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Transpector XPR 3+ Operating Manual
The fraction of ions that are able to travel a distance X in a gas is given as:
[2]
where:
n
is the remaining number of ions after travelling distance X,
n
0
is the original number of ions.
Therefore:
[3]
That is, the fraction of ions in the beam traveling from the ion source decreases with
increasing pressure, P, and increasing length, X, of the ion path. This relationship
indicates that a high pressure sensor must be made small in order to avoid the loss
of ions.
Since the fraction of ion current that is lost is predictable, the data can be linearized
by mathematically compensating for the current loss, provided that the current
output
I
of the ion source is proportional to the partial pressure of the ion of interest.
An additional linearization term, (1+AP), is used to compensate for the effects of
ion space charge in the ion source. Transpector XPR 3+ is equipped to make this
linearizing calculation using the total pressure reading of the ion source. The
linearization factors
F=X/K
and A, the ionizer constant, are empirically determined
for each Transpector XPR 3+ Sensor for the gas being measured and the electron
energy used. The linearized ion current (
I
0
) is proportional to the original ions in the
source,
n
0
, is displayed using the equation:
[4]
where:
I
is the measured raw ion current,
P
is the Transpector XPR 3+ total pressure,
A is the ionizer constant,
F
is the linearization factor.
n
n0
------
X
----
–
exp
=
n
n0
------
P
X
K
----
–
exp
=
I0 I
FP
1 AP
+
exp
=