Transit Time Method - Kamstrup flowIQ 3100 Series Technical Description

TECHNICAL DESCRIPTION

12.3 Transit time method

The transit time method used in flowIQ® 3100 utilizes the fact that it takes an ultrasonic signal sent in the opposite
direction of the flow longer to travel from sender to receiver than a signal sent in the same direction as the flow.
The transit time difference in a flow sensor is very small (nanoseconds). Therefore, the time difference is measured
as a phase difference between the two 1 MHz sound signals in order to obtain the necessary accuracy.
Illustrated on the diagram below.
In principle, flow is determined by measuring the flow velocity and multiplying it by the area of the measuring
pipe:
= 
Q F A
where:
Q is the flow
F
is the flow velocity
A
is the area of the measuring pipe
The area and the length which the signal travels in the sensor are well-known factors. The length which the
signal travels can be expressed by
L
T =
V
where:
L
is the measuring distance
V
is the sound propagation velocity
T
is the time
26
PHASE DIFFERENCE
T
With flow
= 
L T V
, which can also be written as:
Kamstrup A/S • Technical Description • 5512-1242_N1_GB • 10.2019
Against flow
flowIQ® 3100
t