An Automated High
Precision Saturation/Dilution
Calibration System for Trace
Moisture Sensors
The paper discusses the physical construction andoperating principles of this saturation/dilution typecalibration 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 forsettling time, takes readings from sensors, records sensor
readings and creates data tables. The system loads thedata tables into “smart” probes with on-board nonvolatilememory. It also prints the data sheets so that customers
can enter calibration data into analyzer electronics for theiranalog probes. The system is capable of calibrating 128
aluminum oxide moisture sensor probes per week, over arange of –80°C to +10°C dew/frost point temperature. The
system is traceable to the National Institute of Standard andTechnology (NIST). Each aluminum oxide moisture probe hasa dedicated channel for reading the signal from the probe.
A chilled mirror hygrometer is used as a traceable checkstandard.

Introduction

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

The water molecule is polar, it adsorbs to the pore walls of
the Al2O3 structure, thereby changing the dielectric of thesensor.

Various aluminum oxide moisture probes that can becalibrated in the described system.

One of the major challenges, which every manufacturer ofmeasurement equipment must face, is the calibration of theequipment against a secondary standard on a production
line basis while maintaining strict quality control and quality
assurance. As the number of instruments or devices, whichmust be calibrated increases, so does the challenge. Everyeffort must be made to assure consistent accuracy andrepeatability of calibration as throughput increases.
Panametrics manufactures process-oriented hygrometers,
which use aluminum oxide moisture sensors. The aluminumoxide moisture sensor’s basic construction is shown in thegraphic. 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 diffuse through the porous gold layer of the sensor
and into the aluminum oxide dielectric layer of the sensor.

The popularity of the aluminum oxide-based hygrometer
among process industries has increased by several ordersof magnitude since its introduction in the 1960’s. Thus thedevelopment of a sophisticated calibration system capableof calibrating a large number of moisture probes in asingle run is required to meet customer demand for new
moisture probes and provide capacity for customer returnprobes for periodic recalibration. This transducer must be
calibrated against a secondary standard. The latest systemdesign upgrades the automation of the system to provide
faster throughput, more data, better control, and betterdata manipulation while keeping the basis for the moisturegeneration 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.

Water molecules

Gold

Aluminum oxide

Metal

Aluminum oxide moisture sensor construction

The calibration system
The calibration system, against which Panametrics
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 final mixture containing a known concentrationof water vapor. Calibration of the moisture sensor isaccomplished by exposing the probe to a generated carrier
gas/water vapor mixture and recording the probe’s electrical
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.

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 constanttemperature 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 traceablecalibration and is accurate to within ±0.1°C. Pressure ofsaturation 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 calibratetransducers, 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 thecalibration 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 five flow
control valves used in the dilution stage of the calibration
system is of the thermal mass type and the calibrationof each is traceable to NIST. Their rated accuracy is ± 2%
of reading. These flow control valves consist of two units,
a flow sensor and an automatic controller with control
valve. Output of the flow sensor goes to a comparator,
which automatically adjusts the control valve to deliver thequantity of dry or wet gas required to produce the desired
final mixture.

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 electropolished, orbital-welded 316SS tubing into the calibrationlaboratory. Although the theoretical frost point of the nitrogenat 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 refilling, 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 moistureconcentration, which is insignificant compared to the final
mixture that is being generated. The dry gas then enters the
moisture generation system.

Aluminum oxide moisture probe calibration system

The diagram shows the flow mixing system principal for thevarious combinations of “wet” and “dry” gas flowmeters
used to generate carrier gas/water vapor mixtures

Saturator

DP = 10°C@ 1ATMequivalent to various dew/frost point temperatures. The
table below shows the flowrates for the respective valves toachieve dew/frost set points.

FCV
1
FCV
2

Dry gas
N2

Vent

FCV
3

FCV
4

FCV
5
FCV-1
FCV-2
FCV-3
FCV-4
FCV-5

To probetanks

0-500 SCCM wet nitrogen
0-5000 SCCM wet nitrogen
0-5000 SCCM dry nitrogen
0-500 SCCM mixture
0-15000 dry nitrogen
Flow block diagram+/- 0.3
Flowrates and accuracies for generationof dew/frost point temperatures
Dew/frost point

Flowmeter(s) in use

Flowrate (cc/min)

Calculated dew/frost point accuracy (°C)

None (System Dry-Down)

