Thermocouple Vacuum Gages

A thermocouple gage is used for monitoring pressures below atmospheric pressure (vacuum) in the range from 2 to 10–3 torr. A gage consists of a filament heated by a constant-current source and a thermocouple attached to it to monitor its temperature. The filament and the thermocouple are housed in an enclosure with an opening connected to the vacuum chamber whose gas pressure is to be measured. A schematic diagram of the TC gage is shown in Fig. 5.32.

Fig5.32 Thermocouple vacuum gauge.

The basic transduction mechanism can be described as the loss of heat caused when the surrounding gas molecules impinge on the heated filament and take heat away from it, resulting in a decrease in its temperature.79 As the pressure decreases, a smaller number of molecules impinge on the filament, and consequently its temperature rises. This rise in temperature is indicated by the electrical output of the thermocouple and is proportional to the pressure (vacuum). In order to improve the lower limit of 10–3 torr, several TCs are connected in series as is done in a thermopile.

The Pirani gage also operates on the same basic transduction mechanism. The changes in temperature of its tungsten filament are measured as changes in its resistance with a Wheatstone bridge. The out-of-balance voltage is proportional to the vacuum level. The range of the Pirani gage is around 10–3 to 100 torr.86 The Convectron is a Pirani gage with improvements in its accuracy, repeatability, and response time. The upper limit extends to 1000 torr.

Thermocouples

When two dissimilar metal wires are joined at the ends and are kept at different temperature TR and TS, a continuous thermoelectric current flows owing to the Seebeck effect as shown in Fig. 5.31a. If the wires are cut, an open-circuit Seebeck voltage is measured which is proportional to the temperature difference.

If TR is constant, the Seeback voltage is proportional to Ts. The associated Peltier effect is the increase or decrease of the junction temperature when an external current flows in the thermocouple (TC) wires. Reference 77 gives a description of these effects and their interrelationships.

Figure 5.31 Thermocouples. (a) Seebeck effect. (b) Measurement setup for remote sensing.

Figure 5.31b shows an arrangement when the measuring equipment is remote from the sensing TC. Matched extension TC wires are used and the reference junctions are formed at the measuring equipment. The reference junctions must be kept at a constant temperature, mostly at ambient room temperature TA.

This requirement can also be satisfied by electronically providing a reference voltage compensation.

Thermocouples used in industry are made from several combinations of metals.

Table  summarizes the properties of TCs that are commonly used, and a complete set of thermocouple reference tables is given in Ref. 76. The basic measuring instrument used with these thermocouples is the potentiometer, but direct-reading, analog, and digital meters of many kinds are also available from manufacturers.

Thermodynamic Transducers

In thermodynamic systems, heat flows from one system to another whenever there is a temperature difference between them. Heat flow takes place by heat conduction, heat convection, and thermal radiation.

The calibration of platinum resistance thermometers, thermocouples, and the measurement of heat flow require the precise determination of temperature.

In 1989, the International Committee on Weights and Measures adopted the International Temperature Scale of 1990 (ITS-90). The unit of the fundamental physical quantity known as thermodynamic temperature, symbol T90, is the kelvin, symbol K. The relationship between the International Celsius Temperature, symbol t90, and T90 is t90=T90/K-273.15. ITS-90 also defines specific fixed-point temperatures corresponding to various states of substances, like the triple point (TP) temperature of water (0.01°C, or 273.16 kelvins). A complete list of these fixed points is given in Ref. 76. These fixed points are used in the calibration of platinum resistance thermometers and thermocouples by evaluating the coefficients of the polynomials that represent their outputs.

According to ITS-90, a platinum resistance thermometer, in the range from 0°C to the freezing point of aluminum, would be calibrated at the TP of water (0.01°C) and the freezing points of tin (231.928°C), zinc (419.527°C), and aluminum (660.323°C or 933.473 kelvins).

Tunneling Displacement Transducers

In 1986, Binnig and Rohrer were awarded the Nobel Prize in Physics for the scanning tunneling microscope (STM). The physical basis for the surface profiling transducer (SPT) used in the STM is the phenomenon of electron tunneling.

It represents the flow of electrons between two conducting surfaces under the influence of a bias voltage. The resulting tunneling current or the tunneling effect63 is a measure of the separation between these conductors.

Figure 5.29 Transduction mechanism of the tunneling displacement transducer.

