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Aquacultural En gineering 6 (1987) 75-77

A S i m p l e W a te r F l o w S e n s o r fo r E l e c tr o n i c A l a r mS y s t e m sL. G. Ross and J. F. Muir

Institute of Aquaculture, Universityof Stirling, StirlingFK9 4LA, UK

A B S T R A C TThe use o f a ver t ical f loa t swi tch to ind icate w ater f lo w is described. Theun i t can be used to prov ide 'water on~water o f f ' s ignals fo r incorpo rat ioni n to a m o r e genera l e lec tr on ic a l a r m s y s t em a nd m ay be adap t ed f o r u s ein a research en viro nm en t or in intens ive f i sh prod uc t ion units. The deviceis o f r ela ti vely low cos t an d is based on eas il y ava i lab le co mp onen ts .

It is increasingly common practice in intensive fish farming, particularlyin hatchery units, to install some type of alarm system. These systemscontinuously monitor key features of the installation and provide audiblewarning of plant failure, often coupled with some form of telephone link,or dialler unit, to allow for unattended operation. The development oflow-cost digital logic and of microcomputers has resulted in moderatelysophisticated units with high reliability becoming more commonplace inthe industry.A number of well-tried sensors are available for such systems, andthese are usually deployed according to the specific needs of the opera-tor. The most commonly used device is a horizontal float switch whichdetects water level and this can be used to signal low or high water levelsin stock tanks, header tanks or raceways. Additional sensors may beused indicating water temperature changes or mains power failure.Water flow sensors are not easily incorporated into these systems asmost flow detectors are designed to actually measure flow rate ratherthan provide 'water on/water off' information needed for alarm purposesand thus can be quite costly. It is, however, a major concern in intensivesystems that the integrity of the water supply can be monitored. Supplyfailure can occur for a wide variety of reasons and even short-term cessa-tion of flow can have serious effects in heavily stocked systems. Althoughsuitably placed level switches located in outlets or sumps can indicate thecessation of flow from aquaculture units, these are difficult to install for

75Aquaculnral Engineering 0144-8609/87/S03.50- Elsevier Applied SciencePublishers Ltd. England. 1987. Printed n Great Britain

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7 6 L. G. Ross , J . F . Muirr e l ia b l e o p e r a t i o n i n si n g l e t a n k u n i t s. T h i s s h o r t n o t e d e s c r i b e s t h e u s eo f a v e r t i c a l f l o a t s w i t c h a s a w a t e r f l o w s e n s o r .

T h e s w i t c h u n i t c o n s is t s o f a f u l ly s e a le d v e r t ic a l r ee d s w i tc h w h i c h h a sa r e v e r s i b l e f l o a t , a l l o w i n g f o r n o r m a l l y c l o s e d o r n o r m a l l y o p e n o p e r a -t i on ( Fi g . 1 A ) . T h e s w i t ch u n i t is m o u n t e d t h r o u g h a n e n d c a p j o i n e d t o at e e b y a s h o r t p i e c e o f a p p r o p r i a t e p i p i n g ( F i g . 1 B ) . S h o u l d l a r g e - b o r ep i p i n g b e u s e d , t h e s e n s o r ca n. b e i n st a ll e d o n a n a p p r o p r ia t e s a d d l e . T h et e e i s i n s t a l l e d i n t h e s u p p l y l i n e t o t h e t a n k a n d i s a r r a n g e d s o t h a t t h ef l o a t r i s e s v e r t i c a l l y u p w a r d s w h e n w a t e r i s p r e s e n t . A b l e e d h o l e i sd r i l l e d o f f - c e n t r e i n t h e e n d c a p s o t h a t , w h e n w a t e r c e a s e s t o f l o w , t h et e e c a n d r a i n d o w n o v e r a p e r io d o f a f e w s e c o n d s . I n p ra c ti c e it h a s b e e nf o u n d t h a t a b l e e d h o l e o f a b o u t 6 m m i s s u f f i c i e n t t o p r e v e n t c o n s t a n t

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Fig. 1 . A , S c h e m a t i c d i a g r a m o f t h e v e r ti c al fl o at s w i t c h u s e d a s a w a te r f lo w s e n s o r. B ,T h e m e t h o d o f i n s ta l la t i o n o f t h e s e n s o r i n t o a t a n k s u p p l y li n e .

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A s i m p l e wa t e r l ow s e ns or f o r e l ec tr on ic a la r m s y s t e m s 77blockage. As water is constantly emerging from the bleed hole it isobviously essential to install the sensor as close to the end of the deliverypipe as possible, thereby allowing this water to enter the tank. There is abuilt-in mechanical delay in operation of the sensor because of the timetaken to drain the tee. This has the advantage that an electronic delay atthe input of the alarm system need not be provided.These units have been used at cable distances of up to 50 m from thecontrol unit and when powered from a 5 V source can directly provideinput to a TTL or microcomputer-based alarm system. The switches areeasily available from electronic component distributors (R.S. Compo-nents, 331-017; Farnell Electronic Components, 146-301), have anelectrical life of > 5 x 105 operat ions and have a current cost of about 6(USS9). Taking account of the cost of tees and end caps the total materialcost per unit is about 10 (USS 15), which makes it possible to considerinstallation of these units on individual tanks or in other flow locationsfor most aquaculture purposes.