Clamp on Time differential (transit-time) flowmeters normally use either one or two methods for measuring liquid or gas flow in a pipe. These can be categorized as the DOPPLER or TIME DIFFERENTIAL technique. Most people will be familiar with the Doppler technique . There are several types of clamp on flowmeters available these days , ranging from simple low cost analogue devices to products using multiple sensors at different frequencies. Basically, the Doppler technique in a simple form comprises a Doppler sensor, cable and transmitter unit with outputs and a display.

Doppler flow meters will normally emit a continuous frequency. The above slide shows a dual crystal (piezoceramic) clamped onto a pipe . One of the crystals is the emitter and the other listens. If there are small particles or bubbles in the line, they will act as reflectors and reflect back a frequency different to the emitted frequency, depending on their average speed in the pipe. This is known as the Doppler shift. The Doppler shift will enable us to determine the speed of the particles flowing in the pipe. If we know the inner diameter of the pipe, we can determine the volumetric flow rate. Of course, this is a simple explanation and there are some factors to consider when using Doppler flowmeters. One of them is that the particles flowing in the centre of the pipe may differ in speed from the ones at the side. It is almost impossible to determine the exact nature of all these particles with pipe flows of different velocities and different sizes, so manufacturer's may use techniques to try and improve accuracy. One method is using two or more sensors and each sensor having a different frequency. Practically speaking , the intention is to measure the velocity near and away (towards the centre of the pipe) in order to ascertain the flow profile condition.

Another method (employed by EESIFLO) is to wet calibrate each flowmeter on a flow rig containing water and particles and increase/decrease the flow to determine the K factor to be used. There are obviously setbacks and advantages with each method . These can be discussed in a later paper. Doppler flowmeters do not work well on clean liquids (it is possible to make them do so but one can expect some serious loss of accuracy) Some manufacturers will make their sensors extremely sensitive in order to pick up smaller and harder to detect particle flow in a line but there are drawbacks where the sensors can end up reporting a flow rate when none is present . The sensors normally show a flow rate on the screen by simply rubbing their sensor surface with a finger.

The EASZ-10P and 3000 series flowmeters are designed for true Doppler applications. This means that we will normally recommend them to be used on water based fluids with suspended particles and bubbles. These include raw sewage, wastewater, mining slurries, sludge ,aerated liquids in pipes and in conditions where it has been determined that nothing else will work.

The accuracy we can expect will normally be better than +/- 2% within the flowmeter's range. Doppler flowmeters may have a bad reputation with some people because they have been mis-applied over the years by excited sales people. The fact that they are ultrasonic also has made people associate them with all types of ultrasonic meters including time differential meters. This does not mean that Doppler meters should not be used and in fact there are many applications where they are important and have served their purpose well.

Time Differential Meters (transit time) or sometimes called Time of Flight meters operate on a different principle which also uses two sensors. One sensor upstream of the flow and the other downstream.

It is a known fact that at a zero flow velocity, the time taken for an ultrasonic wave to transmit from the downstream sensor to the upstream sensor is the same and vice versa. The time taken at zero flow from one sensor to the next can determine the sonic velocity of the liquid being measured. It is normally important to know the sonic velocity of the liquid as it is an integral part of all manufacturer's calculations. Since it is not always possible to stop the flow, the transmitter performs calculations while the media is still flowing to determine the sonic velocity of the liquid. More advanced time differential meters perform this calculation continually so that a zero offset can be determined. Once the sonic velocity is known, we can then calculate the time it takes for the signal upstream to downstream and vice versa , hence we have a time differential can be used to determine velocity. Cross correlation techniques can be used to determine or scrap signals by various marker or chirp methods and storing larger numbers of sample data to view electronically and take only relevant data pertaining to flow.

Clamp on time differential flowmeters can produce very accurate readings in both low and high flow velocities if installation guidelines have been followed. A good installation point for this type of meter is a point in the piping where one can expect an axially symmetric flow profile.

It is important to avoid bends and pipe disturbances because these disturb flow profiles .

The flight path is also an important factor. The further the distance between transducers, the more flight path is available and a higher accuracy can be expected. Longer flight paths cause signal attenuation or loss, so the norm is to use either direct or reflect methods with clamp on meters . Accuracy can also be improved on smaller pipes with short flight paths by using higher frequency transducers.

Typical accuracies for clamp on time differential meters in pipes with straight lengths and axially symmetric profiles can reach 0.5% and better in laboratory conditions. In actual field conditions this may vary depending on the actual installation but built in diagnostic functions in clamp on meters and a reasonable amount of training in diagnosing problems can improve overall accuracy.

For further information on a flowmeter to suit your particular application, please contact your local EESIFLO representative