Figure 5.29 illustrates the transduction mechanism of the SPT. In practice,

one of the conductors is replaced by an extremely fine tip and the other conductor is the surface to be profiled. When the clouds of electrons surrounding the tip and the sample surface are made to overlap and a bias potential is applied between them, a tunneling current It is established. This current can be represented by

where d is the distance between the tip and the sample surface and S0 is a constant for a given work function. It is an extremely sensitive function of the distance d. In practical terms, when the tunneling current is well established, a change in d equal to one atomic diameter will change It by a factor of 1000.

The SPT utilizes this sensitivity in a STM to profile the surfaces of materials at the atomic level. The scanning can be done in vacuum, liquids, or gases.

The angstrom movements in an STM are achieved with Inchworm* motors and a scanning tube.65,66 The image shown is of atoms on the surface of a silicon substrate.

Based on similar principles, but on a transduction mechanism that involves repulsive forces, an atomic force microscope (AFM) has been developed for surface profiling.

Fig 5.30 Infrared radiation transducer.

The SPT has also been used in an infrared radiation detector68 as shown in Fig. 5.30. The tip can be lowered and raised by means of an electrostatic force between two electroded surfaces of the cantilever. The infrared sensor consists of a gold-coated membrane that traps a small volume of air or helium between itself and the base of the transducer. The radiation that is absorbed by the gold coating causes the gas to expand and deflect the membrane. The deflection, which is measured by the SPT, is a measure of the infrared radiation.

Chemical and Environmental Transducers.

In chemical SAW transducers, the pairs of IDTs are formed on the same side of the substrate as shown in Fig. contrast to the physical SAW transducers described earlier.

In the intervening space L of the sensing oscillator, a (bio)chemical interface layer is deposited. The second oscillator serves as a reference. Mass loading caused by the interface layer will change the sensing oscillator frequency.

Temperature and stress affect both oscillators equally, and they represent common mode signals. The difference in frequency (Fs-FR) will then be proportional to the magnitude of the measurand, to the extent that it perturbs the interface layer.

The transduction mechanisms are otherwise identical to those described for physical transducers. The magnitude of the change in frequency caused by mass loading of a polymeric interface layer is given by

In chemical, environmental, and biochemical transducers, the changes in the frequency Fs are primarily brought about by the mass loading effect of the interface layer. A biochemical transducer uses the selectivity of enzymes and antibodies to mass load the layer, whereas a chemical sensor depends on the adsorption and chemisorption of the analyte gases.

These changes alter the SAW velocity. The transduction takes place when the velocity changes cause changes in the total electrical phase shift _T around the oscillator loop and corresponding changes in the oscillator frequency proportional to the mass loading of the interface layer.

In addition to frequency changes caused by the interface layer, the SAW attenuates as it propagates between the IDTs. Viscoelastic polymer interface layers, on absorption of volatile organic species, become soft because of plasticity effects and the SAW propagation losses increase because of softening of the polymer. The attenuation of the SAW represents yet another transduction mechanism in a chemical sensor which has been used to identify several chemical species for a particular chemical environment.

Physical Transducers.

In transducers used for force, acceleration, and temperature measurements, the length L is modified by the measurands. In crystalline quartz devices, the fractional change in SAW velocity is small compared with the surface strain caused by a change in L.

We can therefore expect a decrease in oscillator frequency when L is increased by direct axial tension or by an increase in temperature.So, in transducers that measure force, the temperature must be maintained constant. Figure 5.27 shows a dual oscillator configuration where the temperature effects are reduced and the force sensitivity is doubled. For a force acting downward, the A1 oscillator increases its frequency and the A2 oscillator decreases its frequency.

Figure 5.27 Dual SAW-oscillator force transducer. (a) Oscillators on opposite faces of quartz substrate achieve temperature compensation. (b) Change in oscillator frequency as a function of the force. Mixer output shows almost double the sensitivity.

The difference in frequency is a measure of the force.

Surface Acoustic Wave (SAW) Transducers

SAW transducers are used to measure physical, chemical, and environmental measurands. These transducers consist of a SAW delay line with two IDTs, inserted in the feedback loop of an amplifier as shown in Fig. 5.26.

Fig 5.26 SAW oscillator.

The SAW, in traveling between the IDTs, undergoes a phase shift _0, and when the total phase shift _T around the loop is 2nand the total attenuation is less than the amplifier loop gain, the circuit goes into oscillation around the center frequency f0 of the IDT, which is given by nVs/L. The total phaseshiftis