HYDROLOGICAL SERVICES HS-55 Instruction Manual

_______________________________________________________________________________________ INSTRUCTION MANUAL GAS PURGE COMPRESSOR MODEL HS-55 Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 100-01

_______________________________________________________________________________________ CONTENTS Page I GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HS55 100-03 II UNPACKING YOUR HS-55 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HS55 100-04 III TESTING YOUR SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HS55 100-04 IV SPECIFICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HS55 100-05 V INSTALLATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HS55 100-014 VI OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HS55 100-017 VII MAINTENANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HS55 100-018 VIII BUBBLE RATE ADJUSTMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HS55 100-018 IX FAULT FINDING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HS55 100-019 X DRILLING TEMPLATE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HS55 100-019 XI APPENDIX A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HS55 110-01 XII APPENDIX B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HS55 110-08 XIII APPENDIX C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . HS55 110-013 Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 100-02

_______________________________________________________________________________________ HYDROLOGICAL SERVICES HS-55 GAS PURGE COMPRESSOR I GENERAL The HS-55 has been designed to replace conventional nitrogen gas bottle supply bubble units/gas purge systems for water level measurement. The design of the unit centres around the tried and proven Hydrological Services’ Model HS23 Dry Bubble Unit with Differential Regulator (BU07). The HS-55 incorporates a maintenance free Air Drying System that utilises a “Self Purging” Membrane Filter Dryer and Auto Drain Moisture Traps to ensure a moisture free system. The effectiveness of the HS-55 Filter Dryer is such that the unit also removes up to 20% of oxygen from the air it dries, thus helping to reduce aquatic growth at the river end. The unit is maintenance free for up to 5 years. GAS PURGE COMPRESSOR- MODEL HS-55 The HS-55 incorporates a 3-Way switching valve for manual purging of the system. Continuous purging occurs when the valve is rotated to “Purge” position. This is similar to Dry Bubble Unit (Model HS23) functions. For instructions how to use the valve please refer to (page 18) of this manual. Auto Drain” Moisture Trap The system features an Auto Drain Moisture Traps. When the traps sense moisture build up, a relief valve is opened and the moisture is purged under pressure to atmosphere. Voltage Regulator Protection If the pressure lines or fittings on the HS-55 develop a leak that cause the loss of air pressure in the system, the air compressor could run continuously as it attempts to fill the air receiver. If this occurs, the system voltage could fall below 12 Volts. At 11V the voltage regulator switches off the power to the motor until the battery is recharged to 12V at which time the system will commence normal operation. This feature protects the compressor pump and motor from damage. Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 100-03

_______________________________________________________________________________________ II UNPACKING YOUR HS-55 The carton should include: C Model HS-55 Gas Purge System C This Instruction Manual C Any Accessories you have ordered Please verify you have received these items. It is recommended that you visually inspect the HS-55 to verify all components and connections are secure. For more details about HS-55 Purge System (see page 8 to 13 for item details ). III TESTING YOUR SYSTEM Before installing the HS-55, you may wish to test the system prior to going to the field. Testing the HS-55 will familiarize you with the compressor in an environment where it is easy to work. The HS-55 is designed to be mounted in a vertical position. The water /oil separator will not function properly if the unit is in a horizontal position. Once the HS-55 is positioned, connect the power leads to an appropriate power supply (12VDC, 38 Amp Hour minimum). Turn on power switch to start the compressor pumping. The pump will take approximately 4 minutes to fill the air tank to maximum pressure of 85 p.s.i, (600 Kpa) and will then automatically switch off. Connect to the outlet port (see figure 6) a short piece of river line tubing and secure the end into a container of water. Check the bubble rate setting as required (factory setting is equivalent to 80 bubbles per minute). If the bubble rate requires adjustment, (see page 16 for instructions). Please note, if the HS-55 is to be tested without a pressure transducer fitted, then the red valve on the instrument port (see figure 6) is to be closed prior to the test operation commencing. There are two pressure gauges fitted to the HS-55. The unit attached to the pressure regulator (figure 2, № 44) indicates the set feed pressure (factory set at 400 Kpa) this pressure should be retained at this setting, however, if adjustment is required, pull the supply regulator knob (figure 2, № 49) and turn it until the required setting is achieved, once finished, push the knob back in place. The second pressure gauge (figure 1, №10) is located on the side of the HS-55, indicates the air tank pressure. To test the purge operation, press the red button. Purge duration should be 5 seconds maximum during testing. Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 100-04

