Temporary or short-term flow measurement of rivers and streams is usually arrived at, by the use of the area of velocity equation (Q = V x A). With V being average of the velocity of the river at a known area that's A. Velocity is usually measured by carrying out a velocity profile of the river using an electromagnetic velocity centre. The area is calculated by physically measuring the widths and depths of the river or stream, this of course can prevent many problems the most fundamental of which being that rivers and streams by their nature are neither uniform in their shape or velocity profile. Accurate flows therefore require careful and time-consuming exercises.

An alternative method of flow measurement is by the use of tracer-type meters, these meters introduce a tracer material or dye into the river and stream and by the use of a down stream sensor directly measure the volumetric flow rate. Tracer materials can include radioactive fluids or fluorescent dyes, which of course can introduce their own environmental concerns. In order to address these concerns the Swiss Environmental Agency put forward a requirement for an "environmentally friendly" flowmeter to measure their many thousand mountain streams and rivers, the system that was eventually chosen was the Flo-Tracer, which is now available from Flowline Manufacturing.

This system uses common table salt as a tracer material and a portable hand-held unit, which measures the salinity in the stream and calculates and displays the volumetric flowrate. Introducing salt into fresh water streams is quite acceptable. The amounts required are very low and do not affect marine life, indeed vast quantities of salt enter natural streams as a result of rock salt being washed off of roads in winter.

The Flo-Tracer has been used on numerous applications in the UK including measuring the final effluent discharge from Cross Nest Sewage Treatment Works in South London, for this application only 9 Kg of salt were required to measure a flow of 8,000 litres per second. The entire operation took less than ten minutes to carry out after a suitable metering point had been selected. The Flo-Tracer's advanced capabilities even allow the measurement of raw sewage flows to be made.

Other recent uses have included the verification of fixed flume and weir installations in small treatment works where flows of around 10 litres per second were measured with the use of less than 100 grams of salt. The Flo-Tracer is totally portable requires no special training to operate and comes complete with software, which allows flow data to be downloaded, stored and analysed.