V. Measurement pressure gauges, flow meters residual gas analyzer (RGA) 1. Pressure gauges

Direct-Reading Gauges Capacitance manometer is a diaphragm gauge in which the deflection of the diaphragm is measured by observing the change in capacitance between it and a fixed counter electrode. Double-sided capacitance manometer head assembly Nulling tech. The diaphragm deflection in the transducer can be as low as 10-9cm ; therefore motion of parts due to temperature change becomes a large source of error  Avoid ambient temperature change !

Indirect-Reading Gauges Thermal Conductivity Gauges In this region the heat flow is linearly proportional to the pressure, provided that the accommodation coefficient and the temperature difference between the heated wire and the case remain constant.

Pirani Gauges is a type of thermal conductivity gauge in which the heated wire forms one arm of a Wheatstone bridge. (1843)

If R1R4 = R2R3, then IM = 0   (1) Constant temperature method : R2  as P   Vdc  ( T  ) (2) Constant voltage or current method : The out-of-balance current method is calibrated to read the pressure. - Tungsten - Nickel large temperature coefficient of resistance - Thermistor (semiconductor)

Thermocouple Gauges T/C gauge measures pressure dependent heat flow . Constant current is delivered to the heated wire and a T/C is spot welded to its mid point. A low resistance dc microammeter is connected to the T/C and its scale is calibrated in pressure units.

2. Ionization Gauges In the high and UHV region where the particle density is extremely small, it is not possible to detect the minute forces that result from the direct transfer of momentum or energy between the gas and a solid wall. Hot Cathode Gauges   - The operation of the ion gauge is based on ionization of gas molecules by electron impact and subsequent collection of these ions by an ion collector. - This positive ion current is proportional to the number of particle density.

Nottingham proposed that soft X-rays generated by the electrons striking the grid wire collided with the ion collector cylinder and caused photoelectrons to flow from the collector to the grid.  X-ray limit   In 1950 Bayard and Alpert designed a gauge in which the large area collector was replaced with a fine wire located in the center of the grid. Because of its smaller area of interception of X-rays, this gauge could measure pressures as low as 10-8 Pa.

Schulz-Phelps type ion gauge for operation at high pressure Unknown pressure Relative sensitivity H2 0.5 He 0.18 H2O 0.9 N2 1.00 Acetone 5 Schulz-Phelps type ion gauge for operation at high pressure 10-4 ~ 100Pa sputtering , reactive ion etching and other plasma process.

Cold Cathode Gauge The long trajectories enhance the ionization efficiency much higher than the hot cathode gauge. disadvantage : high pumping speed due to sputtering 0.1 ~ 0.5 l/sec. advantage : overlap the range of hot cathode and thermal conductivity gauge.

Tubulated vs Nude Gauges - pumping by the walls of tube.( condensible gas) - gas densities may be different (5 ~ 10 times) Tubulated Gauges : more rugged Nude Gauges : UHV systems

Flow Meters

 rotameter  choke  differential pressure techniques   thermal mass flow technique - convenient, accurate ( < 1% ), short response time. control the flow using servo controlled valve.

- T constant, measure H The amount of heat required to warm the gas stream is linearly dependent on the mass flow and the specific heat Flow meter - H constant, measure T - T constant, measure H

- The two thermocouples measure the change in temperature profile between the no flow and flow condition. Another form of the device uses a bridge circuit to keep the temperature profile constant  The mass flow is proportional to the account of power required to maintain a constant profile.   The thermal mass flow gauge will have to be readjusted for gases other than air.  multiply the meter reading by a correction factor.