Apparatus and method for sampling a liquid

An apparatus for sampling a liquid includes a chamber into which liquid is pumped and a pressure sensor that detects the time instant when liquid reaches a sample duct from the container. The pump is controlled by a signal from a timing device that is initiated by the pressure sensor so that the volume of liquid being pumped through the sample duct can be controlled by the timing device switching off the pump. The pump is started at predetermined time intervals by a periodic timer. After the pump is switched off, the chamber is drained of liquid and reoccupied by air.

BACKGROUND OF THE INVENTION 
The invention relates to apparatus and a method for periodically sampling a 
liquid and particularly to sampling wastewaters. 
The purpose of sampling a liquid is to obtain a representative portion of 
the liquid from which its average composition can be determined. When 
sampling a flowing liquid, its average composition over a specific period 
of time is required (for example, a 24 hour average). This is normally 
done by compositing a set of individual sub-samples taken with sufficient 
frequency to include all the effects of changes in composition of the flow 
during the period of the sample. The more frequent the taking of these 
individual samples, the more representative will be the complete sample. 
The frequency of the sampling may be at regular time intervals or at time 
intervals in inverse proportion to the rate of flow of the sampled liquid. 
The latter case being flow-proportional sampling. 
The individual sub-samples may be composited in a single container to give 
a gross average or alternatively, may be distributed into an array of 
separate containers to aportion the flow from hour to hour. 
When sampling wastewaters it is important to include any solids waste that 
is being carried by the water. Wastewaters normally flow along open 
channels or ventilated drains and samples of this liquid are usually 
obtained by pumping a small volume into one or more containers at 
predetermined time intervals. Wastewaters often contain settleable 
particulate matter like washings and grit and also contain fine solids 
matter in suspension within the liquid. To obtain a representative sample 
of these solids, the wastewater must be pumped at high velocity, to 
prevent the settleable matter from being left behind, and be transported 
through tubing of sufficient bore to avoid the risk of internal blockage 
within the sampling apparatus. 
Wastewaters may also contain solids matter in the form of debris like rags, 
leaves and other coarse material which is not normally required to be 
included in the sample. These solids must not be allowed to obstruct the 
inlet or the internal parts of the sampling apparatus. 
Sampling apparatus for wastewaters have hitherto been based on powerful 
pumps delivering large individual samples through large bore delivery 
tubes. However, if the total sample volume is much over one gallon, the 
sample becomes difficult to carry and therefore most practical sampling 
devices have a limited capacity of about one gallon. So if the individual 
samples are of large volume, then the frequency of these samples must be 
correspondingly low and so the accuracy of the total sample may also be 
low. 
SUMMARY OF THE INVENTION 
According to the present invention there is provided apparatus for and a 
method of sampling a liquid. The apparatus for sampling a liquid 
comprising: 
A HOUSING HAVING AN INLET FOR LIQUID TO BE SAMPLED AND AN OUTLET FOR A 
SAMPLE; 
A DISPLACER OF LIQUID IN THE HOUSING LINKING THE INLET TO AN ENTRY TO A 
WORKING CHAMBER; 
THE WORKING CHAMBER HAVING A PLENUM OF SECTIONAL AREA SUCH THAT LIQUID 
BEING DISPLACED INTO THE WORKING CHAMBER THROUGH THE ENTRY IS REDUCED IN 
VELOCITY AND ENABLED TO FORM A UNIFORM RISING SURFACE; 
DRIVE MEANS FOR THE DISPLACER WHICH, IN A FIRST STATE, CAUSE THE DISPLACER 
TO DRIVE LIQUID FROM THE INLET INTO THE CHAMBER BY WAY OF THE ENTRY AND, 
IN A SECOND STATE CAUSE THE DISPLACER TO ALLOW LIQUID IN THE WORKING 
CHAMBER TO PASS FROM THE ENTRY TO THE INLET; 
A SAMPLE DUCT JOINING THE OUTLET TO A SAMPLE TUBE PROJECTING INTO THE 
PLENUM, THE SAMPLE TUBE DEFINING AN OPENING DISPOSED IN AN OPPOSITE 
DIRECTION TO THE UPWARD DIRECTION OF MOVEMENT OF A FREE SURFACE OF LIQUID 
DISPLACED THROUGH THE ENTRY INTO THE WORKING CHAMBER BY THE DISPLACER WITH 
THE DRIVE MEANS IN THE FIRST STATE, THE OPENING BEING SIZED SUCH THAT A 
SENSIBLE PRESSURE INCREASE IS CREATED WITHIN THE CONTAINER WHEN DISPLACED 
LIQUID OBTURATES THE OPENING; 
A SENSOR FOR DETECTING PRESSURE CHANGES IN THE PLENUM; AND 
SWITCH MEANS LINKING THE SENSOR TO THE DRIVE MEANS SO THAT ON THE SENSOR 
DETECTING A PRESSURE INCREASE IN THE PLENUM DUE TO OBTURATION OF THE 
SAMPLE DUCT BY A LIQUID SURFACE THE DRIVE MEANS IS SUBSEQUENTLY CAUSED TO 
SWITCH FROM THE FIRST STATE TO THE SECOND. 