FCV-3
FCV-5

4900
15000

—

10°C

FCV-2

4854

+/- 0.1

0°C

FCV-2
FCV-3

4854
4900

+/- 0.2

-10°C

FCV-2
FCV-5

4029
15000

+/- 0.3

-20°C

FCV-2
FCV-5

1380
15000

+/- 0.3

-30°C

FCV-1
FCV-5

480
15000

+/- 0.3

-40°C

FCV-1
FCV-5

159
15000

+/- 0.3

-50°C

FCV-1
FCV-5

48
15000

+/- 0.4

-60°C

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

159
4900
433
15000

+/- 0.3

-70°C

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

48
4900
337
15000

+/- 0.4

-80°C

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

48
4900
69
15000

+/- 0.4

The accuracy of the calibration system varies with thegenerated 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.

Typical agreement between generated dew/frost pointtemperatures and chilled mirror hygrometer readings

The outlet of the dilution stage of the calibration system isrouted 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 threetanks. These probes are continuously monitored at eachgenerated dew/frost point temperature until the change inimpedance becomes within the error of the A-D converter.
The final check of the system is a condensation typehygrometer (also known as a chilled mirror hygrometer).
This hygrometer is a Model OPTICA/1311-XR manufactured byPanametrics. The condensation hygrometer is located at theend of the calibration chain, downstream of the probe tanks.
The condensation hygrometer monitors the entire system toconfirm the water vapor content of the generated mixture
and to trap for the possibility of minute system leaks, which
can bias the generated volume ratio. The condensation
hygrometer has a specified accuracy of +/- 0.15°C anda range of -80°C to +20°C dew/frost point temperature.
Typical agreement of the condensation hygrometer with thegenerated calibration mixture is shown in the table below.
The variation of these generated dew points from thoseshown in the previous table is within the accuracy toleranceof the specified accuracy of the condensation hygrometer.

Moisture calibration system in Panametrics manifold block diagram
Calibration system located in a temperature and relative humidity
controlled environment

Generated D/F PT, °C

D/F PT, chilled mirror
hygrometer, °C

9.63

9.61

-0.33

-0.26

-10.31-10.19-20.30-20.08-30.29-29.95-40.28-40.72-50.33-50.70-60.23-60.05-70.21-70.35-80.16-80.04Chilled mirror hygrometer

Data acquisition, storage and reduction
Data acquisition, storage and reduction is accomplishedby collecting electronic impedance data for individual
moisture sensors through the use of a direct reading ofdigital moisture probes, such as MISP2 or HygroRTE types
and by direct reading of analog moisture probes by meansof master circuit measurement electronics module withan available channel for each slot in the calibration tank.
The electronics of this module consists of analog signal
processing and analog to digital conversion.
Impedance data for each sensor is accessed by a
dedicated reference hygrometer channel for each probe
on a continuous basis. Providing an individual analog
measurement for each probe is a significant improvement
over the heritage system that had limited analog
measurement circuits and required a complex multiplexing
system to measure all the various probes in the calibrationbank. The net benefit to the system is faster measurement
processing and reduced overall calibration cycle time.
The analog signal processing segment contains a selfcalibration routine which assures that any drift in analog
circuitry of the system does not enter into the probe
calibration data. To accomplish self-calibration, the analog
electronics are commanded by the computer to readseveral high accuracy impedance standards. This data is
fed back to the computer for testing against standard data.
If the test data is within a predetermined maximum error
band, it is accepted by the computer, which then generates
a calibration curve. The calibration curve is automaticallyapplied to the actual probe impedance data. After the
analog data is collected and the calibration curve applied, it
is converted to digital information by the A-D converter.
At completion of the calibration run, the digitized impedance
data is moved to a long-term storage file. The data for atleast the recent two calibration data sets is stored in this file.
Inventory and probe suitability data is stored simultaneously.

