The Light-Bulb Pirani Gage Project
Pirani Gage Information: http://en.wikipedia.org/wiki/Pirani_gauge
The principle is that the resistance of metals increases at
higher temperatures and a heated wire suspended in a gas will lose heat
conduction and convection in proportion to the number of gas molecules
present; the heat loss is thus proportinal to pressure. One of the
best ways to measure the heat loss is to measure the voltage required
to maintain the filament at a fixed temperature. The wire has a
of temperature and, given the value of resistance at
ambient temperature, one can calculate the resistance of the wire at a
chosen elevated temperature. If a Wheatstone Bridge is configured so
reference resistor, in value equal to the resistance that the filament
will reach at an elevated temperature, is paired with the sensor
filament, the bridge will be out of balance at ambient temperature
since these resistances are
not equal (as are the insensitive resistors in the other bridge leg -
as shown in the schematic). If this imbalance is used to control a
voltage applied to the
Wheatstone bridge, an increased voltage will heat the filament and its
resistance will increase, while the reference resistor (far less
temperature sensitive) remains stable.
The bridge and feedback circuit stabilize when the filament resistance
reaches the value of the paired reference; this assumes that the other
leg of the bridge uses equal value resistors as shown in the schematic.
The voltage being applied to the bridge at balance at
atmospheric pressure becomes the "ATM" calibration point. As the
pressure surronding the filament is reduced, less heat is lost than at
atmospheric pressure, and less voltage is needed by the bridge circuit
to maintain the filament at the correct resistance (and temperature);
the bridge excitation voltage is then an indication of the pressure,
the voltage decreasing with decreasing pressure.
The elevated filament temperature can be selected so that the
filament is hot enough to burn off any contaminants that may reach it,
but not incandescent. In all cases the filament will be this
temperature and will be robust and avoid burnout. The filaments of the
best gages are often formed of platinum wire alloys, but tungsten, in
of a light bulb filament makes a nice sensor as well.
This is a simple single-element sensor that utilizes the
tungsten filament of the GE "Indicator-6" 6W/120V lamp as a sensor of
vacuum. This lamp has good balance of characteristics for this
application; the 120V, low wattage lamp filament can operate in the
<20V range at the modest filament temperatures used without much
power consumption. To make the sensor, the aluminum screw base is cut
off with fine wirecutters
exposing the base of the glass envelope as shown. There is some resin
that will need to be scraped away. There is a small
central tubular stem of heat sealed glass visible at the center. This
is where the lamp envelope was evacuated and/or backfilled. A diamond
or tungsten scribe can be used to score the stem and it can then be
snapped off (as shown), allowing the interior of the lamp and filament
to communicate with the "outside" environment; the opened bulb needs to
be mounted inside the vacuum system and the leads connected to
"feedthroughs" that connect to the measuring circuit externally.
- Temperature Coefficient:
The temperature coefficient is the relative change of a
physical property when the temperature
is changed by 1 K.
In the following formula, let R be the physical property to
be measured and T be the temperature at which the property is
measured. T0 is the reference temperature, and ΔT
is the difference between T and T0. Finally,
α is the (linear) temperature coefficient. Given these
definitions, the physical property is:
Here α has the dimensions of an inverse temperature (1/K or K−1).
This equation is linear with respect to temperature.
The GE Indicator-6 lamp used had a resistance of 200 ohms at 20C.
Tungsten has a temperature coefficient of resistivity of 0.0046/K
For an elevation of +100C the calculation is: R(T) = 200Ω(1+0.0046*100)
= 200Ω (1.46) = 292Ω (at 120C, assuming 20C initial)
A 294Ω 1% resistor was selected as the closest value for the bridge (R8
in the schematic).
Description of Bridge Operation
The Wheatstone Bridge is the R8-R9 pair for one leg and R6-R7 pair for
the fixed reference legs. The junction of these bridge pairs is
compared by the amplifier IC1a, and the error signal when R8≠R9 drives
the transistor Q1 to provide the bias to the bridge array. The error
amplifier output will go positive when the (-)input is below the
(+)input - this is inverting operation. When first turned on the sensor
leg of the bridge is the 200Ω filament paired with the 294Ω so the (-)
input is low, therefore the opamp output will be high, driving the
transistor Q1 "ON" and increased voltage is applied to the bridge legs.
The voltage at the junction of R6-R7 will in all cases be half the
applied bridge drive, and the junction at the R8-R9 junction will equal
this value when the increased voltage has heated the filament and
caused it's resistance to reach 294Ω (equal to R8). The voltage driving
the bridge is the output, and a sample of it can be scaled to (for
example) 10V at atm pressure by RV1 adjustment; this could also be done
with a recorder "span" or scaling adjustment. If the pressure
surrounding the sensor filament drops from ATM, less voltage is
required to maintain the sensor filament at 294Ω so in this case the
bridge excitation is a lower voltage, yielding a lower output voltage -
output proportional to pressure.
Schematic of the Light-Bulb Pirani
Spreadsheet: data of the Light-Bulb Pirani
Gage (OpenOffice Spreadsheet)
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Spreadsheet: data of the Light-Bulb Pirani
Gage (Excel Spreadsheet)