
a certain amount of gas remains within the vessel, a 5-milliamp 
(approximately) quiescent current is drawn by the circuit 180. 
The preferred use of the sensor is in an automobile where a Helium-Argon 
gas within an air bag system is checked each time the automobile is 
started. The external monitoring circuit checks the gas density and, if 
too low a density is sensed, the motorist is warned that the air bag 
system needs maintenance. 
The first two circuit inputs 185a, 185b to the circuit 180 are located 
outside the sensor body 116. Leads enter the body through a plastic 
connector 240 (FIGS. 11 and 12). The circuit board 131 is placed into the 
cavity 122 and rests against a recessed ring 241 of the body 116 (FIG. 8). 
The connector 240 is placed into the cavity 122 so that two openings 242a, 
242b of a connector flange 243 align with two tabs 244a, 244b in the ring 
241. When seated against the tabs, the connector 240 extends slightly 
above an upper rim 245 of the body 116. 
Once the connector 240 has been attached to the body with suitable 
connectors, a region between the connector 240 and an inner wall 246 is 
filled with a potting compound P. A throughpassage 247 in the connector 
leaves regions of the circuit exposed for attachment of leads to the 
circuit board 130. These leads are carried by a harness (not shown) 
connected to a source of an input voltage. The harness includes a tab that 
fits into an opening 248 in the connector that prevents rotation of the 
wiring harness connector. 
Alternate Embodiment No. 1 
An alternate embodiment of the invention is shown in FIG. 16 where a sensor 
assembly 220 includes a light source 223 (laser, laser diode, lamp or 
LED), a collimating element 224 (lens and/or aperture stop assembly), two 
optical ports 225, 226, a prism 227, a light collector 228, a 
photodetector 229 (photodiode, phototransistor, photocell etc.), and a 
pressure vessel 230 having an interior 231 filled with a gas, is shown in 
FIG. 16. The collimating element 224 and source 223 are aligned so that 
the right edge of the beam 232 strikes a prism face 233 at the critical 
angle for a reference gas density (nominally the gas density observed at 
760 mm and 20.degree. C.). 
When the gas inside the vessel 230 is at or below the reference density, 
all of the beam 232 will be totally reflected at the prism face 233 and 
will strike a prism face 234 and be absorbed by a black coating 235. When 
the gas inside the vessel 230 rises in density above the reference gas 
density, the right most portion of the beam 232 will begin to be partially 
refracted. A refracted portion 236 of the beam will then strike the light 
collector 228 and be directed through the port 226 to the detector 229. 
As the gas density rises, more and more of the beam will be partially 
refracted and be directed to detector 229. This will result in a 
monotonically increasing signal with increasing density. 
The functional relationship between the signal from the detector and gas 
density are optimized towards a desired behavior by selection of beam 
width and reference gas density. Advantages include a signal that 
increases with increasing gas density as opposed to decreasing with 
increasing gas density (as in the preferred embodiment). As in the 
preferred embodiment, instability in the light source 223 can be canceled 
out with the use of a reference detector 238. The choices of electronics 
will be driven by the type of light source and photodetectors that are 
selected. However, in general, an amplifier would be provided for each 
photodetector 229, 238. The light source could be driven in constant 
current or constant light emission mode in a fashion analogous to that 
described for the preferred embodiment and shown in FIGS. 3 and 4. 
Although not explicitly shown in FIG. 16, a useful deployment for this 
alternate embodiment is to separate the portion of the pressure vessel 
containing the ports, prism and electro-optics. This portion is then 
provided as a threaded or weldable insert that can be installed in any 
suitably ported pressure vessel. 
Further Embodiments and Applications 
Nominally, glass and plastic optics will have indices of refraction on the 
order of 1.5 or greater. Pressurized gasses such as Argon in the 14.22 to 
4600 psi range at 20.degree. C., can be expected to have indexes of 
refraction that are in the 1.000 to 1.1 range. A liquid such as water or 
chemical mixtures with water as a solvent, can be expected to have indices 
of refraction on the order of 1.33. Other liquids such as Cineole, Aniline 
and benzine have indices of refraction of 1.456, 1.584 and 1.498 
respectively. 
The devices described in the preceding embodiments would have ready 
application to measuring the density of other gasses besides Argon and 
Helium. They could also be used to determine the relative concentrations 
of two gasses of different molecular dipole moments at a given pressure 
and temperature. In this technique a gas of higher dielectric constant is 
mixed with one of lower. The overall index of refraction would then vary 
with the relative concentrations of the mixed gasses. 
