Kobold MAS Series, Инструкция по эксплуатации

Страница: 41 / 54

MAS Instruction Manual
Page 40
The mass flow rate,
˙
m
, can also be written as:
˙
m =
ρ
Q (3)
where:
ρ
= The gas mass density at standard conditions (g/1);
ρ
is given
in the tables (at 0
°
C, 760 mm Hg).
Furthermore, the temperature difference, DT, is proportional to
the output voltage, E, of the mass flow meter, or
∆
T = aE (4)
where:
a = A constant.
If we combine Equations (3) and (4), insert them into Equation (2),
and solve for Q, we get:
Q = (bN/
ρ
C
P
) (5)
where:
b = H/aE = A constant if the output voltage is constant.
For our purposes, we want the ratio of the flow rate, Q
1
, for an
actual gas to the flow rate of a reference gas, Q
2
, to produce the
same output voltage in a particular mass flow meter or controller.
We get this by combining Equations (1) and (5):
Q
1
/Q
2
= K
1
/K
2
= (N
1
/
ρ
2
C
P2
) (6)
Please note that the constant b cancels out. Equation (6) is the
fundamental relationship used in the accompanying tables. For
convenience, the tables give “relative” K-factors, which are the
ratios K
1
/K
2
, instead of the K-factors themselves.
In the third column of the tables, the relative K-factor is K
actual
/
K
references
, where the reference gas is a gas molecularly equivalent
to the actual gas. In the fourth column, the relative K-factor is
K
actual
/KN
2
, where the reference gas is the commonly used gas,
nitrogen (N
2
). The remaining columns give C
P
and r, enabling you
to calculate K1/K2 directly using Equation (6). In some instances,
K1/K2 from the tables may be different from that which you
calculate directly. The value from the tables is preferred because
in many cases it was obtained by experiment.
Kobold calibrates every MAS mass flowmeter and controller with
primary standards using the actual gas or a molecularly equivalent
reference gas. The calibration certificate accompanying your MAS