A semi-automatic quantitative filtration assembly is disclosed having a measuring means to measure and present a known quantity of fluid for filtering and a fluid holding means adjacent to a filter medium. The fluid holding means is adapted to receive and hold excess fluid during filtering and to prevent intermixing with a known quantity of fluid to be presented to filtering. The filtering apparatus is designed to filter a quantitative amount of fluid and receive fluid in the fluid holding tank which is excess sample fluid or fluid unwanted as a result of error or operational failure and which drains to a common vacuum/waste means for filtered fluid. A variety of features are also provided to insure proper operation and minimize operator error.

FIELD OF THE INVENTION 
The present invention relates to a filtering apparatus designed to 
semi-automatically accomplish repetitive quantitative filtration. 
BACKGROUND OF THE INVENTION 
In current practice, quantitative filtration is usually accomplished by a 
lab technician using glassware items, tubing, a vacuum pump, tweezers, a 
ring stand, a pipette, petri dishes, and disposable pipette tips. The 
generally accepted procedure is as follows: with a special flat-bladed 
tweezer, a filter is carefully removed from between its protective sheets. 
With a special rolling motion, the filter is laid across a fritted glass 
filter stoppered into an Erylenmeyer side-arm vacuum glass. A cylinder 
with a flared, ground-glass base is carefully set over the filter and 
centered. A spring loaded clamp is then squeezed over the fritted glass 
and flared base of the cylinder to rigidly hold the system together. A 
sample of oil is taken using a pipette technique and squirted into the 
cylinder. Vacuum is applied and the progress of the filtration is 
monitored visually by the technician. Disassembly requires removal of the 
spring clamp, followed by carefully lifting the filter and placing it in 
an individual petri dish. Clean up requires a wash bottle and a waste 
container and sink. 
Some attempts have been made to automate the foregoing processes. See for 
example, Nuxhall U.S. Pat. No. 4,020,676. However, Nuxhall represents a 
fairly complicated apparatus and does not provide a simple measuring means 
or an in-process holding tank to collect and hold unwanted fluids. Nuxhall 
also does not have a cover to prevent fluid introduction at inappropriate 
times. The need still exists for a simple system designed to minimize 
operator error and to simplify obtaining the desired quantity of fluid to 
be filtered. 
SUMMARY OF THE INVENTION 
The filtering apparatus of the present invention includes a simple 
measuring means to measure and present a known quantity of fluid for 
filtering. The present invention also has an in-process fluid holding tank 
adjacent to a filter medium. The in-process fluid holding tank is adapted 
to receive and hold excess fluid during filtering to prevent the excess 
fluid from intermixing with the known quantity of fluid to be filtered. 
The in-process fluid holding tank receives unwanted fluid as a result of 
operator error, e.g., if the operator puts too much fluid into the 
filtering apparatus, or operational failure (e.g., if the filter plugs). 
The holding tank drains into the same vacuum/waste means which draws the 
filtered fluid through the filter medium. The measuring means is moved 
into and out of engagement with the filter medium and has an overflow 
passage to remove excess fluid from the measuring means for subsequent 
delivery into the in-process holding tank. Delivery of fluid from the 
in-process fluid holding tank is controlled by a drain and an outlet 
valve. 
Mechanical interlock means are incorporated into the filtering apparatus to 
prevent--in conjunction with a removable cover--the addition of fluid into 
the filtering apparatus unless the filtering apparatus is in condition to 
receive and filter same. Specifically, the filter apparatus includes a 
movable measuring tube which moves into engagement with the filter medium. 
The mechanical interlock prevents the addition of fluid into the filtering 
apparatus unless the measuring tube is in engagement with the filter 
medium. 
A slidable carrier and a frame for holding the filter medium are provided 
to ease handling of the filter medium. The slidable carrier is locked in 
position during filtering. Additional mechanical interlocks are provided 
to insure proper location of the filter medium and measuring tube and 
registry of the filter medium on the slidable carrier. 
Additional features of the present invention include coarse metering to 
insure sufficient fluid to make up the known quantity of fluid in the 
measuring means. A rinse means and rinse cycle are also provided. 
Various sensors and microswitches may be used to sense positioning of the 
slidable carrier for motor-driven applications, and for sensing the end of 
the vacuum cycle during filtering. 
