16
BG 5171 BEN / B (2017-02) PBR 260
Within certain limits the thermal conductibility of gases is pressure dependent. This
physical phenomenon is used for pressure measurement in the thermal
conductance vacuum meter according to Pirani. A self adjusting bridge is used as
measurement circuit (see Fig. 2). A thin tungsten wire is used as sensor element.
Wire resistance and thus temperature are kept constant through a suitable control
circuit. The electric power supplied to the wire is a measure for the thermal con-
ductance and thus the gas pressure. The basic principle of the self adjusting bridge
circuit is shown in Fig. 2.
B
PBR 260
+
–
Fig. 2
The bridge voltage U
B
is a measure for the gas pressure and is further processed
electronically (linearization, digitizing).
The PBR 260 covers the measurement range 5×10
-10
hPa … 1000 hPa.
• The Pirani constantly monitors the pressure.
• The hot cathode system (controlled by the Pirani) is only switched on when the
pressure drops below the set threshold (p
on
). The hot cathode will be ready for
operation after a few seconds' heating time, when the <EMI ON> lamp is lit.
• When the pressure rises above the setpoint (p
off
) the hot cathode is switched off
and the <EMI ON> lamp turns off.
In the upper pressure range, the Pirani reading and in the lower pressure range,
the hot cathode reading is output. In the overlapping range (p
lower
… p
upper
), a
combined signal of the two measuring systems is supplied (→ Fig. 3).
5 ×
10
-10
hPa
P
l
o
w
e
r
1000 hPa
Pirani
P
o
n
P
o
f
f
P
u
p
p
e
r
Hot cathode
Fig. 3
Pirani
Measurement range
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