Ulvac GI-M2, Instruction Manual

Page: 86 / 101

80
13.2. Type of Measurable Gas and Specific Sensitivity Factor
This gauge has been adjusted so that a correct pressure is indicated for nitrogen gas.
Therefore, if a gas other than nitrogen gas is measured, there will be an error in the
indicated value.
The following describes the correction of the difference in the indicated values by type of
gas.
Given that sensitivity S (N
2
) for nitrogen gas is the reference, sensitivity S can be
expressed by the following equation.
S = S(N
2
)
⋅
r ................................. (2)
S : Sensitivity Pa
-1
S (N
2
) : Sensitivity of nitrogen gas [Pa
-1
]
r : Specific sensitivity factor
Substituting this equation in Eq. (1), we obtain the following equation:
Ii = S(N
2
)
⋅
r
⋅
Ie
⋅
P ................................. (3)
Here, given that Ii (N
2
) is the ion current for nitrogen gas.
Ii = r
⋅
Ii(N
2
) ................................. (4)
Thus, the gauge indicates the correct value multiplied by r (specific sensitivity factor).
Eq. (4) shows that the indicated value on the gauge must be corrected for the type of gas
by dividing it by its specific sensitivity factor (relative sensitivity for nitrogen gas).
(Example) Argon gas is measured by an ionization vacuum gauge.
True pressure P (Torr) is as follows when the indicated value P (Ar) at
this time is 5 × 10
-6
[Pa].
P =
P (Ar)
Specific sensitivity factor for Ar
=
5
×
10
-6
1.34
E
= 3.7
×
10
-6
[Pa]