_______________________________________________________________________________________ IV SPECIFICATION Power: 12VDC. (38 amp hour battery minimum). Maximum Current: 17 Amps Average Current Draw: Approx. 14 mA (Averaged over a week and based on 60 bubbles/min) Compressor: Precision Piston Receiver Capacity: (Air Tank) 13.5 litres (3.5 U.S. Gallons) Normal Operating Pressure Range: 60 p.s.i, (400 kpa) to 85 p.s.i, (600 kpa) Duty Cycle: Typically 10 hours per year at 60 bubbles/minute 2 hours per year at 10 bubbles/minute Maximum Head: 30 metres (100 ft) using 200 metres (656 ft) of river line. Note: Water head increases for shorter river lines [i.e. 45 metres (150 ft) of water head with 100 metes (330 ft) of river line]. Maximum Length: (River Line) 200 Metres (Using 3/8" O.D.x1/8" I.D Tubing) Maximum Purge Pressure: 85 p.s.i, (600 kpa) Minimum Purge Pressure: 60 p.s.i, (400 kpa) Differential Pressure: 3 p.s.i, (20 kpa) to 5 p.s.i, (35 kpa) Purging: Manual - 3-Way Switching Valve Bubble Rate: User selectable from 60 to 120 bubbles/minute. (See page 16 for adjustment details). Air Dryer: 5 year maintenance free “Self Purging”Hydrophobic filter Environment: -30 deg C to +80 deg C (-40 deg C to +80 deg C optional) Pressure Connections: Outlet 3/8" dia tube, instrument 1/4 NPT Adaptor fittings available for most transducers Mass: Approx. 30 lbs, (13 kg) Dimensions: 24" H x 12" W x 15" D (610 mm x 305 mm x 380 mm) Packed Dimensions: 32" H x 16" W x 19" D (810 mm x 410 mm x 480 mm) Mass: Approx. 70.55 lbs, (32 kg) Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 100-05

_______________________________________________________________________________________ OPTIONAL ACCESSORIES: (i) Pressure Transducer (Model W.L 3100/A). Enclosure: Aluminium diecast weatherproof enclosure IP65 Dimensions: 180 x 105 x 100 mm Weight: 1.2 kg Isolation Diaphgram 316 Stainless Steel Operating range: 0-5, 0-10, 0-15 , 0-20, 0-30 metres water head ( Imperial units also available up to 100 feet range). Over pressure: 2.67 x ranges Operating Temperature: WL3100A -40 oC to +70 oC (-40 oF to +158 oF) WL3100 -30 oC to +70 oC (-22 oF to +158oF) (Excluding LCD Display) Humidity : 100 % non condensing Overall Accuracy: ±0.02 % F.S. BSL Long Term Stability: Typically ±0.05 % F.S./annum Pressure Connection: 1/4O Tube Sensor Vent: Vent fitting for Hydrophobic Filters and/or desiccant packs. Calibration: Over full range against water head. Operating Voltage: 9.6 to 16VDC operation ie. from SDI-12 +12 volt supply Current Loop Operating Voltage: 9V to 30V DC for 4-20mA current loop drive ( used as a 2-wire current loop transducer) LCD Backlight: Powered via SDI-12 port (9.6 to 16 VDC) LCD Operating Temperature: -20 oC to +60 oC (-4 oF to +140 oF) Display: range, resolution, time, date and Level reading. Parameters: User selectable via push buttons on the pressure transducer housing LCD Operation: LCD display is standard with auto shutdown after Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 100-06

_______________________________________________________________________________________ 35 seconds idle. Power Consumption: Low standby power < 20 microamps (excluding 4- 20mA loop & LCD) Operating Power: 10mA from SDI-12 supply while taking a measurement and 0.5mA between measurements. Options: All options are user programmable via the SDI-12 interface and saved in an on board EEPROM. Units: Metric or Imperial scaling - (specify when ordering). Output: RS232, SDI-12 and 4-20mA output (ii) Pressure Transducer (Model W.L 2100 Dry). Accuracy: +/- 0.05% B.S.L Operating Temp: -20 deg C to +60 deg C (-4 oF to +140 oF) Combined non linearity, Hysteresis and Repeatability: +/- 0.05% of full scale Long term stability: +/- 0.03% full scale per annum Compensated temp. Range: -20 deg C to +60 deg C (-4 oF to +140 oF) (iii) Pressure Line 1/8" I.D. x 3/8" O.D. heavy wall black polyethylene. SC078-01 - 300M (1000 Ft) Roll SC078-06 - 100M (328 Ft) Roll (iv) Orifice Fitting 1) Copper swivel fitting to mount on 2" G.W.I. pipe (BU07) 2) Gas Chamber Orifice (GCO) (v) Mounting Bench mounting bracket available Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 100-07

