Technical information and measuring principle
26
Thermal Mass Flowmeter FMT200-ECO2
42/14-41 EN
11 Technical information and measuring principle
Mass flow rate
Most of the conventional flow meters determine volumetric flow rates. In this case, it is necessary to cor-
rect the density of the mass flow through additional measurement of pressure and temperature. These
corrective measures make measurements more expensive and they reduce the ultimate accuracy of the
measuring system. FMT200-ECO2 (Sensyflow eco2) measuring systems provide the mass flow rate di-
rectly, i.e. without further measurement or correction.
Example:
If 10 m
3
of oxygen is compressed from 1 to 5 bar at a constant temperature, the volume or volume flow
will change to 2 m³, although the amount of substance and the mass are still the same (14 kg). In this
case, a volume flow meter will only indicate 20 % of the original volume flow.
As a result, a volume flow measurement for gases without a correction of pressure and temperature is
without any meaning. The mass flow meter directly determines the mass per unit of time of a flowing me-
dium; a measured value in kg/h is displayed.
Parameters such as volumetric flow rate (referred to the standard state) can be calculated directly from
the standard density of the medium:
q
n
= q
m
/
ρ
n
in e.g.. m³/h – q
n
with
q
n
= volume flow rate as a function of the normal flow rate (e.g. 0 °C and 1013 hPa)
q
m
= mass flow rate
ρ
n
= density as a function of the normal status (e.g.. 0 °C and 1013 hPa)
Measuring principle
FMT200-ECO2 (Sensyflow eco2) operates according to the
principle of the hot-film anemometer. This method of mea-
surement is based on the abstraction of heat from a heated
body by an enveloping gas flow. The "flow-dependent" cool-
ing impact is used as the measuring impact.
The gas stream flows past two temperature-sensitive resis-
tors R
h
and R
T
which are part of an electrical bridge circuit.
Due to the chosen resistance ratio R
h
<< R
T
, R
h
is heated by
the current I
h
, and R
T
adopts the same temperature as the
gas. The current I
h
is preset by the electronic control circuit
to produce a constant temperature difference between the
heated resistor R
h
and the temperature of the gas.
The electrical power generated with resistor R
h
exactly com-
pensates its loss of heat to the gas flow. As this loss of heat
is dependent on the number of particles which collide with
the surface of resistor R
h
, I
h
represents a measure of mass
flow rate.
V = 10 m
p = 1 bar
= 1,4 kg/m
3
3
ρ
m = 14 kg
V = 2 m
p = 5 bar
= 7,0 kg/m
3
3
ρ
m = 14 kg
ρ
ρ ⋅
= m/V m =
V
m
R
h
R
T
I
h
