Overview; Theory Of Operation; Verification - Ametek 5000 Instrument Manual

The AMETEK 5000 Moisture Analyzer System measures trace concentrations of moisture in gases
such as hydrogen, natural gas, air, oxygen, carbon dioxide, and nitrogen. The multi-point option al-
lows for analysis of up to four samples from different sources. Moisture concentration in the sample
gas is quantified in parts per million by volume (ppmv) or pounds per million cubic feet (lbs/mmft

Theory of Operation

The analyzer measures the oscillation frequencies of two crystal oscillators: Y
or LOC, with a constant frequency (nominal ƒ
with a variable frequency (nominal ƒ
which is coated with a thin film of hygroscopic material and enclosed in a measuring cell. When the
sample or reference gas flows through the cell, the crystal coating sorbs or desorbs moisture, thereby
changing the natural oscillation frequency of the measuring crystal. The difference between the
constant and measurement oscillation frequencies is known as the cell frequency.
A typical measurement cycle (60 seconds) is divided into two 30 second measurement periods
During the first period of the cycle, sample gas flows through the measuring cell. Moisture sorbed
by the crystal coating increases the mass of the crystal which changes the measuring frequency. The
measuring frequency (ƒ
"wet" cell frequency, ƒ
During the second period of the cycle, reference gas (moisture concentration < 1ppm) flows
through the measuring cell. Moisture desorbed by the crystal coating decreases the mass of the crys-
tal which changes the measuring frequency. The measuring frequency (ƒ
with the LOC frequency (ƒ
is proportional to the moisture content of the sample gas and is used in the polynomial expression
used to calculate moisture content.
Like the first cycle, the second cycle starts with the ƒ
cycle ƒ and first cycle ƒ
sample
surement is made and the  between the second cycle ƒ
becomes the new ƒ. In this manner, the value of ƒ is updated and sample moisture concentration
is calculated twice per cycle or approximately every 30 seconds*.

Verification

A portion of the reference gas flows through a moisture generator where a known amount of moisture is sorbed
by the gas. During normal operation, this moisture generator gas is exhausted. When cell verification is required,
the moisture generator gas flows through the cell in place of the sample gas. The ƒ between the cell frequency
measurement made while the moisture generator gas flows through the cell (ƒ
This value, or the resulting moisture value (ppmv), is compared to the known quantity of moisture added to
the reference gas by the moisture generator. Cell verification factor, or span, is then verified or can be adjusted
as required.
*
Timing for typical 5000 system only. Refer to Appendix A for timing of units with asymmetric cycle.

OVERVIEW

(Figure 1)
= 8.982000 MHz). The measuring oscillator contains a crystal
2
) is continuously mixed with the reference frequency (ƒ
2
.
sample
) to produce the "dry" cell frequency, ƒ
1
measurements becomes the new ƒ. Then the second cycle ƒ
ref
= 8.982500 MHz) and Y
1
ref
measurement. The  between the second
sample
and second cycle ƒ
sample
, the local oscillator
1
, the measuring oscillator
2
) to produce the
1
) is continuously mixed
2
. The ƒ between ƒ
sample
ref
measurements
ref
becomes the new ƒ.
) and ƒ
cal ref
Overview 1-1
).
3
.
*
and ƒ
ref
mea-