TECHNICAL DESCRIPTION
MULTICAL
®
61
5512-622 GB/01.2011/Rev. E1
37
9.5
Flow limits
In the meter’s working range from min. cut-off and far beyond Q
4
there is a linear connection between the flow rate
and the measured water flow.
In practice the highest possible water flow through the sensor will be limited by the pressure in the system or
cavitation due to too low back pressure.
If the flow is lower than min. cut-off or negative, ULTRAFLOW
®
24 does not measure any flow.
According to OIML R 49 the upper flow limit Q
4
is the highest flow, at which the flow sensor may operate for short
periods of time without exceeding maximum permissible error. ULTRAFLOW
®
24 has no functional limitations
while operating above Q
4
. Please note, however, that high flow velocities
>
Q
4
involve the risk of cavitation,
especially at low static pressures.
9.6
Guidelines for dimensioning ULTRAFLOW
®
24
In connection with installations it has proved practical to work with larger pressures than the ones mentioned
below:
Nominal flow Q
3
Recommended minimum
operating pressure
Max. flow Q
4
Recommended
operating pressure
[m³/h]
[bar]
[m³/h]
[bar]
1.6
1.5
2
2.5
2.5
1
3.1
2
4
1
5
2
6.3
1.5
7.9
2.5
10
1
12.5
2
16
1.5
20
2.5
25
1
31
2
40
1.5
50
2.5
Table 9
The purpose of recommended minimum operating pressure is to avoid measuring errors as a result of cavitation or
air in the water.
It is not necessarily cavitation in the sensor itself, but also bubbles from cavitating pumps or regulating valves
mounted before the sensor.
In addition, the water may contain air in the form of small bubbles or air in the water.
The risk of influence from these factors is reduced by maintaining a fair pressure in the installation.
Furthermore, it must be taken into consideration that the above-mentioned pressure is the pressure at the sensor
and that the pressure is lower after a contraction than before one (e.g. cones). This means that pressure measured
elsewhere in the system may be different from the pressure at the sensor.
This can be explained by combining the continuity equation and Bernoulli’s equation. The total energy from the
flow will be the same at any cross section. It can be reduced to: P + ½
ρ
v
2
= constant, where P is the pressure,
ρ
is
the density of water and v is the flow velocity.
Dimensioning the flow sensor the above must be taken into consideration, especially if the sensor is used within
the scope of OIML R 49 between Q
3
and Q
4
, and in case of strong contractions of the pipe.
