Sensor

First and second elongated conduits each have an inlet and an outlet. First and second resistive elements having a resistance related to their temperature are disposed in the first and second conduits, respectively, in heat transfer relationship with gas flowing therethrough. A mixture of a first gas and a second gas having a different thermal conductivity than the first gas is supplied to the inlet of the first conduit in a fixed ratio. A mixture of the first and second gases is supplied to the inlet of the second conduit in a variable ratio related to a variable pressure to which a branch of a T-network is exposed. The sensor is specifically employed as a flowmeter, a micrometer, or a pressure sensor.

BACKGROUND OF THE INVENTION 
This invention relates to sensitive instruments and, more particularly, to 
a fluid responsive instrument that can be used as a pressure sensor, a 
flowmeter, or a micrometer among other applications. 
In a hot wire anemometer, the hot wire is connected to serve as one arm of 
an electrical bridge circuit. Current passing through the hot wire heats 
the wire, thereby increasing its resistance. The hot wire is disposed in 
an elongated cavity through which the gas to be measured flows and cools 
the hot wire accordingly. If the type of gas passing through the cavity is 
known, the resistance change of the hot wire is a measure of the gas flow 
rate. If the flow rate of the gas passing through the cavity is known, the 
resistance change of the hot wire is a measure of the thermal conductivity 
of the gas and, hence, the gas type. 
My U.S. Pat. No. 3,971,247, which issued July 27, 1976, discloses a thin 
elongated hot wire bent in half to extend along the length of a cavity 
formed in a housing. The ends of the hot wire are soldered to pads on a 
printed circuit board located at one end of the cavity for support and 
electrical connection to a bridge circuit. The middle of the hot wire is 
wrapped around a rod at the other end of the cavity for support. The rod 
is deformed to exert tension on the hot wire as its length changes. Thus, 
for a cavity having a given length, the length of the hot wire can be 
doubled and a corresponding increase in sensitivity can be achieved. But, 
the probability of a short circuit by contact between halves of the hot 
wire or the hot wire and the sides of the cavity rises, as the length of 
the cavity increases. This type of flowmeter is capable of responding 
rapidly to changes in gas flow, i.e., responding to a change from zero to 
peak voltage in the order of a hundred milliseconds. However, to generate 
signal voltages greater than 500 millivolts, a gas flow of 1.5 liters per 
minute, or greater, past the hot wire is required. Such large gas flow 
introduces turbulence, which increases the probability of a short circuit 
of the hot wire and a noisy signal. 
SUMMARY OF THE INVENTION 
The invention permits the generation of large signal changes with low fluid 
flow, albeit at the expense of the speed of response. First and second 
resistive elements having a resistance related to their temperature are 
disposed in first and second conduits, respectively, in heat transfer 
relationship with fluid flowing therethrough. A mixture of a first fluid 
and a second fluid having a different thermal conductivity than the first 
fluid is supplied to the first conduit at a fixed ratio between 0 and 100% 
of one of the fluids. A mixture of the first and second fluids is supplied 
to the second conduit in a variable ratio related to a variable pressure 
to which one branch of a T-network is exposed. The difference in the 
resistance of the first and second resistive elements is sensed to provide 
an indication of the variable pressure.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS 
The disclosures of my U.S. Pat. No. 3,971,247, which issued July 27, 1976, 
my application Ser. No. 787,467 and application Ser. No. 787,468, which 
were both filed on Apr. 14, 1977, are incorporated herein by reference. 
In FIG. 1, an elongated conduit 10 and an elongated conduit 11 are formed 
in a housing 12 made of a material having high thermal conductivity such 
as aluminum or steel to make the apparatus thermally stable. Conduit 10 
has ports 13 and 14 at its ends and a port 15 midway between ports 13 and 
14 to form a pair of flow passages. Conduit 11 has ports 16 and 17 at its 
ends and a port 18 midway between ports 16 and 17 to form another pair of 
flow passages. A blow tube or breath transmission passage 21 has an end 22 
open to the atmosphere and a closed end 23. A tube 25 having a 
substantially smaller diameter than breath transmission passage 21 passes 
through end 23 and opens to the atmosphere. Tube 25 serves as a 
restriction on gas blown into end 22 of breath transmission passage 21. 
A source of air 30, which supplies air at a low flow rate, e.g. 3 to 5 
scc/sec., is connected to a first arm of a T-network 32 and a first arm of 
a T-network 33, in which an adjustable needle valve 20 is located. A 
second arm of T-network 32 is connected by a section of flexible tubing 34 
to breath transmission passage 21 near the restriction, i.e., between the 
end of tube 25 and end 23, as illustrated in FIG. 1, where the gas 
velocity is low and thus the turbulence is small. Flexible tubing 34 
permits breath transmission passage 21 to be moved about while maintaining 
communication with housing 12, which is stationary. A third arm of 
T-network 32 is connected through an adjustable needle valve 35 to a first 
arm of a T-network 36. 
A second arm of T-network 33 is connected to port 15, and a second arm of 
T-network 36 is connected to port 18. A source of helium 37, which 
supplies helium at a low flow rate, e.g. 1.5scc/sec., to each of ports 15 
and 18 is connected to a third arm of T-network 33 and a third arm of 
T-network 36. 
A hot wire R.sub.1 extends along the length of the flow passage between 
ports 15 and 13, a hot wire R.sub.2 extends along the length of the flow 
passage between ports 15 and 14, a hot wire R.sub.3 extends along the 
length of the flow passage between ports 18 and 16, and a hot wire R.sub.4 
extends along the length of the flow passage between ports 18 and 17. 
Ports 13, 14, 16 and 17 are open to the atmosphere. 