Given a liquid solvent of known index of refraction such as Water, Cineole, 
Benzine or even Oil, the disclosed technique could be used to measure the 
relative abundance of any chemical dissolved in or mixed with the solvent, 
provided the chemical that was added effected the index of refraction. 
Devices based on the preferred embodiment would not even require 
transparency in the tested solution as the device is sensitive to index of 
refraction of the tested medium and not its transparency. Applications 
could include measuring the antifreeze concentration in a radiator, 
gasoline contamination in engine oil, battery and concentration and 
contaminants in other chemicals. 
In instances where the angle of total reflection is large, multifaceted 
prisms 250, 252, such as those shown in FIGS. 17A, 17B, would be used. In 
cases where the index of refraction in a liquid is sensitive to 
temperature, pressure, aging or some other parameter, the embodiments 
described could be used to measure that parameter as well. 
Embodiment for High Index of Refraction Media 
For substances with an index of refraction greater than that of the optics 
material an arrangement such as is shown in FIG. 17C could be used. Here, 
a prism 254 has facets on the *inside of an otherwise solid block of 
dielectric material. The medium trapped within the prism cavity would then 
function as the prism. A disadvantage of this method would be that the 
signal (reflected) light would have to pass through the tested medium and 
would be susceptible to changes in transparency as well as index of 
refraction. 
Single Light Path Embodiment 
In all the previous embodiments, two optical ports are shown. This is not a 
requirement. A major advantage of optics resides in the fact that a "light 
pipe" can conduct both forward and backwards at the same time, i.e. a 
"light circuit" can be achieved without a circular path. An embodiment 
exploiting this behavior of light is shown in FIG. 19. A light source 337, 
photodetector 338 and prism 339 are optically linked by a single light 
pipe 340 which in turn may be optionally provided with an optical cladding 
341. Optical couplers 342 and 343 are used to link the light source 337 
and photodetector 338 with the light pipe 340. 
The source coupler 342 is shown as injecting light at a higher point and 
angled away from the detector coupler 343 to prevent cross-talk between 
the couplers. A housing 344 and the cladding 341 serve to optically and 
physically insulate the light path. Light from the source 337 passes up 
the coupler 342 and light pipe 340 to the prism 339 where it must make two 
reflections (as in the preferred embodiment) in order to return back down 
the light pipe 340, out the coupler 343 and back to the detector 338. Some 
fraction of the reflected light that comes back down towards the detector 
will be lost down the source coupler 342. 
The amount of light that is lost in the FIG. 19 embodiment will be a stable 
fraction of the total amount that is reflected down the light pipe 340 and 
can be safely ignored. In this embodiment the light pipe 340 and cladding 
341 are shown passing through a header 345 which in turn is sealed into a 
suitably sized orifice in the vessel 346 that contains the medium 347 that 
is being tested. A major advantage of this device is that the light pipe 
340 can be all or part of an extended optical fiber which would allow the 
electro-optics to be remotely located from the vessel 346 which could then 
be placed in an environment (such as extreme temperature) that the 
electro-optics could not withstand. As in the previous embodiments, a 
reference detector 348 can be used to cancel out variations in light 
source 337 output due to aging, temperature, variations in power supply, 
etc. 
Use of reflections near the angle of total internal reflection serves to 
maximize the sensitivity to small changes in index of refraction that can 
arise from changes in medium density, temperature, chemical composition 
and or physical state. Prism materials can consist of any of a number of 
glasses (such as crown, BK 7, fused quartz etc.) or plastics (such as 
polycarb). In the embodiment of FIG. 17C, the medium becomes the prism and 
the block of material containing the prism defining cavity need only have 
a suitably high dielectric constant and need not even be transparent. In 
the most similar application (see Optek Inc. application note from a 1989, 
1990 Data book), two reflections are used in a prism that are in total 
internal reflection in a gas (air) and in almost complete refraction in a 
tested liquid (such as water). When the prism is immersed in the liquid, 
the amount of light making both reflections is abruptly and substantially 
reduced. In this manner the prism functions as a detector for the presence 
or non presence of liquid as opposed to gas or vacuum. No effort is made 
to monitor changes in index of refraction in the gas or liquid. 
Multiple alternate embodiments of the invention are disclosed in FIGS. 18A, 
18B and 18C. In FIG. 18A, both the light-emitting diode 60' and photodiode 
62' are supported within a prism 16'. A single reflecting surface 30' is 
in contact with a gas within a vessel interior 18' of FIG. 18A and a 
second reflecting surface 341 is in contact with the gas in FIG. 18C. 
