Basic Hygrometry Principles
Reference Guide

panametrics.com

BH022C81
September 2022

Basic Hygrometry Principles
Reference Guide
BH022C81 Rev. C
September 2022

panametrics.com
Copyright 2022 Baker Hughes company.
This material contains one or more registered trademarks of Baker Hughes Company and its subsidiaries
in one or more countries. All third-party product and company names are trademarks of their respective
holders.

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Contents

1.
2.

3.

4.
5.
6.

7.

8.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
Moisture Monitor Hints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
2.1 Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.2 Response Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.3 Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2.4 Flow Rate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Contaminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1 Non-Conductive Particulates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2 Conductive Particulates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.3 Corrosive Particulates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Aluminum Oxide Probe Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1 Cleaning the Moisture Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Corrosive Gases And Liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Calculations and Useful Formulas in Gas Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1 Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.2 Parts per Million by Volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.3 Parts per Million by Weight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.4 Relative Humidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.5 Weight of Water per Unit Volume of Carrier Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.6 Weight of Water per Unit Weight of Carrier Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.7 Comparison of ppmv Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Liquid Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.1 Theory of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.2 Moisture Content Measurement in Organic Liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.3 Additional Notes for Liquid Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Solids Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Basic Hygrometry Principles Reference Guideiii

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Basic Hygrometry Principles Reference Guide

1.

Introduction

This reference guide contains general information about moisture monitoring techniques. System contaminants,
moisture probe maintenance, process applications and other considerations for ensuring accurate moisturemeasurements are discussed. The following specific topics are covered:•

Moisture Monitor Hints•

Contaminants•

Aluminum Oxide Probe Maintenance•Corrosive Gases and Liquids•

Calculations and Useful Formulas in Gas Applications•

Liquid Applications•

Solids Applications

2.

Moisture Monitor Hints

Panametrics hygrometers, using aluminum oxide moisture probes, have been designed to reliably measure themoisture content of both gases and liquids. The measured dew point will be the real dew point of the system at themeasurement location and at the time of measurement. However, no moisture sensor can determine the origin of
the measured moisture content. In addition to the moisture content of the fluid being analyzed, the water vapor
pressure at the measurement location may include components from sources such as: moisture from the inner wallsof the piping; external moisture through leaks in the piping system; and trapped moisture from fittings, valves, filters,
etc. Although these sources may cause the measured dew point to be higher than expected, it is the actual dew
point of the system at the time of measurement.
The aluminum oxide sensors used with Panametrics hygrometers may be mounted either in-situ or in an external
sample system. In-situ measurements are occasionally used in vacuum applications. While in-situ measurements
reduce the overall system cost and may provide improved transport time, in-situ installations expose the sensor tocontamination from the process. Thus, installation of the moisture probe in a sample system is generally
recommended to control sample pressure, sample temperature and sample flow rate. In addition, the sample
system can filter contaminants and facilitate removal of the sensor for maintenance and calibration. In short,sample systems allow a representative sample of the process gas to be exposed to the sensor for optimum
performance.
Whether the sensor is installed in-situ or in a remote sampling system, the accuracy and speed of measurement
depend on the piping system and the dynamics of the fluid flow. Response times and measurement values will beaffected by the degree of equilibrium reached within the system. Factors such as gas pressure, flow rate, materials of
construction, length and diameter of piping, initial moisture level and final moisture level will greatly influence themeasured moisture levels and the response times.
Assuming that all secondary sources of moisture have been eliminated and the sample system has been allowed tocome to equilibrium, then the measured dew point will equal the actual dew point of the process fluid.
Some of the most frequently encountered problems associated with moisture monitoring sample systems include:•

the moisture content value changes as the total gas pressure changes•

the measurement response time is very slow•

the dew point changes as the fluid temperature changes•

the dew point changes as the fluid flow rate changes.