Air and helium from sources 30 and 37 are mixed in T-network 33 and 
supplied thereby to port 15. Helium has a relatively high coefficient of 
thermal conductivity and air has a relatively low coefficient of thermal 
conductivity. This air helium mixture flows slowly past hot wires R.sub.1 
and R.sub.2 to ports 13 and 14, respectively, to cool hot wires R.sub.1 
and R.sub.2 by an amount depending primarily upon the ratio of air to 
helium in the mixture. Similarly, air and helium from sources 30 and 37 
are mixed in T-network 36 and supplied thereby to port 18. The mixture 
flows slowly past hot wires R.sub.3 and R.sub.4 to ports 16 and 17, 
respectively, to cool hot wires R.sub.3 and R.sub.4 by an amount depending 
primarily upon the ratio of air to helium in the mixture. 
In operation, as gas flows into end 22 of breath transmission passage 21 
the static pressure in the second arm of T-network 32 rises by an amount 
depending upon the rate of gas flow in passage 21. As a result, more of 
the air from source 30 flows through valve 35 to T-network 36 for mixture 
with helium, and the air to helium ratio of the mixture increases. The 
increase in the air to helium ratio in turn decreases the cooling of hot 
wires R.sub.3 and R.sub.4 and increases the temperature thereof. 
As shown in FIG. 2, hot wires R.sub.1 through R.sub.4 serve as arms of an 
electrical bridge. Hot wires R.sub.1 and R.sub.3 are connected in series 
between the output terminals of a voltage source V, with hot wire R.sub.1 
connected to the positive output terminal and hot wire R.sub.3 connected 
to the negative output terminal. Hot wires R.sub.2 and R.sub.4 are 
connected in series between the output terminals of voltage source V with 
hot wire R.sub.4 connected to the positive output terminal and hot wire 
R.sub.2 connected to the negative output terminal. The output of the 
bridge appears between the junction of hot wires R.sub.1 and R.sub.3 and 
the junction of hot wires R.sub.2 and R.sub.4. 
As the ratio of air to helium in the mixture flowing through conduit 11 
increases and the temperature of hot wires R.sub.3 and R.sub.4 increases, 
the resistance of hot wires R.sub.3 and R.sub.4 increases as represented 
by the plus sign in FIG. 2. The resulting voltage at the bridge output 
thus represents the flow rate of the breath blown into end 22 of passage 
21. 
Prior to making a breath flow rate measurement, needle valve 35 is first 
adjusted to supply to port 18 a mixture with a small air to helium ratio, 
e.g., about 10%, then needle valve 20 is adjusted until the bridge output 
is balanced. 
Alternatively, pure helium could be supplied to conduit 10, i.e., the air 
to helium ratio of the mixture could be zero, in which case it would not 
be possible to carry out the described bridge balancing procedure prior to 
measurement. 
By varying the thermal conductivity of the gas passing through conduit 11 
in the described manner rather than the flow rate, the apparatus of FIG. 1 
measures the flow rate through passage 21 by the technique used in gas 
chromatography, i.e. by measuring the changing thermal conductivity of the 
gas mixture flowing through conduit 10 rather than the flow rate thereof. 
The flow rates of the air and helium are so low that the change in the flow 
rate of the mixture supplied to port 18 as more of the air from T-network 
32 passes to T-network 36 rather than to breath transmission passage 21 
has negligible cooling effect upon hot wires R.sub.3 and R.sub.4 relative 
to the cooling effect thereon due to the change in the air to helium 
ratio. 
The apparatus described in connection with FIG. 1 could also be employed as 
a pressure sensor, in which case the second arm of T-network 32 would be 
connected to the region where the pressure is to be sensed. 
Another application of the apparatus in FIG. 1 is as a micrometer, which is 
illustrated in FIG. 3. Instead of opening into breath transmission passage 
21, flexible tubing 34 passes through a stationary micrometer element 37. 
The end of flexible tubing 34 is flush with a flat face 38 of micrometer 
element 37. A micrometer element 39 has a flat face 40 parallel to face 
38. Micrometer element 39 is movable toward and away from micrometer 
element 37 to vary the gap, designated X, between faces 38 and 40. As gap 
X, which is typically of the order of several thousandths of an inch, is 
varied, the air pressure in flexible tubing 34 changes to generate a 
voltage across the bridge output in the manner described above in 
connection with FIGS. 1 and 2. Specifically, if gap X increases, the air 
to helium ratio of the mixture supplied to port 18 decreases and vice 
versa. This micrometer is sensitive enough to sense changes in gap X of 
the order of millionths of an inch. 
The described apparatus achieves high sensitivity. In the limiting case of 
a change in the mixture flowing through conduit 11 from 100% helium to 
100% air, the change in voltage across the bridge output is typically of 
the order of 3 volts. Assuming a stability of 0.1 millivolts for the 
electrical bridge, a range of 10,000 units is easily achievable. On the 
other hand, the response to changes in flow rate is relatively slow, i.e., 
of the order of 1 to 2 seconds, depending upon the air and helium flow 
rate. 
The described embodiments of the invention are only considered to be 
preferred and illustrative of the inventive concept. The scope of the 
invention is not to be restricted to such emboidments. Various and 
numerous other arrangements may be devised by one skilled in the art 
without departing from the spirit and scope of this invention as defined 
in the following claims. For example, instead of hot wires other types of 
resistive elements having a resistance related to temperature could be 
employed, such as thermistor beads. Instead of helium and air other gases 
could be used; for maximum sensitivity the gases should have as large a 
difference in coefficient of thermal conductivity as possible. Further, 
the invention can be practiced in connection with the features disclosed 
and claimed in my above referenced patent and application Ser. No. 
787,468, particularly the means to support the hot wires.