The two data sets are used to calculate the actual change incalibration over the last calibration time periods (minimum
two month interval) and projected over the next six month
period. The pass criteria is that the calculated normalizedchange in calibration rate must support the calibration
warranty, where the change in calibration is projected to beless than ± 2°C dew/frost point over a six month period.
In addition, the calibration data is archived to enable
customers to access data about their sensors from anenterprise server. The historical data for a given sensor
can be accessed and used to determine the long-term
calibration characteristics for a particular application.
All current systems utilize microprocessor-based electronics.
For these microprocessor based hygrometers, the Z (sensor
admittance) reading versus the dew/frost point is tabulated
and a data sheet with all the necessary information for such
instruments is generated.
A local pc-based computer is used to control the calibration
system and to collect the calibration data. A National
Instruments LabView interfaces with the controllers, to view
and print sensor data, and to collect system diagnostics.
The system is duplicated at Panametrics facilities in Boston,
MA; Shannon, Ireland; Abu Dhabi, UAE; Tokyo, Japan; andChangzhou, China.
While this paper is based on the primary calibration systems
designed at Panametrics’ site in Billerica, MA using NIST
traceable calibration reference standards, systems locatedin other geographic locations are traceable to the applicableruling metrology authority. For example, Panametric’s facility
in Shannon, Ireland has their moisture calibration systemuse standards that are calibrated traceable to the National
Physics Laboratory (NPL) in the UK.

Calibration system LabView interface offering intuitive graphical control
and monitoring of the calibration run

Replication of this design, data collection, and system
control globally makes it possible for the data and systemverification diagnostics to be stored on redundant servers ina global database.

Overall aluminum oxide moisture probe calibration system view

Conclusion
The Panametrics primary calibration system is capableof calibrating up to 384 aluminum oxide moisture sensor
probes per calibration run. Each run requires seven days
for completion, including a three and one-half day dry
down period.
The probes are normally calibrated at 10 dew/frost points
over the range from -80°C to +10°C, with an accuracy of thegenerated gas/water vapor mixture of better than +/-0.4°C(see Appendix).
Temperature, pressure and flow measurement devices usedin this system are all traceable to the National Institute ofStandards and Technology. Thus, this calibration systemmeets the stated requirement, to calibrate aluminum oxide
moisture probes in quantity on a production line basis,
while maintaining rigorous quality control and quality
assurance criteria.

Appendix

P°T = Total pressure in saturator

The error analysis of moisture generator system is divided into
an analysis of gas saturation and an analysis of gas dilution.

KW = Enhancement factor
t = Temperature within the saturator

Saturator Section - The vapor pressure of water generated
by the saturator is given by the equation

The absolute accuracy of the saturator, based on theaccuracy of NIST traceable thermometer (+/- 0.10°C) andpressure gauge (+/- 0.05 psi), is +/- 0.7% in PS, or +/- 0.11°C in
dew point.
Dilution Section - The error in the dilution section varies
somewhat for different dew points, due to the different
flow rates and number of flowmeters employed. The vapor
pressure of water generated by dilution, PW, is given by

and the vapor pressure of water delivered to the calibration
banks is given by the equation
where
PS = Vapor pressure output by the saturator
where

FW1 = Wet gas flow for first dilution (FCV-1, FCV-2)

PS = Vapor pressure of water in calibration chamber

FD1 = Dry gas flow for first dilution (FCV-3)

P°S = Vapor pressure of water generated in saturator attemperature

FW2 = Moist gas flow for second dilution (FCV-4)

PT = Total pressure in calibration chamber

FD2 = Dry gas flow for second dilution (FCV-5)

A complete expression for the error in the above equation can be calculated(1). It is given by

The error in PS, +/- 0.7%, has been derived. The flowmeter
accuracy, specified by an independent calibration in service,is +/- 2% of flow or +/- 0.5% full-scale flow, whichever islarger. The application of second equation referenced to thedata in Flowrates and accuracies for generation of dew/frost
point temperatures table yields errors for each dew point.
The errors in water vapor pressure vary from +/- 0.7% at thehighest dew point up to +/- 6.8% at the lowest dew point.
These errors, converted to a dew point scale, are listed in

Flowrates and accuracies for generation of dew/frost pointtemperatures table. The dew point errors vary from +/- 0.1°C
to +/- 0.40°C.
The systematic error in dew point due to moisture in the drygas may be neglected at all dew points wetter than -80°C.
For the -80°C point the error is less than +0.4°C in dew point.
This is based on a maximum -85°C dew point at 90 psig inthe dry gas supply line.

References
1. Dietrich. C.F., “Uncertainty, calibration, and Probability’, John
Wiley, New York, 1973, p. 269.
2. Scelzo, Michael; Pierce, Ralph. “A High Precision Saturation
Dilution System for Water Vapor in a Carrier Gas”.
Proceedings of the 1985 International Symposium on
Moisture & Humidity. Instrument Society of America,
Research Triangle Park, NC. 1985. ISBN 0-87664-865-0.

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