Dwyer Series PUF - Manual (Page 6)

1: General Description
2
1.2 Principles of Operation
When ultrasound is transmitted through a liquid, the speed at which the sound travels through the liquid is
accelerated slightly if it is transmitted in the same direction as the liquid flow, and decelerated slightly if
transmitted against it. The difference in time taken by the sound to travel the same distance but in opposite
directions is therefore directly proportional to the flow velocity of the liquid.
The PUF system employs two ultrasonic transducers attached to the pipe carrying the liquid and compares the
time taken to transmit an ultrasound signal in each direction. If the sound characteristics of the fluid are known,
the PUF microprocessor can use the results of the transit time calculations to compute the fluid flow velocity.
Once the flow velocity is known the volumetric flow can be easily calculated for a given pipe diameter. The PUF
system can be set up to operate in one of four modes, determined mainly by the pipe diameter and the type of
transducer set in use. The diagram below illustrates the importance of applying the correct separation distance
between the transducers to obtain the strongest signal.
Figure 1.1 Operating modes
Fluid flow
Fluid flow
Fluid flow
Fluid flow
Fluid flow
U
U
U
U
D
D
D
Separation
Distance
Separation
Distance
Separation
Distance
Separation
Distance
Reflex mode
Reflex mode (double bounce)
Reflex mode (triple bounce)
Diagonal mode
This is the mode most commonly used.
The two transducers (U & D) are attached
to the pipe in line with each other and the
signals passing between them are reflected
by the opposite pipe wall.
The separation distance is calculated by
the instrument in response to entered data
concerning the pipe and fluid characteristics.
In this mode the separation distance is
calculated to give a double bounce. This is
most likely to occur if the pipe diameter is
so small that the calculated reflex mode
separation distance would be impractical
for the transducers in use.
This illustration goes one step further to show
a triple bounce situation. This would normally
apply when working with very small pipes
relative to the transducer range in use.
This mode might be selected by the
instrument where relatively large pipes are
concerned. In this mode the transducers are
located on opposite sides of the pipe but the
separation distance is still critical in order
for the signals to be received correctly.
This mode might be used with the standard
‘A’ & ‘B’ transducer sets but for really large
pipe installation the optional transducer set ‘D’
might be recommended.
Upstream
transducer