Moisture measurement
technologies fornatural gas
By Gerard McKeogh. Global Product & Sales Manager, Panametrics

The measurement of moisture in natural gas is an important parameter forthe processing, storage and transportation of natural gas globally. Natural
gas is dehydrated prior to introduction into the pipeline and distribution
network. However, attempts to reduce dehydration result in a reduction in“gas quality” and an increase in maintenance costs and transportation as
well as potential safety issues. Consequently, to strike the right balance, it isimportant that the water component of natural gas is measured precisely
and reliably. Moreover, in custody transfer of natural gas between existing
and future owners maximum allowable levels are set by tariff, normally
expressed in terms of absolute humidity (mg/m3 or lbs/mmscfh) or dew
point temperature. Several technologies exist for the online measurement
and for spot sampling of moisture content. This paper reviews themost commonly used moisture measuring instruments and provides acomparison of those technologies.

Introduction

Prior to transportation, water is separated from raw naturalgas. However some water still remains present in thegaseous state as water vapor. If the gas cools or comes incontact with any surface that is colder that the prevailingdew point temperature of the gas, water will condensein the form of liquid or ice. Under pressure, water also
has the unique property of being able to form a lattice
structure around hydrocarbons such as methane to form
solid hydrates. Ice or solid hydrates can cause blockage inpipelines. In addition, water combines with gases such as
Hydrogen Sulfide (H2S) and Carbon Dioxide (CO2) to form
corrosive acids. Water in natural gas also increases the costof transportation in pipelines by adding mass and as water
vapor has no calorific or heating value it also adds to theexpense of compression and transportation.
When natural gas is sold, there are contractual requirements
to limit the concentration of water vapor. In the United States
the limit or tariff is expressed in absolute humidity in unitsof pounds per million standard cubic feet (lbs/mmscf). The
maximum absolute humidity for interstate transfer is set
at 7lbs/mmscf. In Europe, bodies such as EASEE-gas makerecommendations on the maximum permissible amount ofwater vapor in the gas. EASEE-gas has approved a limit of 8°C Dew Point, referenced to a gas pressure of 70 Bar(a). This
recommended limit is generally being adhered to in the gas
industry across Europe.

Instrument technologies formeasuring water vapor in natural gas
Various viable technologies exist for measuring the amountof water vapor in natural gas. These tend to rely on sample
conditioning systems, where a gas sample is extracted,
filtered, the pressures regulated and the flow controlled.
it is not advisable to install a sensor directly in a natural
gas pipeline as it can contain both physical contaminants(rust, scale, etc.), additives (such as odorizers, antifreeze
agents such as methanol) and liquid hydrocarbons. Anotherbenefit of a sampling system is that it can be isolated fromthe main pipeline. However, the sample system must notalter the moisture concentration of the sample via leaks ordesorption/adsorption from the wetted components.
Currently the most widely used measurement technologies
are chilled mirror, impedance sensors, quartz microbalance,
Fabry-Perot inferometer and tuneable diode lasers. Eachtechnology has its advantages and disadvantages.