Panametrics hygrometers measure only water vapor pressure. In addition, the instrument has a very rapid responsetime and it is not affected by changes in fluid flow rate. If any of the above situations occur, then they are almost
always caused by a defect in the sample system, or by a problem with the overall process system in general. The
moisture sensor itself cannot lead to such problems.

Basic Hygrometry Principles Reference Guide

1

2.1

Pressure

Panametrics hygrometers can accurately measure dew points under pressure conditions ranging from vacuums as
low as a few microns of mercury up to pressures of 5000 psig. The calibration data supplied with the moisture probe
is directly applicable over this entire pressure range, without correction.

Note:

Although the moisture probe calibration data is supplied as meter reading vs. dew point, it is important toremember that the moisture probe responds only to water vapor pressure.

When a gas is compressed, the partial pressures of all the gaseous components are proportionally increased.
Conversely, when a gas expands, the partial pressures of the gaseous components are proportionally decreased.
Therefore, increasing the pressure on a closed aqueous system will increase the vapor pressure of the water, andhence, increase the dew point. This is not just a mathematical artifact. The dew point of a gas with 1,000 ppmv ofwater at 200 psig is 13°C, which is considerably higher than the dew point of a gas with 1,000 ppmv of water at 1 atm,
which is -20°C. Gaseous water vapor will actually condense to form liquid water at a higher temperature at the 200
psig pressure than at the 1 atm pressure. Thus, if the moisture probe is exposed to pressure changes, the measured
dew point will be altered by the changed vapor pressure of the water.
It is generally advantageous to operate the hygrometer at the highest possible pressure, especially at very low
moisture concentrations. This minimizes wall effects and results in higher dew point readings, and reduces the timenecessary for an equilibrium dewpoint.

2.2

Response TimeThe response time of the Panametrics standard M Series Aluminum Oxide Moisture Probe is very rapid. Thus, theobserved response time to moisture changes is, in general, limited by the response time of the system as a whole.
Water vapor is absorbed tenaciously by many materials, and a large, complex processing system can take several
days to“dry down” from atmospheric moisture levels to dew points of less than –60°C. Even simple systems consisting of afew feet of stainless steel tubing and a small chamber can take an hour or more to dry down from dew points of+5°C to -70°C. The rate at which the system reaches equilibrium will depend on flow rate, temperature, materials of
construction, initial moisture content, final moisture content and system pressure. Generally speaking, an increase inflow rate and/or temperature will decrease the response time of the system.
To minimize any adverse affects on response time, the preferred materials of construction for moisture monitoring
sample systems are stainless steel, PTFE and glass. Materials to be avoided include rubber elastomers and related
compounds.

2.3

Temperature

2.4

Flow Rate

For best results, it is recommended that the ambient temperature be at least 10°C higher than the measured dew
point, up to a maximum of 70°C. Because an ambient temperature increase may cause water vapor to be desorbed
from the walls of the sample system or process piping, it is possible to observe a diurnal change in moistureconcentration for a system exposed to varying ambient conditions. In the heat of the day, the system walls will bewarmed by the ambient air and an off-gassing of moisture into the process fluid, with a corresponding increase inmeasured moisture content, will occur. The converse will happen during the cooler evening hours.
Panametrics hygrometers are unaffected by the fluid flow rate. The moisture probe is not a mass sensor but
responds only to water vapor pressure. The moisture probe will operate accurately under both static and dynamic
fluid flow conditions. In fact, the specified maximum fluid linear velocities (see Table 1 and Table 2 ) for the aluminumoxide moisture probe indicate a mechanical stability limitation rather than a sensitivity to the fluid flow rate.
If the measured dew point of a system changes with the fluid flow rate, then it can be assumed that off-gassing or aleak in the sample system is causing the variation. If secondary moisture is entering the process fluid (either from an
ambient air leak or the release of previously absorbed moisture from the sample system walls or process piping), an
increase in the flow rate of the process fluid will dilute the secondary moisture source. As a result, the vapor pressure
will be lowered and a lower dew point will be measured.

