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www.thinkSRS.com 1
PG105 vs. Thermocouple Gauges
The two most common thermal conductivity gauge technologies used in modern vacuum applications are
Pirani gauges and thermocouple gauges.
This application note is designed to help vacuum users choose between the two competing gauge
technologies and decide when a pressure measurement setup based on TC gauges should be upgraded to
PG105 convection gauges.
In This Application Note
Introduction 3
Pressure Range Considerations 4
Response Times 5
Ion Gauge Auto Start 5
Remote Sensing 5
Controller/Gauge Interchangeability 6
Contamination Resistance 6
UHV Compatibility 6
Price/Performance ratio 7
Freeze-Drying Processes 7
Leak Testing 7
(408)744-9040 Stanford Research Systems
www.thinkSRS.com
2 PG105 vs. Thermocouple Gauges
Stanford Research Systems (408)744-9040
www.thinkSRS.com
PG105 vs. Thermocouple Gauges 3
Introduction
Pressure measurement in a thermal conductivity gauge is based on the transfer of heat
from a hot wire, located inside the sensor, to the surrounding gases. Since gauge output
depends on the thermal conductivity of the gases as well as their pressure, all thermal
conductivity gauges provide indirect, gas-dependent, pressure readings.
The two most common thermal conductivity gauge technologies used in modern vacuum
applications are:
Pirani Gauges
In the Pirani gauge (PG) the voltage required to maintain the hot wire at a constant
temperature is used as a non-linear, gas-dependent, function of pressure. Traditional
Pirani gauges provide useful pressure readings between 10-3 and 10 Torr. In
convection-enhanced Pirani gauges, the upper range is extended upward to 1000 Torr by
taking advantage of thermal convection currents created at the higher pressures.
Thermocouple Gauges
In the thermocouple gauge (TC) the pressure is indicated by measuring the small voltage
of a thermocouple spot welded directly onto the hot wire. The wire is fed with a constant
current and its temperature depends on the thermal conductivity and pressure of the gases
present. TC gauges display useful pressures between 10-3 and 1 Torr.
TC gauges have long been regarded a cost-effective means to (1) monitor the foreline
pressures of pumping stations and (2) as crossover gauges for vacuum systems in general.
However, they are being systematically replaced in all modern vacuum systems by more
accurate and reliable Pirani gauges, such as the PG105 convection-enhanced Pirani gauge
manufactured by Stanford Research Systems.
This appendix is designed to (1) help vacuum users choose between the two competing
gauge technologies and (2) decide when a pressure measurement setup based on TC
gauges should be upgraded to PG105 convection gauges.
For additional information on this subject consult the following references:
1. J. M. Lafferty, "Foundations of Vacuum Science and Technology", section 6.8.
"Thermal Conductivity Gauges", p. 403-414, Wiley-Interscience, 1998. A great book
with lots of great information on almost every imaginable vacuum subject.
2. J. H. Leck, "Total and Partial Pressure Measurement in Vacuum Systems",
Chapter 2., "Thermal Conductivity Gauges", p. 39, Blackie and Sons, Glasgow, 1989.
3. Stephen P. Hansen , "Pressure measurement and control in loadlocks", Solid State
Technology, Oct. 1997, p. 151.
4. Simplify Rough Pumping with a Wide-Range Gauge", R&D Magazine, May 1999,
p. 57.
(408)744-9040 Stanford Research Systems
www.thinkSRS.com
4 PG105 vs. Thermocouple Gauges
5. J. Zettler and R. Sud, "Extension of thermocouple gauge sensitivity to atmospheric
pressure", J. Vac. Sci. Technol. A6(3) (1988) 1153. Note: this is what it takes to
make a TC tube extend into atmospheric pressures!
6. Vic Comello, "Using Thermal Conductivity Gauges", Back to Basics, R&D
Magazine, Vol 39, Number 8, July 1997, p. 57 .
7. Vic Comello, "When to Choose a Thermocouple Gauge", Back to Basics, R&D
Magazine, May 2000, p. 75.