The advantages of the present invention are that it automates nearly all of 
the conventional manual filtration process steps, thereby improving 
reproducibility of the results and throughput efficiency. An operator need 
not be versed or skilled in chemical laboratory techniques in order to 
obtain good results. Some specific advantages include: 
(a) the filtering apparatus is a self-contained unit with all needed items 
except prepackaged filters; 
(b) the filtering apparatus is compact and takes up less space than 
laboratory apparatus; 
(c) no breakable glassware is included; 
(d) disposable pipette tips are no longer required; 
(e) special laboratory techniques which are eliminated include pipetting, 
tweezer handling of filters, manual rinse, and manual detection of 
filtration completion; 
(f) operator involvement is minimized, allowing the operator to accomplish 
other tasks during filtration; 
(g) throughput efficiency for filtering is faster because the number of 
operator steps is grealy reduced; and 
(h) reproducibility of results no longer depends on the skill, consistency, 
and patience of individual technicians.

DESCRIPTION OF PREFERRED EMBODIMENT 
A total filtration system embodying the present invention in one form is 
schematically shown in FIG. 1. A quantitative filtration system 10 of the 
present invention is connected to a vacuum system through a first passage 
or means to apply a vacuum 12 leading to a waste fluid holding tank 14 for 
filtered fluid. A second passage 16 draws a vacuum from the quantitative 
filtration assembly 10 through passage 12 and waste holding tank 14. The 
vacuum is applied by either a portable vacuum pump 18 or through port 20 
to an auxiliary vacuum system. A valve 22 is provided to connect the 
filter assembly 10 up to either the portable vacuum pump 18 or the 
auxiliary vacuum system through port 20. 
A rinse fluid holding tank 24 is connected to the quantitative filtration 
system 10 through an injection port 26. The rinse fluid holding tank 24 is 
pressurized by taking the pressure outlet from the vacuum pump 18 through 
line 28. A valve 30 is provided in the rinse fluid line 28. 
With reference to FIGS. 2-4, the major internal working parts of the 
quantitative filtration assembly 10 will now be explained. A filter 
assembly is provided with a filter support 32. The filter support 32 is 
preferably a fritted glass material, but may be any other suitable filter 
support, such as a wire mesh. The filter support 32 is adapted to support 
a filter medium 34. Depending on the fluid to be filtered, the filter 
medium may be any suitable filter medium. In the preferred use of the 
present invention in filtering metallic wear particles from engine oil, an 
anisotropic filter (i.e., unidirectional) is preferred. A slidable carrier 
and frame means for holding the filter medium 34 (not shown in FIG. 4) 
will be explained in more detail below. 
A vacuum is drawn through the filter support 32. The vacuum system includes 
a suction funnel 36 which is connected to the vacuum system and is adapted 
to draw a vacuum through the filter support 32. 
An in-processing fluid holding tank 38 is provided. The in-processing fluid 
holding tank 38 is adjacent and below the filter support 32 in the 
preferred form. The in-process fluid holding tank is adapted to receive at 
least a portion of a known quantity of fluid positioned above a filter 
medium in the event a portion of the known quantity of fluid is unable to 
pass through the filter medium and must be released. In this manner, 
excess sample fluid or fluid unwanted as a result of error or operational 
failures can be easily collected and removed. In the preferred form, the 
in-process fluid holding tank 38 is a moat which surrounds the filter 
support 32 and is positioned below the filter support 32. 
Above the filter medium 34 is a measuring means 40. The measuring means 40 
measures and presents a known quantity of fluid for filtering. In the 
preferred form of the present invention, the measuring means 40 is a 
metering tube. The metering tube 40 has an open end 42 which is adapted to 
engage the filter medium 34. A fixed distance above the open end 42 of the 
metering tube 40 is an overflow or outlet passage 44. In this manner, when 
the metering tube 40 engages the filter medium 34 a known volume exists in 
the lower end of the metering tube 40 between the end 42 and the outlet 
passage 44. Thus, the metering tube 40 is designed to hold a known 
quantity of fluid for filtering between the end 42 and the overflow 
passage 44. Metering tube 40 is secured into two metering blocks 46 and 
48. The metering blocks 46, 48 together comprise part of the means to move 
the metering tube 40 into and out of engagement with the filter medium 42. 
The metering blocks 46, 48 are guided for movement on two guides or shafts 
50 and 52. The two metering blocks can also be made as a unitary member. 