Chilled mirrors

There are two basic categories of chilled mirror hygrometers:
manually operated and automated. Automated chilled
mirrors are further categorized into cycling chilled mirrors
and equilibrium chilled mirrors. Chilled mirrors measure
the dew/frost point temperature directly by using a coolant
or thermoelectric heat pump to cool a plane surface until
condensation forms. When the mass of condensate on the
mirror is in equilibrium with the surrounding gas sample, thetemperature of the mirror is by definition equal to dew or
frost point temperature.
Chilled mirrors can also be used to determine thehydrocarbon dew point. In gas mixtures containing heavierhydrocarbons the partial pressure of hydrocarbons are
sufficiently high enough that cooling the gas will result ina phase change from gas to liquid. In a similar principle,
the temperature at which hydrocarbon condensate is inequilibrium with the sample gas is the “hydrocarbon dew
point”.
Manual chilled mirrors typically use the expansion highpressure gas as the coolant. The manual chilled mirror
apparatus (also referred to as the Bureau of Mines type),
is described in ASTM-1142. When high-pressure gases suchas methane or CO2 are decompressed, cooling occurs dueto the Joule-Thomson effect. The user observes the onsetof condensation via a view port while the mirror surface iscooling. The rate of cooling is important. If the cooling rateis too rapid condensation occurs prior to thermal stability.
ASTM-1142 provides a procedure consisting of repeating
the test several times and successively slowing the coolingrate at the observed onset of condensation. The user
also has to learn to identify the difference between waterand hydrocarbon condensate. Water appears either as
fine droplets/fog (water) or opaque crystals (ice) whilehydrocarbon liquids appear as shiny film. In some designs
a matt black or ablated surface is used for hydrocarbons
while a polished metal surface is used for water. The dew/
frost reading is subjective, as each operator must decide inboth instances when condensation occurs and then identify
the condensate. Manual chilled mirrors are typically used forspot-checks and do not lend themselves to providing on-line
continuous readings or telemetry.
Automatic chilled mirrors utilize a thermoelectric coolingmodule coupled to a mirror. The cooling module consists ofa multistage stack of arrays of P-N junctions arranged in a
back-to-back orientation. When direct current is applied to
the P/N junctions electrons flow from the “P” junctions leaving
holes. The energy holes are filled with heat energy that flows
from the mirror. The P-N junctions are additionally thermallycoupled to a metal heat sink. If the polarity of the current
is reversed the mirror is heated. Visible or infrared light isemitted and aligned to reflect off the mirror. The reflected
light is received by a photodetector. When the mirror iscooled sufficiently, water vapor condenses on the mirror
and the light received by the photodetector decreases due
to both absorption and scattering of the incident light. The
signal from the photodector is then utilized in a feedback
control loop to maintain a constant mass. A precision

PRTD (Platinum Resistance Temperature Detector) measures
the temperature of the mirror. The heat pump can also be
augmented by refrigeration (evaporator core) or a liquid
coolant block.
The overall measurement capability of typical chilled
mirror is –80 to +85°C. The number of thermoelectriccooling stages, auxiliary, governs the full range. This system
offers excellent precision and is widely used to provide
laboratory reference standards for calibration and metrology
applications. However, the footprint and design of mostchilled mirror systems means that they have been andare restricted to laboratory applications Some units on
the market, such as Panametrics Optica and OptiSonde,
can be used for applications in non hazardous areas,
thereby offering the precision and repeatability previously
found only in laboratory standard instruments. As a result,these instruments could be used both to calibrate existing
impedance type sensors on-site and as high accuracy,highly stable humidity sensors in their own right.

Figure 1. Automatic chilled mirrorA

IR Emitter Regulator

B

IR EmitterC

IR Detector

D

IR Emitter ReferenceE

IR Detector Reference

F

Iris Adjustment

G

Optical Window

H

Operational AmplifierI

MirrorJ

Mirror BlockK

PRT (Temperature Sensor)

L

Multistage Thermoelectric Cooler

M

Temperature Indicator

N

Heat Sink/Heat Exchanger (Liquid Cooled)Table 1. Pros and cons of chilled mirror hygrometers
Pros

Cons

Precise: typical accuracy of ±0.1 to±0.5°C Td

Limited by cooling capacity Not a
compact system

Direct fundamental measurement ofdew/frost point

Requires containment or installation
in a purged enclosure for hazardous
area use

Inert wetted components. Longterm
stability. 5-20 years of service without
any drift

Not specific to water. Other gases
may condense prior to water such as
heavy hydrocarbons

Some models can measure atprocess pressure

Loses the ability to measure low
frost points as the temperature andpressure increases
Manual chilled mirror is subjective
and requires operator expertise

Impedance Sensors

Figure 3. Aluminum oxide sensor
A

Aluminum BaseB

Aluminium Oxide

C

Porous GoldThe sensor is excited with a low voltage alternating current at
a fixed frequency. The impedance of the sensor relates to thewater vapour pressure by the following relationship
Z−1 = AeBPw + C
Z = Impedance
Pw = Partial pressure of water
A, B, and C are constants

Figure 2. Natural gas compressor dehydration station – (Transport natural
gas moisture analysis)

The most widely used impedance based moisture sensor
technology for natural gas is the metaloxide sensor and,specifically, the aluminium oxide sensor. While there arevariations on design, the most widely used sensors consistof an aluminium base that has a thin layer of aluminumoxide deposited or grown on the surface by means of ananodization process. A thin layer of porous gold is depositedover the oxide. On a microscopic level the aluminium oxideappears as matrix with many parallel pores. When exposedto even small amounts of water vapour the superstructure
enables water molecules to permeate into the matrix
where micro-condensation occurs. Since the dielectric
constant of dry gases are significantly lower than gases
containing moisture (about an 80:1 ratio for nitrogen or
standard air) each pore acts as a micro-capacitor. As the
micro capacitors are in a parallel arrangement the total
capacitance is additive. In essence the sensor acts as awater molecule counter.

