6.3 Ultra High Vacuum Guages: Ionization Gauges
Ionisation Vacuum Gauges (VGs) are the most important instruments for measuring gas pressure in the high and ultrahigh vacuum ranges. They measure the pressure in terms of the number density of particles proportional to the pressure. The gas, the pressure of which is to be measured, enters the gauge head of the instrument and is partially ionised with the help of an electric field. Ionisation takes place when electrons are accelerated in the electric field and attain sufficient energy to form positive ions on impact with gas molecules. These ions transmit their charge to a measuring electrode (the ion collector in the system). The ion current, generated in this way is a measure of the pressure since the ion yield is proportional to the particle number density and thus to the pressure. The ions are formed either in a high electric field (cold-cathode or Penning discharge) or in a hot-cathode discharge at lower voltages.
Under otherwise constant conditions, the ion yield and thus the ion current depend on the type of gas, since the ionisation energy is different for various gases. Therefore also ionisation VGs are usually calibrated to nitrogen as reference gas. To obtain the true pressure for other gases, the read-off pressure must be multiplied by the correction factor. These factors are assumed to be independent of the pressure, though they depend somewhat on the geometry of the electrode system.

Cold-cathode Vacuum Guage (Penning VG)
VGs operated with a cold discharge are therefore called cold-cathode VGs. Since the measuring gas volume is embedded in a strong magnetic field, they are also called Penning VGs. The discharge process in the measuring tube is, in principle, the same as in the electrode system of a sputter ion pump.
Construction & Working: A common feature of all types of cold-cathode VGs is that they contain just two unheated electrodes, a cathode and an anode, between which a cold discharge or Penning discharge is ignited and maintained by means of a d.c. voltage of around 2 kV. Penning discharges operate under a strong magnetic field and can therefore be maintained down to very low pressures. The magnetic field extends the electron trajectories so that their rate of ionising collisions with gas molecules is sufficiently large for the number of charge carriers required to maintain the discharge. The magnetic field is arranged such that the magnetic field lines mainly cross the electric field lines. In this way the electrons are forced on spiral paths.
The positive and negative charge carriers produced by collisions move to the corresponding electrodes and form the pressure dependent discharge current, which is indicated on the meter. The reading in mbar depends on the type of gas. The upper limit of the measuring range is given by the fact that above several 10-2 mbar the Penning discharge changes to a glow discharge with intense light emission, in which the current (at constant voltage) depends only weakly on the pressure and is therefore not suitable for measurements.
Figure: Cold Cathode Vacuum Guage (Penning Vacuum Guage)
In all Penning gauges there is a considerably higher gas sorption than in hot cathode ionisation VGs. This is due to the fact that the principle of a Penning gauge is similar to that of a sputter ion pump and therefore the pump speed can be up to S~ 10-2 l/s. This is due to the effect that the ions are accelerated towards the cathode where they are partly retained and partly cause sputtering of the cathode material. The sputtered cathode material, albeit not titanium, forms a gettering surface film on the walls of the gauge tube. In spite of these disadvantages, which result in a relatively high degree of inaccuracy in the pressure reading (up to 50%), the cold-cathode ionisation gauge has three outstanding advantages:
(i) It is the least expensive of all high vacuum measuring instruments.
(ii) The measuring system is insensitive to the sudden admission of air and to vibrations.
(iii) The instrument is easy to operate

Chapter 7 Sensor technology
Sensor: A sensor is a device that converts a physical phenomenon into an electrical signal. As such, sensors represent part of the interface between the physical world and the world of electrical devices, such as computers. The other part of this interface is represented by actuators, which convert electrical signals into physical phenomena.
7.1 Sensor Performance Characteristics Definitions
Transfer Function
The transfer function shows the functional relationship between physical input signal and electrical output signal. Usually, this relationship is represented as a graph or calibration curve showing the relationship between the input and output signal, and the details of this relationship may constitute a complete description of the sensor characteristics.
Sensitivity
The sensitivity is defined in terms of the relationship between input physical signal and output electrical signal. It is generally the ratio between a small change in electrical signal to a small change in physical signal. Typical units are volts/kelvin, millivolts/kilopascal, etc. A thermometer would have “high sensitivity” if a small temperature change resulted in a large voltage change.
Span or Dynamic Range
The range of input physical signals that may be converted to electrical signals by the sensor is the dynamic range or span. Signals outside of this range are expected to cause unacceptably large inaccuracy. This span or dynamic range is usually specified by the sensor supplier. Typical units are kelvin, pascal, newtons, etc.
Accuracy or Uncertainty
Uncertainty is generally defined as the largest expected error between actual and ideal output signals. Typical units are kelvin. “Accuracy” is generally considered by metrologists to be a qualitative term, while “uncertainty” is quantitative. For example one sensor might have better accuracy than another if its uncertainty is 1% compared to the other with an uncertainty of 3%.
Noise
All sensors produce some output noise in addition to the output signal. In some cases, the noise of the sensor is less than the fluctuations in the physical signal, in which case it is not important. Many other cases exist in which the noise of the sensor limits the performance of the system based on the sensor.
Resolution
The resolution of a sensor is defined as the minimum detectable signal fluctuation.