The inlet funnel assembly 58 is provided with a slidable cover 62, which 
has a viewing glass 64 in the center thereof. The slidable cover 62 pivots 
or slides on a pin near the perimeter of the inlet funnel 58 so as to 
allow the addition of fluid into the funnel. The inlet funnel assembly 58 
is also provided with an injection port 66 which permits the injection of 
rinse fluid into the inlet funnel. The inlet funnel cover 62 is also 
provided with a rinse safety interrupt switch 68 which automatically stops 
the addition of rinse fluid through rinse injection port 66 if the 
slidable cover 62 is opened during the rinse cycle of the filtering 
apparatus 10. The inlet funnel 58 is also provided with a coarse metering 
line 69 which will be explained in more detail below. 
The outlet passage 44 in the metering tube 40 leads to an overflow tube 70 
which drains by gravity into moat 38. As shown in FIG. 4, the moat 38 is 
sloped toward an outlet 72 which in turn is controlled by an outlet valve 
74. The outlet valve 74 for the moat drain 72 is a normally-open valve 
which is closed by moat pin 76 acting against the valve. The outlet valve 
74 uses a leaf spring to bias the valve to a normally open position. Moat 
pin 76 is a spring biased pin secured within the lower metering block 46. 
The moat pin 76 travels in conjunction with the metering tube 40. 
A breakaway in FIG. 4 shows that the moat pin 76 is spring loaded in the 
following manner. The lower metering block 46 contains a bore 78 
therethrough. The bore 78 has a narrow end 80 with an opening only big 
enough to allow passage of the pin 76. A collar 82 attached to the moat 
pin 76 acts against a spring 84. A set screw 86 determines the tension on 
the spring at rest. When the moat pin 76 comes in contact with the outlet 
valve 74, movement of the metering blocks 46, 48 will tend to move the 
moat pin 76 downwardly under pressure against the outlet valve 74. Upward 
movement of the moat pin 76 is resisted by the spring 84, thus biasing the 
moat pin 76 by spring tension against the outlet valve 74. 
A part of the means for moving the metering blocks 46, 48 together with the 
metering tube 40 into and out of engagement with the filter medium 34 will 
be explained with reference to FIG. 5. An outer knob 90 is connected to a 
shaft 92. The shaft 92 is supported for rotation through openings in a 
front support 94 and a second inner support 96. Supports 94 and 96 are in 
turn connected to side frames 98 as seen in FIGS. 2 and 3. Secured to the 
other end of shaft 92 is a cam 100. The cam 100 is mounted eccentrically 
about shaft 92. 
As seen in FIG. 6, cam 100 engages a block 102 which in turn is secured to 
metering blocks 46, 48. Cam 100 engages a projecting surface 104 from the 
block 102, and cam 100 is biased against said surface 104 by a spring 106. 
If desirable, blocks 102, 46 and 48 could be a unitary member so long as 
member 104 is adjustable to allow tension adjustment between metering tube 
40 and filter medium 34. 
As is clear from FIGS. 4-6, rotation of knob 90 moves cam 100 eccentrically 
about shaft 92. Rotation of the cam 100 against the block 104 attached to 
metering blocks 46, 48 by means of plate 102, causes movement of the 
metering tube into and out of engagement with the filter medium 34. One 
advantage of using cam 100 is that knob 90 may be moved in either 
direction to raise or lower the metering tube 40. If desirable, a 
mechanical stop can be used limiting rotation of the knob 90 in only one 
direction and through only 180.degree.. 
The cam mechanism shown in FIGS. 5 and 6 is one suitable method for raising 
and lowering the metering tube 40. Other suitable arrangements could be 
employed, such as a rack and pinion. 
The slidable cover 62 is provided with mechanical interlock which is 
schematically shown in FIGS. 7A-B. At one edge of the slidable cover 62, a 
pin 108 is secured thereto. At the lower end of pin 108 is a projecting 
flange 110. Secured to the filtration apparatus adjacent the upper 
metering block 48 is a slotted receptacle 112. FIG. 7A schematically shows 
the metering block 48 in its upper position. When the metering block 48 is 
in its upper position, the slotted receptacle 112 receives the projecting 
flange 110 of the slidable cover 62. When the slidable cover 62 is in 
place covering the inlet funnel assembly 58 and the metering block 48 is 
in its upper position as shown in FIG. 7A, the mechanical interlock 
between parts 110 and 112 prevents removal of the slidable cover 62. That 
mechanical interlock prevents the introduction of fluid when the metering 
tube 40 is in its upper position and, thus, not ready to accept addition 
of sample fluid because tube 40 is not engaged with filter medium 34. The 
pin 108 is provided with a spring 111 and a washer 113. The spring 111 
causes the funnel cap to seal with the funnel tight enough to prevent 
solvent from coming out during the rinse cycle. The up and down movement 
of the washer 113 provides actuation of switch 68 when the funnel cap is 
opened. The upward movement of funnel cap 62 is caused by the lower edge 
of the funnel cap moving over a corresponding edge on the funnel. 
