GE Sensing Moisture Probe User Manual

GE Oil & Gas

An Automated High Precision Saturation/Dilution
Calibration System for Trace Moisture Sensors

The paper discusses the physical construction and
operating principles of this saturation/dilution type
calibration system for water vapor in a nitrogen
carrier gas used for the purpose of trace moisture
sensor calibration. The system generates known
moisture standards, allows for settling time, takes
readings from sensors, records sensor readings
and creates data tables. The data tables are either
loaded into “smart” probes with on-board
non-volatile memory and/or printed as data sheets
provided to customers so that calibration data may
be inputted into analyzer electronics. The system
is capable of calibrating 128 aluminum oxide
moisture sensor probes per week, over a range
of –80°C to +10°C dew/frost point temperature.
The system is traceable to the National Institute of
Standard and Technology (NIST). Each aluminum
oxide moisture probe has a dedicated channel for
reading the signal from the probe speeding data
acquisition time. A chilled mirror hygrometer is used
as a traceable transfer standard.

The heritage design, in operation since the early
1980’s, is upgraded with a modern data acquisition,
computer controls, data historian, human-machine­interface, and report generation based around
National Instruments LabView measurement
and automation software. The newly automated
system provides faster system throughput and
more diagnostic information about calibration
performance than the heritage systems. In
addition, the system can provide local and remote
operator alarms should errors in calibration system
operation be detected during the calibration run.

Various aluminum oxide moisture probes that can

be calibrated in the described system

Introduction

One of the major challenges, which every
manufacturer of measurement equipment must
face, is the calibration of the equipment against
a secondary standard on a production line basis
while maintaining strict quality control and quality
assurance. As the number of instruments or
devices, which must be calibrated increases, so

does the challenge. Every eort must be made to

assure consistent accuracy and repeatability of
calibration as throughput increases.

GE manufactures process-oriented hygrometers,
which use aluminum oxide moisture sensors.
The aluminum oxide moisture sensor’s basic
construction is shown in the graphic. The aluminum
oxide moisture sensor is a transducer, which
responds to the vapor pressure of water in the
medium to which it is exposed, either gas phase

or liquid phase. Water molecules diuse through

the porous gold layer of the sensor and into the
aluminum oxide dielectric layer of the sensor.

Because the water molecule is a polar molecule, it
adsorbs to the pore walls of the AlO2 layer, thereby
changing the dielectric of the sensor.

The popularity of the aluminum oxide-based
hygrometer among process industries has
increased by several orders of magnitude since its
introduction in the 1960’s. Thus the development
of a sophisticated calibration system capable of
calibrating a large number of moisture probes in
a single run is required to meet customer demand
for new moisture probes and provide capacity for
customer return probes for periodic recalibration.
This transducer must be calibrated against a
secondary standard. The latest system design
upgrades the automation of the system to provide
faster thru-put, more data, better control, and
better data manipulation while keeping the basis
for the moisture generation calibration gas on
the proven double-dilution system. The system is
also scalable as a stand-alone system that can be
replicated easily and provided to global production
and service locations, thus providing customers with
regionally located turn-around for their service needs.

impedance. The volumetric mixing ratio of the gas/water
vapor mixture is then altered and the process is
repeated. The process of calibration consists of two
main sections.

1. Production of the carrier gas/water vapor mixture

2. Data acquisition, storage and reduction

Shown below is a block diagram of the calibration
system. Gaseous nitrogen is withdrawn from a large
tank of liquid nitrogen and transported via a single
section of electro-polished, orbital-welded 316SS
tubing into the calibration laboratory. Although the
theoretical frost point of the nitrogen at this point is
equivalent to the boiling point of nitrogen (-198°C),
due to trace moisture ingress from various sources

(transportation from the gas supplier, tank relling,

etc.), its frost point temperature is actually -85°C
or less at 85 to 95 psig (5.9 to 6.6 bar) pressure.
The pressure of the nitrogen is reduced to ambient
pressure, which further reduces the frost point
to approximately -95°C. The dry nitrogen then
travels through a chamber, which contains several
“standard” aluminum oxide moisture sensor probes.
These standard probes have been previously
calibrated in a laboratory, which is independent
of the facility being described here. They are
used to assure that the nitrogen entering the
calibration system does in fact contain a moisture

concentration, which is insignicant compared to
the nal mixture that is being generated. The dry

gas then enters the moisture generation system.