In FIG. 18B, the vessel 11' supports a multi-faceted prism 16". A single 
reflecting surface 30" directs light to a photodetector 62". FIG. 18C is 
similar to the FIG. 18A embodiment except that two reflecting surfaces 
30', 34' intercept the light beam as it travels from the light-emitting 
diode to the detector. 
Referring to FIG. 21, a cylindrical rod 400 of optically transmissive 
material (such as glass) replaces the prism of the previous embodiments. 
Art optical detector 402 backed by an aperture stop 404 is situated at a 
low pressure (or non-immersed) end of the rod 400 so that a receptive 
portion of the detector 402 faces one end of the rod. 
An optical source 406 supported by an insert 407 is situated below the 
aperture stop 404 and is oriented to direct radiation along an angled or 
beveled surface 405 of the aperture stop 404 through a ring lens 408 and 
into the rod 400. The diameter of the aperture stop 404, the shape of the 
lens 408, and the lens' location relative to the end of the rod as well as 
the location of the source 406 are controlled to allow only large angle 
rays 410 to enter the rod 400. 
The angles of rays that are accepted into the rod 400 are constrained by 
the controlled parameters to be less than or equal to the angle of total 
reflection .theta. between the cylindrical outer surface 412 of the rod 
and the medium within a vessel 413 that supports the rod. This angle of 
total reflection is of course dependant on the index of refraction of the 
test medium which is the index of refraction R at the condition of 
interest, i.e, a given density or composition. 
Rays that would strike the surface 412 at angles greater than the angle of 
total reflection .theta. are stopped by the aperture stop 404. Rays that 
are at or below the angle of total reflection will be totally or partially 
reflected each time they strike the outer surface of the rod. 
By making multiple reflections the rays 410 that enter the rod 400 will 
eventually reach a far end of the rod where they are reflected by a 
reflective coating 414 applied to the far or distal end of the rod. The 
rays will then reflect off the coating 414 and reflect back down the rod. 
A certain percentage or proportion of the rays 410 that are emitted to 
enter the rod will strike the detector having made multiple reflections at 
the medium/rod interface. 
Rays that are only partially reflected at this interface will be 
substantially attenuated if they make multiple reflections. The rays in 
total reflection will suffer little attenuation as they reflect off the 
cylindrical wall of the rod. When the condition of the medium that contact 
the rode changes (for example when the gas density changes) to cause the 
index of refraction of the medium to fall below R, rays close to the angle 
of total reflection will begin to go into total reflection and the light 
signal reaching the detector 402 will increase. As the index of refraction 
continues to drop a progressively greater fraction of rays will go into 
total reflection resulting in an even great light signal. 
The rod and detector/source assembly are supported by the vessel 413 
containing the medium for testing. FIG. 21 also uses a reference detector 
416 mounted relative to the source 406 for use in eliminating source or 
detector response variations from the determination of the medium 
composition. Suitable electrical signals for activating the source 406, 
and responsive to the detector 402 and detector 416 are transmitted by 
conductors 420. 
FIGS. 22, 23, 23A, 24 and 24A depict an alternate means of positioning a 
prism with respect to a vessel wall 450. The vessel depicted in FIG. 22 
has two ports 452, 454 that are filled with a glass and direct light to a 
prism 456. The prism is attached to the vessel wall 450 by means of a 
glass preform 460. The preform 460 is a generally square (in plan) support 
made of a powdered glass molded to shape with a volatile binder. A 
preferred powdered glass is Corning 1990 which softens at around 
500.degree. C. The prism 456 is made of a higher softening point glass 
such as Crown Glass which softens around 730.degree. C. When raised to a 
temperature above 500.degree. C., the preform 460 fuses the prism 456 to 
the vessel wall to secure the prism 456 in place. 
In FIGS. 23 and 24, two preforms 470, 480 support prisms 472, 482 that seat 
against a shoulder 476 of the wall 450. When fused, the preform and prism 
extend into the cavity in the wall and have exposed surfaces 474, 484 
outside the vessel. Note, the prism 482 is a cylinder having a cone-shaped 
end 486 exposed to a medium inside the vessel interior. 
While a preferred embodiment of the invention has been described with a 
degree of particularity, it is the intent that the invention include all 
modifications from the disclosed design falling within the spirit of scope 
of the appended claims.