FIG. 7B shows the metering block 48 in its lower position when metering 
tube 40 is adjacent and engaging the filter medium 34. Disengagement of 
parts 110 and 112 deactivates the mechanical interlock and permits an 
operator to remove the slidable cover 62 to add sample fluid for 
filtering. 
The operation of the moat pin 76 and the moat outlet valve 74 will be 
described in more detail in conjunction with FIG. 8, which schematically 
shows the moat pin 76 in its lower position biasing outlet valve 74 to its 
closed position over drain outlet 72. The lower floor of moat 38 slopes 
toward drain outlet 72. Along one side of the moat 38 a leaf spring 114 is 
attached to the moat outlet valve 74. The leaf spring 114 is secured to a 
support 116 such that without any pressure on outlet valve 74, the leaf 
spring 114 biases outlet valve 74 away from drain 72. 
The outlet drain 72 leads to the suction funnel 36 which is the same outlet 
waste passage for filtered fluid drawn through the filter medium 34. 
In addition to moat pin 76 serving as a means of closing the moat outlet 
valve 74, the moat pin 76 also serves to insure that the filter medium 34 
is properly placed between metering tube 40 and filter support 32 before 
the metering tube 40 is lowered. As shown in FIG. 8, and also FIG. 10, a 
slidable carrier 120 is provided. The slidable carrier 120 has grooved 
side channels for receiving the filter medium 34 mounted in a filter 
holder or frame 124. For convenience, the filter holder 124 can be a 
conventional 35 mm slide holder. A recess or hole 126 is provided in the 
slidable carrier 120, and it is designed to allow moat pin 76 to register 
therewith. Similarly, a recess or notch 128 is provided in one edge of 
filter frame 124. 
The moat pin 76 helps insure proper placement of the filter medium 34 and 
prevents undesired operation of the filtration assembly. Unless the 
slidable carrier 120 and the filter frame 124 are in proper registry, the 
moat pin 76 cannot pass through the opening 126 in the slidable carrier 
120 or the notch 128 in the filter frame 124. If the moat pin 76 cannot 
pass through those recesses 126, 128, the operator will be unable to lower 
the metering blocks 46 and 48 because the moat pin 76 wll prevent complete 
lowering of the metering blocks and the metering tube 40. If the metering 
tube 40 and the metering blocks 46, 48 are not lowered into their 
lowermost position so that metering tube 40 engages filter medium 34, the 
mechanical interlock 110, 112 on the cover 62 will prevent the operator 
from adding fluid to the inlet funnel 58. Thus, the moat pin 76 is 
designed to insure in conjunction with the slidable carrier 120 and the 
filter frame 124 that the apparatus 10 is ready for operation and the 
filter is in place before any fluid is added to the system. 
FIG. 9 is a top plan view of the moat 38 surrounding the filter support 32 
and being located below filter support 32. The slidable carrier 120 slides 
along surface 130 above the moat 38. Two sets of rollers 132-3 and 134-5 
are provided on either side of filter support 32 and spaced above the 
surface 130 and from each other by space bars 137, 139. The slidable 
carrier 120 slides between the rollers and surface 130. The purpose of the 
rollers 132 and 134 is to serve both as a guide for the slidable carrier 
and also to prevent vertical movement of the slidable carrier relative to 
the filtration assembly. The purpose of the separate rollers 132 and 133 
(and also 134-5) provide clearance for the optional insertion of thick 
samples and transport to the analysis area 140. 
If desired, the moat is provided with an extension 138. The purpose of the 
extension 138 is to provide a second opening 140 for analysis of any 
materials retained on the filter medium 34. For example, after the filter 
medium 34 is removed from the filtering station above filter support 32, 
the slidable carrier 120 can be moved to position the filter medium 34 
over opening 140 for analysis. One type of analysis could be an X-ray 
analysis of wear metal particles in engine oil. 
