GXCM07-12 PROPELLER TYPE CURRENT METER
GXCM07-12 Propeller Type Current Meter is used to measure hourly average flow velocity on given points. Being light, impact and portable, this instrument applies to rivers, pipelines, irrigation and drainage channels, hydrological investigation, and runoff experiments.
|The Rotary Diameter for Propeller
|The Hydraulic Thread Pitch for Propeller
unbiased variance: m≤±1.2%(Normal)
||two signals on every rotary
|Continuous Running Time
|Wading Rod (additional)
CG16-1 wading rod for open channel: 1600 mm in four sections;
Inner supporting type special wading rod for pipeline
1) GXCM07-12 Propeller Type Current Meter is composed of a sensing part, signal transition mechanism, a tail fin, and a wading rod. Instruction of counter is in last page of the text.
2) Sensing Part
Installed on the rotary axis at the front part of the instrument, the sensing part is a three-leaf screw propeller used to sense the flow velocity. Rotate speed of the propeller is proportional to flow rate. The proportional constants are measured in flume testing.
3) Signal Transition Mechanism
Signal transition mechanism is to transform rotating number of propeller into electrical pulse signals for counting by magnet—reed switch. Magnet is vertically installed at the tail of the rotary axis, synchronous rotating with propeller. Reed switch is encapsulated in a water-proof component fixed on the main body close to magnet. When propeller is driven by current, magnet and propeller are rotating synchronously. Every propeller’s rotation induces two times of magnetization between magnet and reed switch. The rotating number is transformed into signals and transmitted to the counter.
4) Tail Fin(Additional)
Tail fin is used to level instrument in open channel during flow measurement. If in shallow, and slow-rate flow, or pipelines, instruments can be leveled by eyeballing, tail fins are not necessary.
5) CG16-1 Type Wading Rod(Additional)
Wading Rod is used to measure water depth, and fixed instrument at the given measuring points. There is a signal transition socket on top of the wading.
4. Operational Principles and Calculation
The functional relationship between flow rate and rotary speed of propeller is determined by flume testing.
V = Kn + C
K—hydraulic thread pitch of propeller,m；
n—rotary rate of propeller,n= N /T,1/s
( N—rotating number of propeller; T—measuring duration)；
Flow rate actually is average rotary rate of propeller in given measuring duration.
The material of the propeller is excellent engineering plastic. After more than ten years of various experiments, this material is proved to be characteristic of impact-resistance, corrosion-resistance, atmosphere-caused deterioration-resistance and good thermal stability, up to technical standards.
Quality of Propeller dominates the capability of the instrument. In order to guarantee accurate value of K in above formula, maintenance of propeller should be emphasized.
1) Prevent it from fierce impact during measurement.
2) Clean out oil and sediment from propeller by water or gas, keeping it clean and dry.
3) In ice seasons, boiled water can be poured on propeller to thaw the ice.
Never use fire to roast it.
4) Propeller is fixed on rotary axis by screw threads. If under fast flow rate, hook spanner should be used to screw it down. Especially for measurement in pipelines of pumping stations, when measurement instantly finishes, reverse current will make the propeller rotate rightabout. Screw thread is easy to loose. Never use wire cutter to screw it down for fear of laceration.
As a crucial component of the supporting system of the current meter, the ball bearing, especially the low-rate section influences the capacity of the instrument. Technical schedule should be conformed to cleanse the ball bearing. Material of ball bearing is 9Cr18 stainless steel. Although it is rust-resistant, careful maintenance for GCr15 stainless steel should be conducted on it. If damaged, the ball bearing can be purchased according to code: D25MX.
Cleansing the ball bearing:
1) Gas Number 120(SY1207—67) or Number 200(GB444—64) is good cleanser.
2) Cleansing should be conducted in a covered porcelain box or aluminum mess tin. Set a copper screen (65 eyes/3.3cm) at 10mm from box bottom to filter the sediment. Prepare three boxes for rough cleansing and finish cleansing.
3) Operators should wash hands with warm water and soap. Other people must not touch the ball bearing by hand.
4) The ball bearing chamber should be cleansed carefully to prevent dust, the archenemy for ball bearing.
5) GB487—84 Instrument oil is 6.3—8.5 centistroke at 50℃. It has excellent viscosity-temperature characteristics and lean harmful substances such as acid, moisture, dust and mechanical impurity, up to strict standards. If other types of instrument oil are used, new calibration on it should be made.
6) Cleanser and instrument oil should accord with international standards, properly deposited, and strictly prevented from water and impurity.