The method of sampling a liquid comprising the steps of: 
pumping a liquid into a chamber to provide in the chamber a rising free 
surface of the liquid acting to displace air or other gas in the chamber 
through a tube projecting towards the rising surface in the chamber; 
allowing the rising liquid to enter and so close the tube end; 
sizing the entry to the tube end such that a flow of liquid therein creates 
a sensible pressure rise within the chamber; 
conveying liquid in the tube out of the chamber to provide a sample; 
ceasing to pump the liquid after a predetermined time interval following 
the pressure rise within the chamber; and 
thereafter allowing residual liquid in the tube to fall back into the 
chamber and liquid in the chamber to fall, under the action of gravity, 
out of the chamber. 
It is an object of this invention to provide a means of obtaining liquid 
and wastewater samples in small volumes from a liquid periodically flowing 
at high velocity through tubing of large bore. 
Other objects of this invention will be apparent hereinafter from the 
specification and from the recital of the appended claims, particularly 
when read in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a housing 1 with an inlet 2 leading to a displacer 3. A 
delivery tube 5 communicates the displacer to a working chamber 4 fixed to 
the housing. The working chamber 4 leading to the outlet 6 through a 
sample duct 7 which protrudes into the working chamber and has an opening 
8 with an axis in a generally vertical attitude. Outlet 6 is positioned 
above a collection vessel 9 fixed to the housing 1. Inlet 2 is immersed in 
the liquid 10 to be sampled. A pressure sensor 11 detects the pressure 
within the working chamber 4 and can communicate a signal to switching 
means 12 when a sensible pressure rise occurs within the working chamber. 
The switching means 12 encorporates a timing device 13 to delay the 
transmission of a switching signal to drive means 14. Drive means 14, 
encorporates a periodic timer 15 which intermittently operates the 
displacer 3. Switching means 12 interrupts the driving of the displacer 3 
and re-sets the periodic timer 15. Switch 16 switches the apparatus on or 
off. 
When switch 16 is put into the "on" position the periodic timer 15 operates 
the drive means 14 after a predetermined time period. This causes 
displacer 3 to operate in its first state and draw liquid 10 in through 
the inlet 2 and force it through delivery tube 5 into the working chamber 
4 through entry 17, where it forms, without splashing, a rising uniform 
surface within the working chamber and displaces air out through delivery 
duct 7. When the liquid surfaces reaches the opening 8, liquid flows into 
the sample duct 7. Because liquid has a higher viscosity and density than 
air, the acceleration of the liquid into the sample duct 7 creates a 
pronounced increase in pressure within the working chamber. This causes 
pressure sensor 11 to send a signal to switching means 12. After a 
predetermined time delay set by timing device 13 a second signal is sent 
from the switching means to the drive means 3 causing the drive means to 
change from its first state to operate in its second state and operate the 
displacer to drive liquid from the working chamber entry 17 backwards to 
the inlet 2, thereby emptying sample duct 7 and working chamber 4. During 
the predetermined time delay, a predetermined volume of liquid is being 
forced through sample duct 7 into the collection vessel 9. This volume of 
liquid is an individual sample. 
This series of operations is repeated cyclically by the periodic timer 15 
to produce a set of individual samples over a period of time to form a 
composite sample within the collection vessel 9. 
Referring to FIG. 2 of the drawings, the apparatus of this embodiment of 
the invention, which is intended mainly for sampling from open channels, 
stands on a floor 39 of a channel. The apparatus has a vertical pump 
housing 30 and a pair of rigid legs 35 (only one shown) forming a self 
supporting tripod. The two legs 35 are secured to a rigid frame 33 by four 
leg struts 36. The pump housing 30 is attached to the rigid frame by two 
brackets 32. To protect the contents of the frame 33 from the environment 
a weatherproof cover 34 encloses the frame. A supporting member 31 rigidly 
locates a working chamber 9, an outlet 17 of a sample duct 11 and locates 
a sample collection vessel 18. The channel contains a liquid flowing in a 
direction indicated by arrow 40. 
An inlet opening 6, angled to the flow direction, faces downstream to avoid 
any debris adhering to the inlet. The length of an inlet tube 5 locates 
the opening 6 at a distance from the heel 37 of the housing 30 so that any 
elongate debris that may be present in the liquid and may collect on the 
heel 37 will not usually be long enough to trail alongside and be drawn 
into the opening 6. 
A two bladed centrifugal impeller 1 mounted on a stainless steel shaft 2 is 
driven by a low voltage electric motor 3. A seal 4 prevents liquid 
entering the motor 3 and pump housing 30. A pump outlet 7 leads through a 
delivery tube 8 to the working chamber 9 having a diffusing section 10, a 
splash ring 13 and an opening 14. A flexible sample duct 11 forms an 
airtight fit within opening 14 and extends below splash ring 13. A 
bellmouth piece 12 fits onto the inlet of the sample duct 11; an outlet 17 
to the sample duct being held over the neck of a collection vessel 18 by 
the supporting member 31. 