FIGS. 10 and 11 illustrate the slidable carrier 120 of the present 
invention in more detail. When using an anisotropic filter, the present 
invention can be adapted to insure that the filter medium 34 is placed in 
its proper orientation in the slidable carrier 120. A blocking screw 142 
may be positioned in the channel in the slidable carrier 120 for receiving 
the filter frame 124. If the filter frame 124 is notched at one corner 
144, then the filter frame 124 can only be placed in one orientation in 
the slidable carrier 120 which will permit the notch 128 in the frame 124 
to be in registry with the opening 126 in the slidable carrier 120. The 
metering tube 40 cannot be brought into engagement with the filter medium 
34 unless the filter frame 124 containing the filter medium is properly 
placed within the slidable carrier 120. 
The present invention can also be adapted to prevent inadvertent filtering 
when no filter medium is in place. A blocking mechanism 148 can be added 
to the slidable carrier and biased away from a fixed stop 150 by a spring 
152. The spring 152 and the blocking means 148 should be positioned so 
that when the spring is at rest, the blocking means 148 covers the recess 
126 in slidable carrier 120. If no filter frame 124 is positioned properly 
in slidable carrier 120, the blocking means 148 would prevent the moat pin 
76 from passing through recess 126 and therefore prevent operation of the 
apparatus. Thus, filter operation is only possible when filter frame 124 
is properly positioned in slidable frame 120 with the filter frame 124 
pushing the blocking means 148 away from its blocking position at recess 
126 in slidable carrier 120. 
As shown in FIG. 11, the upper surface of slidable carrier 120 may be 
provided with a gear rack 154. The gear rack 154 is designed to engage a 
pinion gear attached to motor 156. Thus, if motorized movement of the 
slide 120 is desired, the motor 156 and the gear rack 154 may be utilized. 
If manual operation of the slidable carrier 120 is desired, scribes on the 
upper surface of carrier 120 can be provided to mark proper positioning of 
the carrier relative to the filtration assembly frame. 
The operating sequence of the filtration assembly 10 of the present 
invention will now be described. The slidable carrier 120 is removed from 
the filtration assembly, and the filter frame 124 with a properly mounted 
filter medium 34 is inserted into the carrier 120. If anisotropic filters 
are used, the proper orientation of the filter must be achieved. Blocking 
means 148 will prevent operation of the apparatus unless a filter frame 
124 is properly placed in the frame 120. Blocking screw 142 will insure 
proper orientation of an anisotropic filter. When the metering blocks 46, 
48 are in their upper position, the slidable frame 120 with the properly 
oriented filter medium 34 is placed underneath rollers 132, 134 into the 
apparatus 10 until opening 126 is positioned over outlet valve 74. Proper 
registry between the notch 128 in the filter frame 124 and the outlet 
valve 74 with moat pin 76 is necessary before the metering tube 40 can be 
lowered. While the metering blocks 46, 48 are in their raised position, 
the mechanical interlock 110, 112 prevents removal of the slidable cover 
62 and therefore addition of any fluid into the apparatus. When the 
slidable carrier 120 is properly positioned, the knob 90 can be rotated to 
lower the metering blocks 46, 48 and tube 40 so that metering tube 40 
engages the filter medium 34. When the metering tube is properly engaged 
with the filter medium, the slidable cover 62 can be removed. 
The present invention is designed to insure adequate addition of sample to 
provide sufficient fluid for measuring a known quantity of fluid in the 
lower end of metering tube 40. Coarse metering is provided by the scribe 
69 in inlet funnel 58 and the structure of inlet funnel 58. Accurate and 
reproducible coarse metering occurs provided that the filter is 
sufficiently impermeable so that negligible filtration occurs during 
coarse and final metering. This is easily accomplished in most cases as 
long as no vacuum is applied. Coarse metering is obtained by pouring 
sufficient sample in the inlet funnel 58 until it reaches the scribe 69. 
The length and the shape of the outlet 56 of the inlet funnel 58 are 
adjusted to slow delivery of the liquid to the metering tube 40. With 
suitable adjustments of the scribe 69 and the structure of the inlet 
funnel for the viscosity of various fluids, it is possible to obtain 
predictable coarse metering. Coarse metering also tends to be 
self-compensating: if a person pours fast, the person tends to fill 
somewhat above the scribe 69 before stopping; if a person pours slowly, a 
certain portion of the liquid will already have passed through the inlet 
funnel. Coarse metering has found to be accurate and reproducible and 
permits conservation of sample fluid while assuring a sufficient supply 
for the final metering at the lower end of the metering tube 40. 