Each sensor is calibrated at multiple dew/frost points,
the partial pressure of water being a function of the dew/
frost point temperature. The impedance at each dew/frost
point is recorded and entered into a digital look-up table
either embedded in the memory of the sensor module
or programmed into an analyzer. The analyzer utilizes apolynomial expansion equation to convert the measured
impedance by reference to the look-up table to produce
direct readout in dew/frost point temperature. Typicalaccuracy is ±2°C Td from +60 to -65°C Td and ±3°C Td from–66 to -110°C Td.
In general impedance sensors provide excellent response tomoisture changes in the dry-to-wet direction. They however
have significant response times in the wet-to-dry direction.
Aluminium Oxide sensors are subject to drift over time.
The typical drift is around 2°C per year and this can be
managed by a regime of recalibration. Since Aluminium
Oxide sensors are economical, very often users maintainadditional sensors that are rotated in and out of service,thus always maintaining the in-service sensors within theirrecommended recalibration interval (typically one year).
Aluminium Oxide sensors have the capability to be installedat high pressure (up to 5000psig) and their footprint is quite
compact. The sensors however are seldom installed directlyin the pipeline. Instead, an extraction type sampling system
is utilized, allowing the gas to be filtered, the pressure to be
regulated and the flow rate to be controlled.

Table 2. Pros and cons of impedance sensors
Pros

Cons

Available with hazardous area
certification (XP and IS)

Sensors have significant wet to dry
response time particularly after
process upsets

The sensor has a small footprint andmay be installed long distances fromthe analyzer

Other polar gases such as alcohols
or amines also change the sensor
impedance

Sensor may be installed at linepressure. No adjustment required forvariation in natural gas composition.

Yearly recalibration is required

Large dynamic measurement range(-110°C to +60°C)

May be attacked by some Sulphur
compounds

Sensors are economically priced

Manual chilled mirror is subjective
and requires operator expertise

Sensors are easy to replace in thefield

Quartz microbalance hygrometers

Quartz microbalance hygrometers consist of a quartz
substrate that is coated with a hygroscopic polymer film.
When a voltage is applied, the quartz oscillates at a resonant
frequency. When the sensor is exposed to gas with water
vapour, water is adsorbed by the hygroscopic coating andthe resonant frequency changes in accordance with theincreased mass of the sensor. The adsorption of water into
the sensor’s substrate is proportional to the partial pressureof the surrounding water vapor.
Quartz microbalance sensors have a certain degreeof hysteresis and must be “re-zeroed” periodically. The
measurement system therefore requires a “zero gas”. Whileno gas supply can have an absolute value of zero, the zerogas may be defined as a gas that is closer to zero than any
significant amount of water. Typically accuracy is ±10% of
reading from 1-2,500 ppmv (parts per million by volume).
Some measurement modes employ a non-equilibrium
technique where the sensor alternates from being exposedto the zero gas and the process gas. The offline time spenton the zero gas should be factored into response timerequirements.
The sensing surface is also susceptible to contaminationand must remain clean. A suitable sampling system must be
employed. Quartz Microbalance analyzers are characterizedby having relatively fast response times.

Figure 4. Quartz microbalance sensor
A

Quartz Substrate (Oscillator)

B

Hygroscopic Coating

C

ElectrodesE

Gas InletF

Gas Outlet

G

Stainless Steel Flow Cell

Table 3. Pros and cons of quartz microbalance
Pros

Cons

Wide range ability to measure to sub
1 ppmv levels

Relatively expensive

Moderately fast response time

Supply of zero and span gases
neededReadings are step changes above
zero baseline not absolute values
Flow rate, temperature and pressuremust be precisely controlled

