Ametek 5000 Instrument Manual

Overview 1-1
*
Timing for typical 5000 system only. Refer to Appendix A for timing of units with asymmetric cycle.
OVERVIEW
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
3
).
Theory of Operation (Figure 1)
The analyzer measures the oscillation frequencies of two crystal oscillators: Y
1
, the local oscillator
or LOC, with a constant frequency (nominal ƒ
1
= 8.982500 MHz) and Y
2
, the measuring oscillator
with a variable frequency (nominal ƒ
2
= 8.982000 MHz). The measuring oscillator contains a crystal
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
2
) is continuously mixed with the reference frequency (ƒ
1
) to produce the
"wet" cell frequency, ƒ
sample
.
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
2
) is continuously mixed
with the LOC frequency (ƒ
1
) to produce the "dry" cell frequency, ƒ
ref
. The ƒ between ƒ
sample
and ƒ
ref
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
sample
measurement. The  between the second
cycle ƒ
sample
and first cycle ƒ
ref
measurements becomes the new ƒ. Then the second cycle ƒ
ref
mea-
surement is made and the  between the second cycle ƒ
sample
and second cycle ƒ
ref
measurements
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
cal
) and ƒ
ref
becomes the new 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.

Ametek 5000 Specifications

General

Brand	Ametek
Model	5000
Category	Measuring Instruments
Language	English