The final metering at the lower end of metering tube 40 is accomplished by 
allowing the sample fluid to fill the known volume between the open end 42 
of the metering tube 40 and the outlet passage 44. Any fluid in excess of 
the known quantity of fluid to be filtered flows out passage 44 through 
overflow passage 70 into moat 38. At that point, moat pin 76 has closed 
outlet valve 74 and the excess fluid is retained in the moat 38 and not 
intermixed with the sample fluid. 
The inlet cover 62 is closed and the vacuum is applied to the system. As 
the vacuum is applied to the suction funnel 36, filtered fluid is drawn 
through the filter support 32 into waste line 12 and into a waste holding 
tank 14. A vacuum transducer 160 (FIG. 1) on waste holding tank 14 can be 
provided to sense an increase in pressure and thus an indication that the 
known quantity of fluid in metering tube 40 has passed through the filter. 
The vacuum transducer 160 can be designed to automatically shut off the 
vacuum system indicating completion of filtration. 
After the completion of filtration, the rinse cycle commences. Rinse fluid 
is inroduced from rinse fluid holding tank 24 through injection port 66 
into inlet funnel 58. The round inlet funnel 58 in conjunction with the 
angle injection port 66 produces a swirling rinse to completely rinse out 
the apparatus and insure that residual sample fluid of the known quantity 
of fluid on the walls of metering tube 40 is passed through the filter 
medium 34. 
At the completion of the rinse cycle, the metering blocks 46, 48 can be 
raised. Raising the metering blocks in turn raises the moat pin 76 which 
allows any excess fluid in the moat 38 to drain into the same suction 
funnel and waste line as the filtered fluid. 
The in-process holding tank or moat 38 of the present invention serves 
several purposes. First, it serves to hold any excess sample fluid flowing 
out of the overflow passage from the metering tube 40, and it prevents the 
excess fluid from intermixing with the known quantity of fluid to be 
filtered. Second, the moat 38 is designed to receive any fluid that spills 
as a result of operator error or operational failure. Third, the moat 38 
is designed to receive fluid in the event the filter medium 34 becomes 
plugged and filtration cannot be completed. In that event, the metering 
tube 40 would have to be raised even though a quantity of fluid still 
remains in the lower end thereof. When the metering tube is raised, any 
remaining fluid in metering tube 40 simply flows into the moat 38. Another 
feature of the moat 38 is that the fluid therein drains into the same 
waste system using the same line and vacuum system as the fluid filtered 
through the filter medium 34. 
During the solvent rinse, it is possible to slide the cover 62 off of the 
inlet funnel 58 because of disengagement of the mechanical interlock 110, 
112. In that event, a rinse safety interrupt switch 68 may be provided to 
shut off the flow of rinse fluid. 
Another feature of the present invention is that, during the filtering 
cycle, movement of the filter medium 34 is minimized. The rollers 132-135 
prevent vertical movement of the filter medium 34 relative to the filter 
support 32 and the metering tube 40. The moat pin 76--as a result of its 
engagement in opening 126 in slidable carrier 120--prevents linear 
movement of the slidable carrier 120. 
Another feature of the present invention is that a filter frame 124 is 
provided to minimize handling of the filter paper. Many filter media or 
papers are very fragile and utilization of a filter frame 124 minimizes 
damage and contamination of the filter medium during handling. Framed 
filters may also be easily stored for future reference and cataloging. 
Another feature of the present invention is the structure of the slidable 
inlet cover 62. The purpose of the slidable cover 62 is threefold: First, 
during the rinse cycle, rinse fluid is pumped through the angled injection 
port 66 up against the cover to produce a dispersion and a swirling 
action; second, the cover prevents splash-out during rinse; and third, the 
inlet cover prevents through the mechanical interlock 110, 112, any 
introduction of sample at an inappropriate time. 
The filtration assembly of the present invention can be operated either 
manually or with a motorized drive using the rack and pinion gear shown in 
FIG. 11. The motor can be electrically interlocked through microswitches 
to prevent operation of the motor unless the metering block is in the 
upper or clear position. Automatic position monitoring of both the 
slidable carrier 120 and the metering tube 40 can be achieved by 
mechanically operated switches or by strategically placed 
phototransducers. An additional positioning function is served by the moat 
plug pin 76, which must pass through the recess 126 in the slidable 
carrier 120.