_______________________________________________________________________________________ V INSTALLATION (i) Installing the Model HS-55 For proper installation of the HS-55, you will need: C The HS-55 C This Instruction Manual C Installation Kit (provided) Installation for your HS-55 may vary according to your application. Figure 6 illustrates the physical input/output connections on your HS-55. This drawing will help in determining where you should make your connections for power, ground, communications, outlet to orifice and pressure to your sensor (ii) Field Installation (i) The HS-55 must be wall mounted in a vertical position, with the compressor at the top. A mounting bracket can be provided for bench mounting (see drilling template page 17). (ii) Connect your pressure sensor directly to the manifold or connect a pressure line between the manifold and your pressure measurement device. If your measurement device has non- compatible connections, you will need to request adaptor fittings to suit. The manifold port for the transducer is ¼ BSP (iii) The HS-55 requires only one power source to power the compressor. This supply is typically a 12V, 38 amp hour minimum, long life re-chargeable battery with solar or mains charger. (iv) Connecting a Pressure Sensor The pressure sensor can be fitted directly to the manifold or connected using pressure fittings and connecting tube. Figure 7 shows typical installations of a complete pressure measurement system. Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 100-014

_______________________________________________________________________________________ VII MAINTENANCE The HS-55 is almost maintenance-free. You will need to ensure that your power supply is sufficient for the equipment that you have installed. (See Specification Page HS-55 100-06). As the compressor pump’s duty cycle is a few minutes every 48 hours (set at 80 bubbles/minutes), it will last for many years. The voltage sensor prevents the HS-55 from running from running the battery voltage below 11 Volts for long periods of time. This will protect the pump and many other components of the HS-55 in case of a system leaks or other malfunctions. All fittings must be secure. At 80 bubbles/minutes, even a tiny leak will allow the entire gas flow to escape. VIII BUBBLE RATE ADJUSTMENT The chart below has been prepared to show the relationship between the bubble rate at the test orifice and the equivalent rates that would be observed on a sight glass bubble unit. Once the desired bubble rate has been achieved tighten the screw to lock the metering valve. Bubble rate at Equivalent Sight Test Orifice Glass Rate 15 60 21 80 27 100 35 120 Note: Bubble rates detailed above are only an approximate guide for the purpose of conserving gas and have no influence on the performance of the transducer. Example: To adjust the HS-55 to the equivalent of 80 bubbles per minutes, the metering valve should be rotated anticlockwise if the bubble rate is below this amount, or clockwise if bubble rate is above 80 per minute. There should be 21 bubbles per minute emerging from the test orifice. Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 100-018

_______________________________________________________________________________________ IX FAULT FINDING (i) HS-55 Not Powering Up (a) Check battery voltage (b) Check all connections including POWER and GROUND. Both the compressor and control power sources must be connected. (ii) Intermittent Operation C Check your power and ground connection. Moisture over time, will oxidize and corrode connectors and pins; C Verify your power supply or battery voltage. X DRILLING TEMPLATE (See page 20). Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 100-019

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_______________________________________________________________________________________ XI APPENDIX A Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 110-01