Aluminum Oxide Moisture

Sensor Construction

The Calibration System

The calibration system, against which GE aluminum
oxide moisture sensor probes are calibrated, operates
on the principal of saturation of a gas with water
vapor, and then dilution of that saturated gas with

dry gas to achieve a nal mixture containing a

known concentration of water vapor. Calibration of
the moisture sensor is accomplished by exposing
the probe to a generated carrier gas/water
vapor mixture and recording the probe’s electrical

2

Aluminum Oxide Moisture Probe

Calibration System

Dry gas stream is here split into two parts. One
part remains dry while the other part is saturated
with water vapor by passing it through a large
packed tower containing distilled water. The
tower is maintained at a constant temperature
of 15.2°C and a constant pressure of 20.7 psia
(244Kpa). Temperature of saturation is monitored
by a laboratory thermometer, which has a NIST
traceable calibration and is accurate to within
±0.1°C. Pressure of saturation is monitored
by a pressure transducer regularly calibrated
against a pressure gauge with an NIST traceable
accuracy of ±0.05 psi. This gauge is also used
to calibrate transducers, which monitor the
pressure in the calibration tanks. The saturated
gas mixture will produce a gas mixture with a
dew point of 10°C ±0.1°C when the pressure is
reduced to one atmosphere.

The 10°C dew point gas exits the saturator section of
the calibration system and enters the dilution section.
To produce dew/frost point temperatures above

-50°C, only one dilution stage is necessary. Frost point
generation below -50°C requires two-stage dilution.

Each of the ve ow control valves used in the dilution

stage of the calibration system is of the thermal mass
type and the calibration of each is traceable to NIST.

Their rated accuracy is ± 2% of reading. These ow
control valves consist of two units, a ow sensor and

an automatic controller with control valve. Output

of the ow sensor goes to a comparator, which

automatically adjusts the control valve to deliver the
quantity of dry or wet gas required to produce the

desired nal mixture.

The diagram shows the ow mixing system principal

for the various combinations of “wet” and “dry” gas

owmeters used to generate carrier gas/water

vapor mixtures equivalent to various dew/frost point

temperatures. The table below shows the owrates for

the respective valves to achieve dew/frost set points.

Flow Block Diagram

3

Flowrates & Accuracies for Generation of Dew/Frost Point Temperatures

Dew/Frost Point Flowmeter(s) in Use Flowrate (cc/min)

None

(System Dry-Down)

10°C FCV-2 4854 +/- 0.1

0°C

-10°C

-20°C

-30°C

-40°C

-50°C

-60°C

-70°C

-80°C

FCV-3
FCV-5

FCV-2
FCV-3

FCV-2
FCV-5

FCV-2
FCV-5

FCV-1
FCV-5

FCV-1
FCV-5

FCV-1
FCV-5

FCV-1
FCV-3
FCV-4
FCV-5

FCV-1
FCV-3
FCV-4
FCV-5

FCV-1
FCV-3
FCV-4
FCV-5

4900

15000

4854
4900

4029

15000

1380

15000

480

15000

159

15000

48

15000

159

4900

433

15000

48

4900

337

15000

48

4900

69

15000

Calculated Dew/Frost

Point Accuracy (°C)

-

+/- 0.2

+/- 0.3

+/- 0.3

+/- 0.3

+/- 0.3

+/- 0.4

+/- 0.3

+/- 0.4

+/- 0.4

The accuracy of the calibration system varies with
the generated dew point, as shown in the table.
These accuracy estimates were obtained from an
error analysis of the moisture mixing system, given
in the Appendix of this paper. The overall accuracy
of the calibration system is better than ±0.4°C
dew/frost point.

The outlet of the dilution stage of the calibration
system is routed to the calibration tank capable of
holding 128 probes. Each system can support up
to 3 calibration tanks providing an overall capacity
of 384 probes. Equilibrium conditions in each tank
during calibration are assured by random sampling
of the impedance of probes from each of the three
tanks. These probes are continuously monitored at
each generated dew/frost point temperature until
the change in impedance becomes within the error
of the A-D converter.

4

Chilled Mirror Hygrometer