Abstrict A disposable flow cell for use in a thermal pulse flow meter of
the type that incorporates a pulse heated thermal probe and a detecting
thermal probe. The disposable cell has two side ports covered with
a stretchable film barrier. The thermal probes extend into the flow
cell through the ports stretching the barrier ahead of them so that
the probes can transmit and then detect thermal pulses to a fluid
flowing through the flow cell while at the same time maintaining
isolation between the probes and the fluid. The invention is also
a thermal pulse flow meter incorporating the above cell.
Claims What is claimed is:
1. In an improved thermal pulse flow meter comprising a flow cell,
the flow cell having a channel therethrough, the channel having
an inlet end and an outlet end, the flow cell having a first port
therethrough to the channel near the inlet end and a second port
therethrough to the channel near the outlet end, a pulse heated
thermal probe positioned in the first port so that the tip of the
thermal heated probe is positioned in the channel, a detecting thermal
probe positioned in the second port so that the tip of the probe
is positioned in the channel, wherein the improvement comprises:
stretchable rubber barriers sealably positioned across each of
the ports so that the barriers partition the probes from the channel
and so that the barriers conform to the tips of the probes by barrier
tension.
Description FIELD OF THE INVENTION
The invention is in the field of the measurement of fluid flow
and more specifically relates to a disposable flow cell for a thermal
pulse flow meter and an improved thermal pulse flow meter.
BACKGROUND OF THE INVENTION
The thermal pulse flow meter described in U.S. Pat. No. 4491024
to Miller, Jr. and in U.S. Pat. Nos. 4532811 and 4628743 to
Miller, Jr. et al. was an advancement in the art of flow measurement
because, for example, it used no moving parts and was extremely
accurate.
The gist of the thermal pulse flow meter is that an upstream thermal
probe located in the flow cell of the flow meter is pulse heated
to thereby generate a thermal pulse in the fluid flowing through
the flow cell. A thermal probe located downstream in the flow cell
then detects the passage of the thermal pulse. The time period between
successive thermal pulses is a mathematical function of the flow
rate of the fluid flowing through the flow cell. The commercial
thermal pulse flow meter sold by the Molytek Corporation, also incorporates
a temperature compensation thermistor probe to determine the temperature
of the fluid flowing through the cell. The above mentioned patents
are fully incorporated herein by reference.
When used, for example, for the flow measurement of sterile fluids
for biological applications, the flow cell and thermal probes of
the thermal pulse flow meter required cleaning and sterilization
before use. The present invention is a solution to this and similar
problems.
SUMMARY OF THE INVENTION
The invention is an improved thermal pulse flow meter comprising
a flow cell which has a channel through it. The channel has an inlet
end and an outlet end. The flow cell also has a first port therethrough
to the channel near the inlet end, and a second port therethrough
to the channel near the outlet end. A pulse heated thermal probe
is positioned in the first port so that the tip of the heated thermal
probe is positioned in the channel. A detecting thermal probe is
positioned in the second port so that the tip of the detecting thermal
probe is positioned in the channel. The improvement comprises positioning
barriers across each of the ports so that the barriers sealably
partition the probes from the channel and so that the the barriers
conform to the tips of the probes by barrier tension. The barriers
can be constructed of a stretchable material, such as rubber, so
that the probes stretch the barriers ahead of them during assembly
of the flow meter. The barriers can be formed from separate parts
or can be formed integrally with the body of the cell.
The invention is also a disposable flow cell adapted for use in
the improved flow meter as described above. The disposable flow
cell comprises a body defining a flow channel therethrough, the
channel having an inlet end and an outlet end. The body of the flow
cell also has a first port therethrough to the channel located near
the inlet end of the channel and a second port therethrough to the
channel located near the outlet end of the channel. A barrier is
sealably positioned across each of the ports so that when the flow
cell is normally engaged with the probes, the probes extend through
the ports in such a manner that the tips of the probes are positioned
in the channel, so that the barriers partition the probes from the
flow channel and so that the barriers conform to the tips of the
probes by barrier tension. The barriers can be stretchable barriers
such as barriers made from rubber. The barriers can be a separate
part from the body or can be integral with the body.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an enlarged cross sectional view of a preferred embodiment
of the invention.
FIG. 2 shows a further enlarged cross sectional view of the cell
shown in FIG. 1.
FIG. 3 shows an enlarged view of the cell of FIG. 1 normally engaged
with the thermal probes of a thermal pulse flow meter.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1 therein is shown an enlarged cross sectional
view of a disposable flow cell 1 of the invention which is a preferred
embodiment. The disposable flow cell 1 comprises a tubular body
9 defining a flow through channel 3 having an inlet end 4 and an
outlet end 5. The body 9 has a first port 6 therethrough located
near the inlet end 4 of the channel 3. The body 9 also has a second
port 7 therethrough located near the outlet end 5 of the channel
3. A flexible tube barrier 8 is positioned around the body 9. Preferably,
the tube barrier 8 is rubber such as latex rubber or silicone rubber.
The body 9 terminates at each end in a standard male luer tip 10
so that each end of the body 9 can be connected to a standard female
luer connection not shown. Preferably, the body 9 is molded of a
plastic such as polycarbonate.
Referring to FIG. 2 therein is shown a further enlarged cross
sectional view of the flow cell 1 of FIG. 1.