Fabry-Perot hygrometers

The sensor head in Fabry-Perot type hygrometers consists ofa multi-layered structure comprising materials with high andlow refractive indices. Typical materials used are SiO2 andZrO2. The sensor head is coated with a glass substrate witha maximum surface pore size no bigger than 0.4 nm, making
the structure specific to water molecules (pore size 0.28 nm).
A light beam is transmitted through the sensor via fibre optic
cable. The light source is generally a light–emitting diode(LED). As water molecules penetrate the sensor surface, theychange the refractive index of the light beam (refractive
index of Air 1: Water 1.33) causing a change in wavelength.
The wavelength change is proportional to the amount ofwater molecules equilibrated on the sensor. The refracted
light is detected by a Polychromator and the reading iscalibrated in terms of dew point temperature vs. wavelength
shift.
The sensor itself is mounted on the end of a stainless steel
probe and connected via fiber optic cable to the control unit.
The unit requires temperature compensation and pressurecompensation if a ppmv readout is required.

The Beer-Lambert principle states that when light energy at
certain wavelengths travel through gas a certain amountof the energy is absorbed by the water within the path. The
amount of light energy lost is related to the concentration ofwater.
A diode laser is very similar to an LED in that when a current
is injected into a p-n junction, holes and electrons recombine
and release photons.

Figure 5. Fabry-Perot hygrometer schematicA

IR Emitter

B

Fibre Optic Beam WaveguideC

Glass Carrier (Quartz)

D

Multi-Layered Structure (SiO2 ZrO2)

E

Diffraction GridF

CCD Array

Table 4. Pros and cons of Fabry-Perot hygrometer
Pros

Cons

Intrinsically safe – light signal

Expensive fiber optic cable needed toconnect probe to analyzer

Sensor may be installed at linepressure

Slow response
Hygroscopic coating may degrade
after prolonged exposure to naturalgas

TDLAS hygrometers

Tunable Diode Laser Absorption Spectrometers (TDLAS)
offer a fully non-contact method of continuous moisturemeasurement in natural gas. The measuring principle isbased on the Beer-Lambert Law.
I
A = ln( 0) = S • L • N
I
A = Absorption

A diode laser stimulates the release of these photons andincorporates an optical cavity to create laser oscillationand the release of a beam of coherent light at a single
wavelength or frequency.
It is possible to change the frequency of the emitted lightby changing the temperature of the “injection current”.
The monochromatic frequency can also be modulated.
Consequently by passing light at the water absorption
frequency through a sample chamber containing natural
gas of a certain moisture content, it is possible to precisely
establish the water content by measuring the amount of loss
in the absorption spectrum.
In practice, a TDL system measures the water concentrationin natural gas by sweeping a narrow band laser diodeand changing its wavelength by ramping the injection
current while holding the temperature constant by use of athermoelectric (Peltier) heat pump array. The laser is also
modulated at high frequency. At the center frequency thesecond harmonic (known as 2F) peak height is measured.
The 2F peak height is directly proportional to the partialpressure of water in the absorption cell. By simultaneously
measuring the cell total pressure the concentration in
ppmv is determined. Boyle’s law, which relates the pressureof a gas to the volume, is then applied. The simultaneous
measurement of the gas temperature and process pressure
enable other humidity parameters such as absolute
humidity, dew point and process dew point to be determinedwith a high degree of precision.

I = The measurement of beam intensity when tuned to theabsorbing wavelength of moisture
Io = The reference measurement or beam intensity when
tuned away from the moisture absorbing wavelength
S is the fundamental absorption line strength and is a fixed
constant
L = the path length of the beam through the sample and is a
fixed constantN = the number of water molecules contained in the beam
path passing through the sample
ln is the natural log

Figure 6. TDLAS hygrometer

The typical accuracy of a TDLAS hygrometer for natural gas
is 2% of reading in terms of the mole fraction or ppmv. By
simultaneously measuring the temperature and pressure,the absolute humidity and dew/frost point temperature ismeasured with high precision by the use of psychometricequations. Measurement of the process line pressure alsoenables these units to calculate the pressure dew point.
TDLAS hygrometers are characterized by having very fast
response times. The optical response is in <2 seconds.
However it takes time to purge the absorption cell andsampling systems. Typical system response times are lessthan 5 minutes for a 90% step change.
Table 5. Pros and cons of TDLAS hygrometers
Pros

Cons

Very fast response in both directions.
Dry to wet and wet to dry Noncontact.

Relatively expensive

No sensing surface to degrade due
to exposure

Must be calibrated using a testgas with the same basic major
components of the process gas

High long-term stability

Measurement is made a close toatmospheric pressure.