7) Cleansing Procedure
Rotate and clean the ball bearing in first box with gas 10 times, half in one direction. Then turn to the second and third boxes to repeat the same process. Shake off gas, and install the ball bearing instantly,avoiding long-time exposure in air.
If there is any water entering into the ball bearing chamber (judged by sensitivity of propeller’s rotation) after measurement, clean it in time. Hold the circular circumference of ball bearing, shaking the water or sediment out of the ball bearing in axial direction. Then repeat the cleansing procedure in (7).
8) Keep the instrument in airy, dry and noncorrosive room. After 3 –6 months, ball bearing should be checked, cleaned, and oiled.
Dismantling, Washing, Assembling and Oil Filling
Although the bearing chamber and the rotary axis have excellent sealing performance, sordid condition and ignorance of technical principles will induce moisture entering into the chamber. Special attention must be paid during dismantling, washing, assembling and oiling the instrument.
(1) Dismantling and Washing
After measurement, dismantle the current meter body from the bracket. Check the quality of oil in bearing chamber from transparent cap. If the oil is clear without water in it, and sensibility of propeller is normal, the instrument is not requisite to be washed. However, in order to guarantee the accuracy of the instrument, open the transparent cap, add oil into the bearing chamber and squeeze out the water drops from the front part. If the oil in bearing chamber is a little feculent, the instrument should be dismantled completely.
According to reverse sequence of dismantling, assemble the components washed. Tighten the screw caps by hand or hook spanner. Assembling should be checked as followed.
1) The assemblage gap between propeller’s rotary set and main body of the instrument is 0.3—0.4mm which can be measured by thickness gauge. If the gap is too narrow, a gasket can be added to the tread of rotary set to avoid the friction possible to influence sensibility of the instrument. If the gap is too wide, water is easy to enter into the bearing chamber. The two abnormal situations are mainly caused by nonstandard assembling procedure.
2) The bounce gap of rotary set should not surpass 0.03mm checked by dial gauge. Dial the propeller by hand. If rotation of propeller is not so smooth but a little shaky, it is not accordant with standards. Check the bounce gap of propeller axis and rotary set by dial gauge to determine repair methods. Special notice: extrusion of eccentric driven pump is most possible to cause water intake.
3) Sensibility of the instrument is checked by mouth’s blowing. If a slight blowing drives the propeller rotate smoothly without sense of blocking, the sensibility is considered to be eligible. We recommend JGM-1 type flywheel sensibility checking-gauge developed by our institute. The flywheel is working with a suspended cable and a weight. Average moment of friction is determined by rotation duration of the flywheel. Thus, sensibility of the instrument can be judged. It is more reliable to use the gauge to measure sensibility.
4) The instrument assembled should be placed on table. Lay wood block under the body, keeping the propeller suspending. Otherwise, put it into housing immediately.
(3) Oil Filling
Bearing and distance sleeves are washed, then assembled together. If the rotary set is proved to be accordant with technical demands, fill oil in the chamber assembled with rotary set, and the cap. Couple them together instantly and tighten the cap. If the instrument oil throws from the rotary set and there is no air bubble in the cap, the chamber must be filled up. In order to prevent chamber from air bubbles, keep the head of instrument on the top when you tighten the cap. Fill oil in holes of bracket, then assemble it with the main body. Thus, the instrument is full of oil inside.
Signal Transition Mechanism
Signal transition mechanism is composed of magnet and reed switch. The magnet is hold in the magnet box. Remanence is 700—1000Gauss. The reed switch is sealed with epoxy. The minimum ampere-turns is 12—18AT. The magnet and reed switch on one instrument are not exclusively coupled, but exchangeable with other instruments. They are also disposable an purchasable.
Life span of reed switch is greatly influenced by power system load. At 3V and 30mA, it can work 5×105times. After that it should be replaced. 1minute is only needed for replacement, even in field. It’s not necessary to lift the instrument up from water, because reed switch is completely sealed.
Notice the value of power and current of the counter connected. Never choose inductance type counter.
In water with high salinity or industrial polluted wastewater, such as electroplating effluent, Adhesive plaster can be used to wrap the binding posts (positive) of reed switch, avoiding short circuit with main body of the instrument (negative) in water.
After measurement, lift the instrument up from water. Dry it with towels. Put it into housing according to original position. Other accessories should be well placed. Check the placement of components if the housing can’t be well-covered. Never forcedly press the cover of housing.
6. Complete set
Main body (with screws used to fix main body
on wading rod)
|Spare reed switch (with cap to contain line)
7. Life Span
1. 5years under correct operation and maintenance.
2. Under correct operation, calibration formula should be checked after 1—2years.