Thermocouple gauge & pirani gauge

1. PRESSURE
MEASUREMENT
DEVICES
Submitted by,
Navaneeth Krishnan A R
B130592CH
B Batch
S5 CHED

2. Thermocouple Gauge
&
Pirani Gauge
• Both works on the same principle of detecting
thermal conductivity of the gas by a change in
temperature.
• In the thermocouple gauge, the temperature is
sensed by a thermocouple and in Pirani gauge by
the change in resistance of the heated wire.
•

3. THERMOCOUPLE
GAUGE
• A Thermocouple is kept in contact with the heated
wire and the temperature of the wire is directly
measured as a measure of pressure.
• For different pressures, the temperature is
measured by the fine-wire thermocouple, the
hating current being initially fixed by the resistance
as shown in the figure. This device is usually used for
comparison purposes. The sensitivity of such an
instrument depends on the pressure and the wire
current.

4. WORKING
• Two sets of thermocouples are used to measure temperatures of
heater wires in the two chambers and oppose each other.
When there is a difference in pressures, there occurs an
unbalance which is measured by a potentiometer circuit.
Instead of a single thermocouple per wire, a thermopile is often
chosen to increase sensitivity. The thermocouple gauge is also
composition dependent and needs empirical calibration for the
high vacuum range.

5. PRINCIPLE
• The T/C gauge contains two elements: a heater (filament) and a
thermocouple junction which contacts the filament. With the filament
current held constant, as the pressure within the tube is decreased the
filament will become hotter because of the improved thermal
insulation provided by the increasingly rarefied gas. This temperature is
sensed by the thermocouple junction. Measurement is accomplished
by reading the thermocouple junction voltage on a sensitive meter
which has previously been calibrated against a manometer

6. CALIBRATION
• Thermocouple gauges are calibrated such that the
wire’s temperature is displayed as a pressure
reading. This allows such problems as variations in
heat flow through the supporting electrodes to be
taken into account.
• T/C gauges are not particularly accurate
instruments. Most often they are used only as rough
indicators of pressure where 10 to 20 percent
accuracy is acceptable.

7. PIRANI GAUGE
• The Pirani gauge is a robust thermal
conductivity gauge used for the measurement of
the pressures in vacuum systems.
• The thermocouple (or T/C) gauge is one of the
more common and cost effective gauges for
vacuum pressure measurement in the 1 Torr to 1
milliTorr range

8. STRUCTURE
• The Pirani gauge consists of a metal filament
(usually platinum) suspended in a tube which is
connected to the system whose vacuum is to be
measured. Connection is usually made either by a
ground glass joint or a flanged metal connector,
sealed with an o-ring. The filament is connected to
an electrical circuit from which, after calibration, a
pressure reading may be taken

9. PRINCIPLE
• A conducting wire gets heated when electric
current flows through it. The rate at which heat is
dissipated from this wire depends on the
conductivity of the surrounding media. The
conductivity of the surrounding media in turn
depends on the density of the surrounding media
(that is, lower pressure of the surrounding media,
lower will be its density). If the density of the
surrounding media is low, its conductivity also will be
low causing the wire to become hotter for a given
current flow, and vice versa.

10. DESCRIPTION
The main parts of the arrangement are:
• A pirani gauge chamber which encloses a platinum filament.
• A compensating cell to minimize variation caused due to
ambient temperature changes.
• The pirani gauge chamber and the compensating cell is
housed on a wheat stone bridge circuit as shown in diagram.

11. OPERATION
• A constant current is passed through the filament in the pirani
gauge chamber. Due to this current, the filament gets heated
and assumes a resistance which is measured using the bridge.
• Now the pressure to be measured (applied pressure) is
connected to the pirani gauge chamber. Due to the applied
pressure the density of the surrounding of the pirani gauge
filament changes. Due to this change in density of the
surrounding of the filament its conductivity changes causing
the temperature of the filament to change.
• When the temperature of the filament changes, the resistance
of the filament also changes.
• Now the change in resistance of the filament is determined
using the bridge.
• This change in resistance of the pirani gauge filament
becomes a measure of the applied pressure when calibrated.
• Note: higher pressure – higher density – higher conductivity –
reduced filament temperature – less resistance of filament &
vise versa

12. • The gauge may be used for pressures between
0.5 Torr to 10−4 Torr. The thermal conductivity and
heat capacity of the gas may affect the readout
from the meter, and therefore the apparatus may
need calibrating before accurate readings are
obtainable. For lower pressure measurement other
instruments such as a Penning gauge are used.

13. Applications of Pirani gauge
• Used to measure low vacuum and ultra high
vacuum pressures.
Advantages of Pirani gauge
• They are rugged and inexpensive
• Give accurate results
• Good response to pressure changes.
• Relation between pressure and resistance is linear
for the range of use.
• Readings can be taken from a distance.
Limitations of Pirani gauge
• Pirani gauge must be checked frequently.
• Pirani gauge must be calibrated from different
gases.
• Electric power is a must for its operation.

14. WHICH IS PREFFERED
PIRANI GAUGE
OR
THERMOCOUPLE GAUGE

15. ANSWER :
• PIRANI GAUGE IS PREFFERED OVER THERMOCOUPLE
GAUGE BECAUSE:
o ITS TEN TIMES FASTER THAN THEROCOUPLE GAUGE
o WIDER RANGE OF PRESSURES CAN BE MEASURED
WITH A PIRANI GAUGE.
Well designed Pirani gauges offer better accuracy
and response time than do thermocouple gauges
(often tens of milliseconds vs. several seconds).