Pressure Range Considerations
TC gauges deliver useful pressure readings between 10-3 and 10 Torr. Pressure readings
above the upper limit are virtually useless, making it impossible, for example, to tell the
difference between an overpressure condition caused by (1) a malfunctioning pump or
(2) an accidental venting to ambient air by improper use of the foreline valves. While
pumping down a system, a TC gauge cannot indicate if the pumps are working until
pressures in the 1-10 Torr range are achieved and valid readings start to be displayed.
This forces the operator to wait in front of the vacuum system until a reading of vacuum
is obtained before being able to move on to something new!
PG105 convection gauges deliver useful pressure readings between 10-3 and 103 Torr.
This extended pressure range makes the PG105 convection gauge ideal for monitoring
the pumpdown of vacuum systems from atmosphere to the base pressure of most
mechanical pumps, without any blind pressure spots. Convection gauges are found in
virtually every modern semiconductor and thin film process system, for monitoring
pumping system performance. The vacuum operator gets an immediate indication of
pumping action as soon as the pumpdown begins! Atmospheric pressure response is what
makes convection gauges one of the most popular sensors found in loadlock systems.
Most loadlocks must be open to atmosphere under a positive internal pressure of dry
nitrogen or air to ensure a gentle flow of gas out of the chamber once the door is open. A
convection gauge is often used to decide whether it is safe to open the gate and expose
the loadlock chamber to air! Many users even combine their convection gauges with
differential pressure devices called atmospheric pressure switches for added reliability.
Thermocouple gauges should definitely not be used to monitor the backfilling of
loadlocks!
WARNING!
Claims of TC Gauge readings extending to atmospheric pressures must be treated with
extreme caution!
Modern oil-free high vacuum systems increasingly rely on hybrid turbo pumps backed by
oil-free mechanical pumps. As the compression ratios of turbo pumps continue to
increase so do the foreline pressures those systems require. Convection gauges are better
suited to monitor pressures in modern turbo pumped systems. It is not uncommon to cold
start a turbo pumped station from atmosphere and use a convection gauge to follow the
pressure in the foreline from atmosphere to the base pressure of a diaphragm or scroll
pump. The ultimate pressure of the mechanical (diaphragm) pump is one of the numbers
that can be used to define if the system is properly pumped down.
Stanford Research Systems (408)744-9040
www.thinkSRS.com
PG105 vs. Thermocouple Gauges 5
With proper precautions, the PG105 lower range can also be extended further down into
the 10-4 Torr decade, providing an amazing seven orders of magnitude of dynamic range
from one gauge!
Response Times
Operation at constant wire temperature provides the PG105 convection gauge the
advantage of a faster response to pressure transients. The response time is very fast
(milliseconds in most cases) because components do not have to change temperature as
pressure changes. Response time to a pressure step-function is pressure dependent, but it
is roughly about an order of magnitude faster than in TC gauges.
Fast response time makes the PG105 convection gauge ideally suited for protective
functions, as in determining when ionization gauge emission current should be
deactivated or turned off. They are also well suited to control valves, heaters, bakeout
ovens and safety interlocks.
Ion Gauge Auto Start
The IGC100 has a built in IG Auto-Start mode that makes it possible to automatically
link the emission status of an ionization gauge to the pressure readings of a PG105 gauge
exposed to the same vacuum environment. The ion gauge emission is immediately turned
off as soon as the pressure goes above a user specified threshold value. This protects the
filament from accidental burnouts. The emission is automatically reestablished as soon as
the PG105 pressure readings goes below the threshold value, making it possible to
automate pressure measurements from atmosphere down to UHV during pumpdown.
Remote Sensing
Compatibility with long cabling and immunity to electrical noise are important
specifications for thermal conductivity gauges used in vacuum setups where the sensor
must be placed far away from the controller.
The bridge circuit used to set the wire temperature is built right into the PG105 head, and
the voltages are read using a Kelvin probe (4 wire) arrangement making them
independent of cable length. Up to 150 m long cables can be used with PG105 gauges.
The output of the PG105 convection gauge is between 0.3 and 6 V as opposed to the
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