Explanation of the criteria for classification and numbering of components      C67.2.01
   (DEC 00)
Explanation of the dates on the component's description



1.   Purpose and objectives

     To measure the vertical water velocities in boreholes. Exceptionally high sensitivity down to 1mm/sec permits measurement down to true zero flow.

2.   Description

     SENSA Electromagnetic flow meters measure the velocity of water or other aqueous fluid flowing over a sensing probe. The measurement method uses the Faraday Principle of Electromagnetic Induction, where a conductor moving in a magnetic field has induced in it a voltage proportional to the 
speed of movement. In this case the water is the moving conductor, and the induced voltage is sensed by two electrodes on the probe. The probes have no moving parts, and the initial calibration remains valid for life. They are also unaffected by debris and suspended solids in the flow. 

     SENSA-B is based on the SENSA-RC2 (see component C79.2.09), adapted to measure the vertical velocity of water in boreholes. It may be mains or battery powered, measuring flow velocity over the range 0 to 4 metres/sec, with resolution down to 1mm/sec. LCD display of instantaneous or averaged 
readings, with RS232 output as standard.

     The measured parameter is the velocity at the centreline of the borehole, and the relationship between this measurement and the mean velocity has been determined for a typical velocity profile in boreholes from 80 to 400 mm diameter. To ensure correct probe positioning, mounting rings to suit different borehole sizes can be supplied.

     The probe cable is tough polyurethane protective jacket, with internal Kevlar suspension cores to prevent stretching, and systems with cable lengths up to 300m have been supplied. The cable and probe are fitted to a suitable cable reel, with a flying lead to the control box.

     Only manual cable reels have been supplied to date, but powered reels are possible.

     Operating temperature is - 5 to + 50 C, and the control boxes are rated to IP67.

3.   Input

     Water flow along the axis of a centralised vertically mounted probe.

4.   Output

     Direct reading in metres/second of the average velocity in the borehole, displayed on the internal LCD display and also transmitted as an ASCII string from the RS232 output. Using depth information from the cable reel, the velocity/depth graph can be plotted to enable borehole yield from different levels to be determined.

5.   Operational requirements and restrictions

     For short cable lengths (< 50m), the instruments may be used by a single operator. With longer cables, at least two people are required for handling, and at 300m mounting on a vehicle is advisable. Velocity measurement is extremely easy, and little training is required for basic measurements. Training is suggested for mechanical handling of longer cable lengths. Instruments with cable length <60m may be powered by internal batteries, but longer cables require external power from mains or battery.

     Operation and testing has been in boreholes with non-magnetic liners (usually PVC). Operation in magnetic liners such as mild steel has not been evaluated, and may affect results.      

6.   Form of presentation

     Catalogues and specifications of the instruments are available.

7.   Operational experience

     Units have been in use since 1996, with good results.

8.   Originator and technical support

     Aqua Data Services Ltd. England.

9.   Availability

     From Aqua Data Services Ltd
     Aquatec House
     SN15 4PP
     Tel:    +44 1249 890724 
     Fax:    +44 1249 891329
     Web:  (also lists agents world-wide)

10.  Conditions of use

     Instruments are for sale direct from manufacturer or from agents.



(First entered: 1 DEC 00

Last updated: 1 DEC 00)