An air tube 15 leads from an aperture 16 in the top of the working chamber 
9 to diaphragm chamber 19 to isolate the damp air within the working 
chamber from a pressure switch 21. Damp air can adversly affect electric 
pressure switches. The diaphragm chamber 19, which can be opened to clean 
out any condensate, contains a flexible diaphragm 20 through which 
pressure changes are freely communicated to the pressure switch 21. Such 
pressure changes close electric contacts within the pressure switch 
thereby signalling electronically the pressure change through a cable 22 
to control box 27. 
The control box 27 contains a periodic secondary timer (not shown) linked 
to a control knob 28 for switching on the motor 3 at predetermined time 
intervals that can be preset from one minute to sixty minutes. A knob 23 
is linked to a primary timer (not shown) to control the time interval 
between the signal from pressure switch 21 and the switching off of the 
electric motor 3. Socket 24 enables the periodic timer (secondary) to be 
externally governed by a sensor monitoring the flow rate of liquid within 
the channel so that the frequency of sampling is in proportion to the flow 
rate. Socket 25 is a power input socket for a battery cable (not shown). A 
cable 26 supplies power to the electric motor 3. An on-off switch 29 
controls the supply of electrical power to the primary and secondary 
timers. 
When a sample of the flow is required switch 29 is put in the ON position 
thereby activating the secondary timer. After the period of time preset by 
sample interval knob 28 power is switched through the cable 26 to the 
motor 3 which rotates the centrifugal impeller 1 at high speed forcing 
liquid up through delivery pipe 8 into the diffusing section 10 of the 
working chamber 9. Diffusing section 10 reduces the velocity of inflowing 
liquid to avoid spurious obturation of the inlet 12 by liquid splashing 
onto that inlet. Splash ring 13 inhibits liquid reaching the aperture 16 
at the top of the working chamber. The continuing inflow of liquid causes 
a liquid surface to rise until it reaches the bell mouth opening 12. 
Liquid then obturates the opening preventing air from leaving the working 
chamber. The higher viscosity and density of the flowing liquid creates an 
increase in pressure in air trapped above the free surface of liquid 
within the working chamber 9. 
This increase in pressure is transmitted through aperture 16 and tube 15 to 
diaphragm chamber 19 causing the diaphragm 20 to deflect and thereby 
communicating the pressure through tube 29 to the pressure switch 21. 
Electrical contact within switch 21 signals the primary timer within the 
control box 27 through cable 22. After a small time interval predetermined 
by the setting of control knob 23, the power to motor 3 is switched off 
and the secondary timer is reset. During this primary time interval the 
pump impeller 1 has been forcing liquid through the sample duct 11 into 
the collection vessel 18. When the impeller 1 stops rotating the pumped 
liquid decelerates and then flows backwards under gravity returning 
through sample duct 11, down through working chamber 9, delivery tube 8 
and out through the intake 5. 
All the liquid that has passed sample duct opening 17 will be retained in 
the collection vessel. The remainder of the pumped liquid will have flowed 
out through inlet 6 thereby dislodging any debris that may have been 
attracted to the inlet opening. A small volume of liquid will remain 
within the pump because the pump is submersed beneath the flow surface. 
This sequence of events is repeated cyclically by time intervals 
predetermined by the setting of control knob 28. 
A portable version of the embodiment of the invention has in its 
construction an anodised aluminium alloy for the legs 35, leg struts 36, 
frame 33 and cover 34. The pump housing 30, brackets 31, working chamber 9 
and bottle 18 are made from polypropylene. Both aluminium alloy and 
polypropylene are reasonably resistant to adverse environments. 
A 12 volt permanent magnet DC motor of 20 watts power drives the impeller 1 
at 4500 rpm to deliver about 250 cubic centimeters per second of liquid 
through delivery pipe 8. The bore diameter of the delivery pipe 8 being 12 
millimeters and of the sample duct being 10 millimeters. Collection vessel 
18 has a capacity of 5000 cubic centimeters. 
Solid state integrated circuits are used for the primary and secondary 
timers and these control the power to the motor 3 through a 
Darlington-type power transistor switch. 
A 12 volt sealed lead gel rechargeable battery of 4.5 Ampere hour capacity 
is used to supply power to the timers (0.008 amperes -- continuous) and 
the motor (1.7 amperes -- intermittent). The time taken for the motor to 
complete a sampling cycle is typically less than 3 seconds so that small 
4.5 ampere hour batteries can give over 2500 individual samples. Sample 
interval knob 28 has ten settings ranging from 1 sample per minute to 1 
sample per 60 minutes. The settings for control knob 23 are graduated in 
terms of cubic centimeters of liquid delivered through opening 17 on each 
operation of motor 3 rather than in terms of time delay on switching off 
the motor. To avoid continuous running of the motor 3 when liquid level in 
the channel falls below the inlet 6, the motor is switched on for a 
limited period of only 6 seconds on each cycle of the secondary timer.