7.2 Classification of sensors
A logical way to classify sensors is with respect to the physical property the sensor is designed to measure. Thus, we have temperature sensors, force sensors, pressure sensors, motion sensors, etc. However, sensors which measure different properties may have the same type of electrical output.
We must consider the environment of the sensor. Environmental effects are perhaps the biggest contributor to measurement errors in most measurement systems. The environment includes not only such parameters as temperature, pressure and vibration, but also, the electromagnetic and electrostatic effects, and the rates of change of the various environments. For example, a sensor may be little affected by extreme temperatures, but may produce huge errors in a rapidly changing temperature.

7.3 Temperature sensors
Because temperature can have such a significant effect on materials and processes at the molecular level, it is the most widely sensed of all variables. Temperature is defined as a specific degree of hotness or coldness as referenced to a specific scale. It can also be defined as the amount of heat energy in an object or system.
Temperature sensors detect a change in a physical parameter such as resistance or output voltage that corresponds to a temperature change. There are two basic types of temperature sensing:
■ Contact temperature sensing requires the sensor to be in direct physical contact with the media or object being sensed. It can be used to monitor the temperature of solids, liquids or gases over an extremely wide temperature range.
■ Non-contact temperature sensors interprets the radiant energy of a heat source in the form of energy emitted in the infrared portion of the electromagnetic spectrum. This method can be used to monitor non-reflective solids and liquids but is not effective with gases due to their natural transparency.

Types of Temperature Sensors
Temperature sensors comprise three families: electro-mechanical, electronic, and resistive.
Electro-mechanical
Bi-metal thermostats are exactly what the name implies: two different metals bonded together under heat and pressure to form a single strip of material. By employing the different expansion rates of the two materials, thermal energy can be converted into electro-mechanical motion.
Bi-metal thermostats are also available in adjustable versions. By turning a screw, a change in internal geometry takes place that changes the temperature setpoint.
Bulb and capillary thermostats make use of the capillary action of expanding or contracting fluid to make or break a set of electrical contacts. The fluid is encapsulated in a reservoir tube. This allows for slightly higher operating temperatures than most electro-mechanical devices.
Electro-mechanical sensors are typically the simplest components to interface with their applications. Since they are capable of either opening or closing with increasing temperature, they are capable of interrupting a power circuit to control or shut down a circuit or of closing a circuit to sound an alarm, turn on a fan, etc.

In most circumstances, thermostats are connected to one leg of the power source. When the application temperature is reached, the device will function to either make or break the circuit.
Electronic sensors
Silicon sensors make use of the bulk electrical resistance properties of semiconductor mate-rials, rather than the junction of two differently doped areas. Especially at low temperatures, silicon sensors provide a nearly linear increase in resistance versus temperature or a positive temperature coefficient (PTC). IC-type devices can provide a direct, digital temperature reading, so there’s no need for an A/D converter.
Infrared (IR) pyrometry. All objects emit infrared energy provided their temperature is above absolute zero (0 Kelvin). There is a direct correlation between the infrared energy an object emits and its temperature.
IR sensors measure the infrared energy emitted from an object in the 4–20 micron wavelength and convert the reading to a voltage. Typical IR technology uses a lens to concentrate radiated energy onto a thermopile. The resulting voltage output is amplified and conditioned to provide a temperature reading.
Thermocouples produces a voltage when the temperature of one of the spots differs from the reference temperature at other parts of the circuit. Thermocouples can also convert a temperature gradient into electricity. They are formed when two electrical conductors of dissimilar metals or alloys are joined at one end of a circuit. Thermocouples do not have sensing elements, so they are less limited than resistive temperature devices (RTDs) in terms of materials used and can handle much higher temperatures. Typically, they are built around bare conductors and insulated by ceramic powder or formed ceramic.
All thermocouples have what are referred to as a “hot” (or measurement) junction and a “cold” (or reference) junction. One end of the conductor (the measurement junction) is exposed to the process temperature, while the other end is maintained at a known reference temperature. The cold junction can be a reference junction that is maintained at 0°C (32°F).
Resistive temperature Devices
Thermistors (or thermally sensitive resistors) are devices that change their electrical resistance in relation to their temperature. They typically consist of a combination of two or three metal oxides that are sintered in a ceramic base material.
Thermistors are available in two different types: positive temperature coefficient (PTC) and negative temperature coefficient (NTC). PTC devices exhibit a positive change or increase in resistance as temperature rises, while NTC devices exhibit a negative change or decrease in resistance when temperature increases. The change in resistance of NTC devices is typically quite large, providing a high degree of sensitivity.

They also have the advantage of being available in extremely small configurations for extremely rapid thermal response.
In addition to metal oxide technology, PTC devices can also be produced using conductive polymers. These devices make use of a phase change in the material to provide a rapid increase in electrical resistance. This allows for their