Technical Report; Measurement Principle; Sensitivity Coefficient - Ulvac G-TRAN Series Instruction Manual

18.

Technical report

18.1.

Measurement principle

When a gas molecule collides with a particle having an energy greater than a certain
level, it emits electrons and becomes an ion. This is called the ionization phenomenon
of gas.
The filament is heated to ionize the gas, and the accelerated thermal electrons that
are generated are used for the measurement. Since the collision frequency of the
thermal electrons and gas molecules is proportional to the gas density, if the
collisions generate ions at a constant rate, the number of ions generated determines
the gas molecule density (gas pressure).
The relationship between the number of thermal electrons (emission current: Ie)
emitted from the filament, the number of ions generated(ion current: Ii), and the
density of gas molecules (pressure: P) is as follows:
The sensitivity S in the above equation is determined by the sensor's structure and
size, conditions such as the voltage applied to each electrode, and the type of
measuring gas.
This unit indicates the value that is obtained by electrically amplifying and
calculating Ii in the above equation. Therefore, if the sensitivity S changes, the
measured value output of the vacuum gauge will differ, even if the emission current Ie
and pressure P are the same. Typical factors that change the sensitivity S are the
following:
1) Type of Sensor
2) Type of measuring gas
3) Deterioration of Sensor
The sensitivity of the sensor of this unit in a nitrogen gas atmosphere is 0.06 Pa
18.2. Type of measuring gas and specific sensitivity coefficient
This unit is adjusted to measure the correct pressure with nitrogen gas.
Therefore, when this unit measures a gas atmosphere other than nitrogen gas, an error
occurs in the measured value output.
The following explains how to correct the errors in measured value output depending
on the type of gas.
With the sensitivity Srj(N
expressed as the equation below.
Ii＝S×Ie×P
Ii ：Ion current【A】
S ：Sensitivity【Pa
Ie ：Emission current【A】
P ：Pressure【Pa】
) for nitrogen gas as a standard, the sensitivity S is
2
S ＝ Srj(N ) × Srj
2
S ：Sensitivity【Pa
Srj(N ) ：Nitrogen gas sensitivity【Pa
2
Srj ：Specific sensitivity coefficient
54
-1
】
-1
】
-1
】
――(1)
.
-1
――(2)