_______________________________________________________________________________________ INSTALLATION OF POLYTHENE TUBING AND ORIFICE FITTINGS A. General The polythene tubing - SC078-01 is provided in three hundred metre rolls. It is an extremely durable material that has been used successfully in this application world wide for many years. The tubing is flexible, easy to handle and can be cut with a knife. The orifice fitting is comprised of a moulded polythene threaded cap, an orifice outlet and a tube fitting. The moulded cap will screw onto a standard 50 nominal bore galvanised pipe. B. Orifice Fittings The site and reference level for the orifice fittings should be resolved before installation proceeds. The length of tubing from the HS55 to the orifice should not exceed 150 metres. A suggested method for installation of the orifice fitting is shown below. The site of the orifice should be positioned in quiet water out of the mainstream to provide protection. The area should be free of silting and aquatic growth. The mounting for the orifice fitting should consist of a section of 50 nominal bore galvanised pipe threaded at one end and secured at the appropriate reference level. The securing of the orifice mounting should be such that it can withstand the forces applied by the flowing stream and associated floating debris. C. Preparation of Orifice Tubing Do not open the carton or remove the strapping. To prepare the roll of tubing for use, cut a 300 mm diameter hole in the centre of the cardboard carton, retrieve the end of the tubing from the middle of the coil, then cut the strapping retaining the coil. This method will reduce the possibility of tangling the tubing. Prevent the ingress of dirt etc. from entering the tubing, by taping up the ends. At no time should the open end of the tubing be permitted to contact the soil. D. Orifice Tubing It is recommended that a trench be dug (minimum 600 deep) between the equipment shelter and the proposed positions of the orifice The trenches should not have any low points in their length, they should have a continuous fall to the river. Pull the free end of the coiled tubing to the orifice position laying the tubing in the trench as you go. Refer to diagrams below Remove the protective tape from the end of the tubing, and push the tubing through the 50 nominal bore pipe mounting. Insert the tubing into the tube fitting on the rear of the orifice fitting, ensuring that both ferrules are in place. Tighten the fitting nut. (Refer to the following diagram for tightening procedure). Place the orifice fitting onto the mounting pipe and tighten. Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 110-02

_______________________________________________________________________________________ Cut the tube at the coil end (tape both ends), ensuring that there is sufficient length to make the joint at the HS55 within the shelter. Check the lay of the tubing in the trench to ensure that it is free of coils and kinks. Under no circumstances should the tube form hollows where moisture can be trapped within the tube. Lay the tubing into position in the equipment shelter as required. Remove the protective tape from the end of river line tube. Insert the end of the tube into the appropriate river line fitting on the HS55 and tighten both fittings. (Refer to diagram for tightening procedure). Open the valve to the orifice. Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 110-03

_______________________________________________________________________________________ XII APPENDIX B Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 110-08

_______________________________________________________________________________________ INSTALLATION OF POLYTHENE TUBING AND GAS CHAMBER ORIFICE GC01 E. General Description The G.C.O.1 permits the use of extremely low bubble rates with increased accuracy and near total reduction of lag between actual level rise and orifice pressure. The polythene tubing - SC078-01 can be supplied in 100 or 300 metre rolls. It is an extremely durable material that has been used successfully in this application world wide for many years. The tubing is flexible, easy to handle and can be cut with a knife. The copper construction of the GCO1 deters aquatic growth while the plastic insulating bush prevents contact between the copper chamber and steel pipe. Bubble rates as low as 40 bubbles per minutes (bpm) can be used on fast rising water levels, thereby reducing gas consumption by as much as 60%. The GC01 chamber orifice is constructed of copper and welded to a 2" copper connection . The connection mates with a 2" plastic insulating bush and is taped for a 1/4" National pipe thread (NPT) copper Swagelok a fitting which allows adaptability to the existing standard USGS orifice configuration. The bottom of the chamber include a removable copper screen for cleaning purposes. F. Performance: The following table summaries the results of comparative tests on the H.S demonstration rig with water level rising at 9 meters/hr. Float Operated Encoder (Level in mm) Standard Orifice 100 Bubble/Min Gas Chamber Orifice 10 Bubbles/Min Reading Lag Reading Lag 0 0 -- 0 -- 810 578 232 810 0 1700 1200 500 1700 0 2400 1540 860 2398 2 3600 2294 1306 3588 12 At 3 meters/hr rate of rise the standard orifice still lagged significantly whereas the gas chamber orifice exhibited no appreciable error. Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 110-09