Referring to FIG. 3 therein is shown an enlarged cross sectional
view of the flow cell shown in FIG. 1 normally engaged with the
heated and detecting probes of a thermal pulse flow meter. The heated
probe 12 and the detecting probe 13 of a thermal pulse flow meter
are permanently secured in a mount 14 with epoxy cement 15. The
tip of the sending probe 12 and the tip of the detecting probe 13
have distended the flexible tube barrier 8 into the flow channel
3 so that no liquid communication can occur from the channel 3 through
the ports 6 and 7 to the outside of the body 9 so that the thermal
probe 12 can transmit a thermal pulse through the tube barrier 8
into a fluid flowing through the channel 3 and so that the thermal
probe 13 can then detect the resulting thermal pulse through the
tube barrier 8 when the thermal pulse encounters the thermal probe
13. Additionally, the stretching of the tube barrier 8 by the tips
of the probes 12 and 13 helps to insure a seal between the barrier
8 and the ports 6 and 7 and insures that the barrier 8 conforms
closely to the tips of the probes 12 and 13 and thins the barrier
8 at the tips of the probes 12 and 13. Preferably, the tube barrier
8 is thin so that the heat pulses can be readily transmitted and
detected. The critical nature of a thin barrier 8 that closely conforms
to the tips of the probes 12 and 13 can be understood by reference
to U.S. Pat. No. 4491024. The thinner the barrier 8 the smaller
would be the additional increase in the "K" factor caused
by the increased time for the thermal pulse to travel from the tip
of the probe 12 through the barrier 8 and then into the liquid in
the channel 3. The thinner the barrier 8 the better the detecting
probe 13 can sense the passing transient thermal pulse. Preferably,
the barrier 8 is about 0.05 inches thick or less. More preferably
the barrier 8 is about 0.02 inches thick or less. Most preferably
the barrier 8 is about 0.005 inches thick or less. In addition to
being thin, the barrier 8 needs to be under tension at the tips
of the probes 12 and 13 so that the barrier 8 closely conforms to
the tips of the probes 12 and 13 so that the thermal pulse can be
efficiently transmitted and detected through the barrier 8. The
temperature compensation probe 16 from the flow meter is secured
to the mount 14 with epoxy cement 17.
Several alternatives to the use of the tube barrier 8 are believed
to be generally equivalent. For example, it should be possible to
heat seal or adhesively seal flexible disks across the ports 6 and
7. Another approach that is believed to be feasible would be to
mold a thin barrier integral with the body 9 across the ports 6
and 7 from a molding material that would be stretchable in thin
section such as polyethylene. Further, yet another approach that
is believed to be feasible would be to mold indented thin barriers
integral with the body 9 across the ports 6 and 7 from a molding
material that is relatively nonstretchable in thin section such
as polycarbonate plastic. In this case the dimensions of the indentations
would have to closely match the dimensions of the thermal probes
so that the barriers conformed to the tips of the probes by barrier
tension without breaking the barrier.
EXAMPLE 1
An apparatus as generally shown in FIG. 3 is assembled. A 3.5 inch
long, 1 inch outside diameter, 0.1 inch inside diameter cylinder
of polycarbonate is machined at each end to form the luer tips 10
on the body 9 The ports 6 and 7 are drilled equidistant from the
center of the body 9 and are spaced 1.5 inches from each other.
The ports 6 and 7 are 0.094 inch in diameter. A 2 inch long latex
rubber tube barrier 8 that is 0.75 inch in diameter and has a wall
thickness of 0.003 inch is stretched over the body 9 so that the
ports 6 and 7 are covered by the tube barrier 8. The tube 8 is obtained
from a "finger cot", generally obtained from retail outlets
of medical supplies.
The three thermistor probes of a Molytek Thermalpulse Flow Meter
Model TPII (a pulse heated probe, a detecting probe and a temperature
compensation probe) are removed from the flow meter's original flow
cell. A 2.5 inch long, 1 inch inside diameter, 1.25 inch outside
diameter polycarbonate tube is sawed in half lengthwise to produce
two pieces, one of which is not used here. The other piece is then
sawed in half lengthwise to produce two quarter round sections,
one of which is not used here. Two holes are then drilled through
the remaining quarter round section (called a mount 14) to receive
the sending probe 12 and the detecting probe 13. The probes are
secured therein by epoxy cement 15. The holes are drilled equidistant
from the center of the mount 14 and are 1.15 inches from each other.
The probes extend 0.5 inch from the concave surface of the mount
14. The temperature compensation probe 16 is secured within the
mount 14 by the epoxy resin 17.
The mounted probes are then clamped to the disposable cell so that
the sending probe 12 stretches the tube barrier 8 ahead of it through
the port 6 and so that the detecting probe 13 stretches the tube
barrier 8 ahead of it through the port 7. The clamp is designed
to hold the cell and the mounted probes in alignment so that the
probes 12 and 13 line up with the ports 6 and 7 and so that the
probe tips are essentially at the axis of the flow channel 3. This
example teaches how to make the invention.
EXAMPLE 2
The apparatus of Example 1 is connected to an adjustable flow rate
metering pump which is pumping methanol The flow of methanol is
directed into the flow channel 3 of the cell at the inlet end 4
and is conducted away from the cell from the outlet end 5. The tube
barrier 8 sealably partitions the methanol from the sending and
detecting probes 12 and 13. The flow rate of methanol is adjusted
to be 0.80 1.15 1.50 1.82 2.14 and 2.41 ml per minute. The
respective average time period (T, as described in detail in U.S.
Pat. No. 4491024) between thermal pulses measured by the thermal
pulse flow meter are 3.64 2.98 2.60 2.33 2.19 and 2.02 seconds.
This data indicates that a linear relationship exists between the
inverse of the flow rate and the thermal pulse period (T) and shows
that the improved flow meter of the invention can be calibrated
to accurately measure flow rate over a broad flow rate range despite
the isolation of the methanol in the channel 3 from the thermal
probes 12 and 13 by the barrier 8. This example teaches how to use
the invention.
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