D-FL 100 properties - Why does a detection limit exist for D-FL 100?

D-FL 100 DS is a MPPT (multipoint pitot tube), which uses the impact pressure measurement method.

The method is very accurate however at low flow rates, the measurable pressure difference is very small. As a result, the measured threshold is determined by the uncertainty of the differential pressure transmitter.

Basis of D-FL 100 is a specially designed probe with two measuring chambers, each equipped with several bores.

The measuring chambers are pneumatically separated from each other.
The measuring chambers are connected to a differential pressure transmitter.

The probe generates a differential pressure in flowing sample gas dependant on:

• V  the flow velocity of the gas
• R  the normalized density of the sample gas
• T  the sample gas temperature
• P  the absolute pressure of the sample gas.
• K1 the flow profile of the sample gas in the flow channel
• K2 the probe profile
• K3 the arrangement of the port openings

DURAG offers a precise differential pressure transmitter as an option to the probe. The selected transmitter has a basic range of 0-10 hPa and can be set to a minimum output range of 0 to 0.5 hPa corresponding to 4-20mA output .

Ex works the transmitter is configured to:

• output the dP measured value square rooted.
• suppress output values < 6% of the assigned mA output range (= 0.002 hPa at 0.5 hPa).

The zero suppression can be parametrized different or disabled.For special applications DURAG configurates the zero suppression to 3% (= 0.001 hPa).

As a result of this supression function of the dP transmitter those measuring gas flows, which produce a differential pressure signal, which is smaller than the threshold are indicated as 0 m/s (= 4mA).

Consequently the setpoint of the threshold determines the minimum detection limit of D-FL 100.

The detection limit of a D-FL 100 typically is a function of the measured gas temperature as follows:

values valid for 6% (3%) threshold of zero suppression

• Gas velocity 0.4 (0.3) m / s at T =  20 ° C
• Gas velocity 0.5 (0.3) m / s at T = 100° C
• Gas velocity 0.5 (0.3) m / s at T = 200° C
• Gas velocity 0.6 (0.4) m / s at T = 300° C
• Gas velocity 0.7 (0.4) m / s at T = 400° C
• Gas velocity 0.7 (0.4) m / s at T = 500° C
• Gas velocity 0.8 (0.5) m / s at T = 600° C

These detection limits depend on the principle but are no special property of D-FL 100!

Measured values higher than the listed detection limit are precise, because the measurable difference pressure of the probe is increasing quadratic with gas velocity.