_______________________________________________________________________________________ G. Orifice Fittings The site and reference level for the GCO1 should be resolved before installation proceeds. The length of tubing from the (HS23A or HS23DB) to the orifice should not exceed 150 metres. A suggested method for installation of the GCO1 is shown in (fig1,page 100-03) The site of the orifice should be positioned in quiet water out of the mainstream to provide protection. The area should be free of silting and aquatic growth. The mounting for the GCO1 should consist of a section of 50 nominal bore galvanised pipe threaded at one end and secured at the appropriate reference level. The securing of the Gas Chamber Orifice (GCO1) mounting should be such that it can withstand the forces applied by the flowing stream and associated floating debris. The immersion of the GCO1 in the water should be at level equal or higher to that of CTF point ( see fig 1, page 100-03) H. Preparation of Orifice Tubing Do not open the carton or remove the strapping. To prepare the roll of tubing for use, cut a 300 mm diameter hole in the centre of the cardboard carton, retrieve the end of the tubing from the middle of the coil, then cut the strapping retaining the coil. This method will reduce the possibility of tangling the tubing. Prevent the ingress of dirt etc. from entering the tubing, by taping up the ends. At no time should the open end of the tubing be permitted to contact the soil. I. Orifice Tubing It is recommended that a trench be dug (minimum 600 deep) between the equipment shelter and the proposed positions of the orifice The trenches should not have any low points in their length, they should have a continuous fall to the river. Pull the free end of the coiled tubing to the orifice position laying the tubing in the trench as you go. Remove the protective tape from the end of the tubing, and push the tubing through the 50 nominal bore pipe mounting. Insert the tubing into the tube fitting on the rear of the GCO1 orifice fitting, ensuring that both ferrules are in place. Tighten the fitting nut first and then place the GCO1 assembly onto the mounting pipe and tighten the barrel union. (See fig 2, page 100- 04). Cut the tube at the coil end (tape both ends), ensuring that there is sufficient length to make the joint at the (HS23A, HS23DB) within the shelter. Check the lay of the tubing in the trench to ensure that it is free of coils and kinks. Under no circumstances should the tube form hollows where moisture can be trapped within the tube. Lay the tubing into position in the equipment shelter as required. Remove the protective tape from the end of river line tube. Insert the end of the tube into the appropriate river line fitting on the (HS23A, HS23DB) and tighten both fittings. Finally, Open the valve to the orifice. Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 110-010

_______________________________________________________________________________________ PRESSURE SYSTEM OF HEAD MEASUREMENT GAS PURGE PRINCIPLE The behaviour of a gas flowing under a variable back pressure is utilised as an accurate means of measuring liquid head. Provided that: 1. The density of the gas is nearly the same as air 2. The velocity of gas is low enough so as not to produce significant pressure drop down the line then the system pressure will be equal to the back pressure. This principle is illustrated in the following series of diagrams. In the first example, the tube exhausts to atmosphere and hence the water manometer indicate zero pressure in the system because it is acted on by two equal pressures- the system pressure and the atmospheric pressure Fig.1 - Depicts first stage in application of gas purge principle. Fig.1 also illustrate another feature of all pressure systems, namely the gas endeavours to escape through any material that it is in contact with to find a lower pressure, but for practical purposes, the system pressure reflects the lowest of the outlet pressures. In this case, gas endeavours to pass through the walls of the tubing and fittings, through the water, but elects to take the easier path to atmosphere, which is in effect a “massive leak”. Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 110-014

_______________________________________________________________________________________ Therefore, it follows that in any pressure system that can exhaust to atmosphere, either through a valve or by a large leak, the system pressure is atmospheric and will not reflect any other pressure. Provided air or a gas that has a very similar density to air is used, then the errors from this source can be neglected. Nitrogen is preferred as it does not promote the formation of aquatic growth. The effect of the velocity of the gas can be observed by opening valve A, a higher pressure will be needed to force a high volume of gas through the line, resulting in a pressure drop between valve A and the outlet. Depending force a high volume of gas through the line, resulting in a pressure between valve A and the outlet. Depending on the location of the manometer connection, the manometer will reflect a proportion of this increase in pressure. Fig. 2 – High gas flow * Refer “Bubbler System Instrumentation for Water Level Measurement” - Report of Investigation No. 23 state of Illinois, U.S.A Head To Pressure Conversion As we are not seeking to measure atmospheric pressure, but liquid head, how is the principle applied ? Simply by using two physical characteristics. 1. The tendency of the gas under pressure to “seek” a lower pressure. 2. The ability of a gas to “pass through” liquids. The first characteristic has already been demonstrated . The second characteristic can be appreciated by blowing through a hollow tube (straw) that is immersed in water. No bubble will appear until the pressure in the tube is sufficient to expel all the water from the tube. Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 110-015

_______________________________________________________________________________________ Fig.3 (ii If a tube is immersed in a liquid, it can be seen that the pressure at the bottom of the tube is that exerted by the head of liquid above it. (ii If we connect a gas supply to the tube, the gas will act on the surface of the liquid in the tube and tend to force it down. Assuming there is no change in level, the pressure at the end of the tube must be still the same. (ii If we increase the pressure so that the gas can push out all the liquid against pressure level A, then it follows that the pressure at A will be the back pressure in the system. Also the pressure of each bubble of gas as it emerges from the end of the tube will a that point have an internal pressure equal to the head, and this internal pressure reduces until the bubble emerges at the surface at atmospheric pressure. Therefore the gas acts like a plunger to keep the water out of the tube and in effect, precisely balances the head of water that would naturally occur if the tube was opened to atmospheric pressure. In other words, the system pressure (back-pressure) is supporting a column of water, equal to the pressure head. Hence, any pressure measuring device, if connected to the pressure system will record the pressure head over the tube outlet (orifice). Fig. 4 depicts a water manometer connected to the system and it is recommended that such an arrangement be set up in a workshop for staff training. Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 110-016