No zero or span gases needed
Based on fundamental measurement
Immune to glycol, H2S, methanol andother contaminants found in naturalgas.

Conclusions

The technology with the widest measurement range istypically the impedance type sensor, which can measurefrom -110°C to +60°C. The narrowest measurement range isconfined to the automatic chilled mirror, which is constrainedby the number of stages (cooling capacity) of the sensor
installed with the device. Fabry-Perot type analyzers haverange capability similar to impedance type sensors. The
TDLAS technology range of measurement is determined by
the type of measurement cell used in the device. A standard
measurement cell has a typical lower detectable limit of
5 ppmv, while the latest instruments extend this to sub 100
ppbv. Upper ranges can be from 2000 to 5000ppm. Quartz
microbalance ranges down to 0.1/1ppm with upper ranges of1000 to 2000 ppmv.
In terms of accuracy, the automatic chilled mirror technology
is the most precise offering a typical accuracy of 0.1°C to
0.5°C dew point. The TDLAS unit is the next most precise
instrument with a typical accuracy of +/-2% of reading(accuracy will vary in terms of dew point due to the nonlinear relationship).
The most stable or drift free technologies can be considered
to be TDLAS and Chilled Mirror.
Lasers, by their nature are inherently stable and theremaining components in the device can essentiallybe considered drift free. The non-contact nature of themeasurement ensures that there is no process relateddegradation of the measurement circuitry, laser light sourceor detectors. At the other extreme, the measurement layer in
impedance type sensors is in a continual state of drift, whichneeds to be continually corrected by regular calibrations.
In terms of response time, TDLAS comes out on top. The
technologies that require equilibrium of moisture in thegas sample with a sensing surface/layer suffer in thiscategory due to the polar nature of the water molecule andits tendency to stick to surfaces. A significant contact timewith the gas to be measured is required, more specifically ingoing from a wet sample gas to a dry sample gas.
Maintenance is an important consideration when evaluating
the lifetime costs of the different measurement technologies.
As more customers outsource the maintenance function,
they continually look to install low maintenance equipment.
Contact based sensors will always require more in termsof maintenance than non-contact based measurements
as their successful operation is much more dependent
on a clean sample gas reaching the sensor. Corrosive
components in the natural gas stream, like Sulphur
compounds will also add to the maintenance requirements
for contact based sensors. Maintenance requirements
range from periodic inspection/replacement of sample
system filters to annual or bi-annual recalibration of thesensors themselves. The TDLAS technology does not havean annual recalibration requirement and is typically sold asa maintenance-free technology, with the exception of any
associated sample system filter maintenance. A planned
factory calibration check every three to five years is typicalwith TDLAS technology.
All technologies require a clean gaseous phase sample
to reach the sensor, hence a sample handling system isalways recommended, although some vendors promote
direct inline measurement as an advantage. Mixed
phase sample, condensate or liquid glycol carryover, cancoat contact based sensors, causing them to becomeunresponsive or read erroneously, or in extreme cases, canrequire sensor replacement. Liquid contaminant can also
deposit in the TDLAS measuring cell, causing dispersal of
the light signal, resulting in an erroneous measurement.
The TDLAS technology has the capability to alert the user
if contamination occurs, by comparing the measuring
photo-detector tuned to a non-absorbing wavelengthwith a reference photo-detector, to determine if a shift has
occurred (within some specified limits).
Contact based sensors may be partially contaminated andcontinue reading, although experienced users may be able
to determine contamination has taken place by observing
sensor behavior in terms of response to step changes inmoisture or an actual step change in process readings after
the contamination event. In most cases, if sensors becomecontaminated they can be cleaned, purged with a dry gas,and returned to service.

Table 6. Scorecard of moisture measurement technologies for natural gas
A

B

C

D

E

Range

1

5

3

4

3

Precision

5

2

3

3

4

Stability

5

2

4

4

5

Speed of response

3

2

3

2

5

Maintenance

3

3

2

3

5

Price

1

5

1

2

1

TOTAL

18

19

16

18

23

5 = Most Desirable, 1 = Least Desirable
A = Chilled Mirror
B = Impedance
C = Quartz Microbalance
D = Fabry=Perot
E = Tuneable Diode Laser

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(11/2020)

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