_______________________________________________________________________________________ Gas Purge Operational Characteristics In the foregoing, the discussion has been limited to static head conditions, whereas in practice, the levels in stream are constantly fluctuating. “Rising” Situations It is the “rising” or “increasing head” situation that is critical for the correct operation system. Consider a situation where the gas supply is cut off and the level rises- refer Fig.5. Fig.5 - With gas supply cut off. Head increased from H1 to H2, gas is forced down the tube by an amount X. In this situation, it can be seen that whilst the system pressure has increased, it is not equal to the new head H2, but rather H2-x. This condition will remain as long as the gas is shut off. In other words, we now have a “closed” system, which respond to changes in accordance with universal gas law- a constant PV T i i i  To revert back to the open system, we must restore gas supply, which will eventually build up pressure in the system, until the gas escape from the orifice and the system pressure again becomes the precise head measurement. The time taken for the pressure system to respond to a given head pressure is commonly referred to as “follow rate”. The normal procedure is to establish the maximum rate of rise to be measured and adjust the gas flow so that there is no significant lag in the pressure system. However, in order to conserve the gas supply, the flow rate (bubble rate) is set to the aluminium that will meet these conditions. Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 110-017

_______________________________________________________________________________________ Constant Flow Regulator 3 4metersH2O  MAX Constant Supply Pressure Set at 2 to 3 times The head being MIN P R ES S U R E Sherlock Pressure Sensing Unit MAX Measured Head MIN Automatically Regulated pressure Maintained at 3 to 4 metres of Water above head “Falling “ Situation The “falling” situation is of course the reverse of the rising situation except that there is now excess pressure in the system. If we were to suddenly lift up the orifice in Fig. 4, the pressure would drop to the new head, seen by the sudden rush of bubbles. Accordingly, even the lowest bubble rate will suffice for falling conditions. In practice, it may take some time to the pressure to “bleed” away”, but this is usually less than the response time of the sensing unit. Field Requirement of Gas Purge Systems The principle requirements of gas purge are- 1. a fine metering valve and sight feed for accurately setting the bubble rate. 2. Constant regulator to maintain a constant bubble rate with changes of head. 3. Pressure connections and valves for connecting the tubing to the river and the instrument. With the overall provision that the unit must be “leak proof”, otherwise the true head will not be recorded. The configuration most widely used is illustrated in Fig.6 Fig. 6 - Diagramatic view of field gas purge, single line without purge facilities. Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 110-018

_______________________________________________________________________________________ The Hydrological Services bubble unit follows this layout closely, with the optional facilities of bypassing the constant flow regulator valve and supplying the delivery pressure to river line to expedite cleaning of line. Dual Line Connections For long line, the effect referred to in Fig.2 becomes significant and it is usual to make the connection to the system near the orifice, so in effect there are two lines running the orifice. Hence the term dual line. The operation of Hydrological Services units is described in more details in diagrams 15 and 16 of “instruction Manual for Sherlock Pressure Sensing Units”. Pressure Bulb or Closed System The pressure bulb system relies on the normal PV = constant relationship for gases under T i i i pressure. Using similar diagrams to before, the closed system can be illustrated follows:- Fig. 7- Atmosphere- up to the level at which the water seals off the bottom of the bulb. Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 110-019

_______________________________________________________________________________________ Fig.8 - water seals off air and at this point system pressure is still atmosphere. Fig. 9 - system operating under head conditions. As the water rises, the system pressure increases to the equivalent of the head H minus the amount the water rises within the bulb. This amount x, is non-linear and therefore cannot be completely compensated for by simple calibration. Provided the head H does not fall below say 300 mm, error from this cause is minimal. Basically the closed system will give good results subject to gradual changes of water temperature being experienced and the system being completely gas tight. A full discussion of this principle in given in “Low Cost Stream Height Recorders” - Unisearch (N.S.W) publication. Gas Purge Compressor Issue 7: 16/12/03 © Copyright HS55 110-020