Note:

2

Refer to the Specifications section of the Hygrometer User’s Manual or Datasheet for the maximum
allowable flow rate for your instrument.

Basic Hygrometry Principles Reference Guide

Note:

Note:

Table 1: Maximum Gas Flow Rates
Based on the physical characteristics of air at a temperature of 77°F and a pressure of
1 atm, the following flow rates will produce the maximum allowable gas stream linear
velocity of 10,000 cm/sec in the corresponding pipe sizes.
Inside Pipe Diameter (in.)

Gas Flow Rate (cfm)

0.25

7

0.50

27

0.75

60

1.0

107

2.0

429

3.0

966

4.0

1,718

5.0

2,684

6.0

3,865

7.0

5,261

8.0

6,871

9.0

8,697

10.0

10,73711.0

12,991

12.0

15,461

Table 2: Maximum Liquid Flow Rates
Based on the physical characteristics of benzene at a temperature of 77°F, thefollowing flow rates will produce the maximum allowable fluid linear velocity of 10
cm/sec in the corresponding pipe sizes.

Inside Pipe Diameter (in.)(gal/hr)

0.25

3Flow Rate(l/hr)
11

0.50

12

46

0.75

27

103

1.0

48

182

2.0

193

730

3.0

434

1,642

4.0

771

2,919

5.0

1,205

4,561

6.0

1,735

6,567

7.0

2,361

8,939

8.0

3,084

11,675

9.0

3,903

14,776

10.0

4,819

18,243

11.0

5,831

22,074

12.0

6,939

26,269Basic Hygrometry Principles Reference Guide3

3.

ContaminantsIndustrial gases and liquids often contain fine particulate matter. Particulates of the following types are commonly
found in such process fluids:•

carbon particles

•

salts

•

rust particles•

polymerized substances•

organic liquid droplets•

dust particles•

molecular sieve particles•

alumina dust

For convenience, the above particulates have been divided into three broad categories. Refer to the appropriate
section that follows for a discussion of their affect on the Panametrics moisture probe.

3.1

Note:

Non-Conductive Particulates

Molecular sieve particles, organic liquid droplets and oil droplets are typical of this category.

In general, the performance of the moisture probe will not be seriously hindered by the condensation of
non-conductive, non-corrosive liquids. However, a slower response to moisture changes will probably be observed,
because the contaminating liquid barrier will decrease the rate of transport of the water vapor to the sensor andreduce its response time.
Particulate matter with a high density and/or a high flow rate may cause abrasion or pitting of the sensor surface.
This can drastically alter the calibration of the moisture probe and, in extreme cases, cause moisture probe failure. A
stainless steel shield is supplied with the moisture probe to minimize this effect, but in severe cases, it is advisable toinstall a PTFE or stainless steel filter in the fluid stream. Also, a good sample system can mitigate the effects ofparticulate matter.
On rare occasions, non-conductive particulate material may become lodged under the contact arm of the sensor,
creating an open circuit. If this condition is suspected, refer to “Aluminum Oxide Probe Maintenance” on page 5 and“Cleaning the Moisture Probe” on page 5, for the recommended cleaning procedure.

3.2

Note:

Conductive Particulates

Metallic particles, carbon particles, alcohols and other conductive liquid droplets are typical of thiscategory.

Since the hygrometer reading is inversely proportional to the impedance of the sensor, a decrease in sensor
impedance will cause an increase in the meter reading. Thus, trapped conductive particles across the sensor leads
or on the sensor surface, which will decrease the sensor impedance, will cause an erroneously high dew point
reading. The most common particulates of this type are carbon (from furnaces), iron scale (from pipe walls),
alcohols (used as water scavengers) and glycol droplets (from glycol-based dehydrators).
If the system contains conductive particulates, it is advisable to install an appropriate filter to minimize these effects.
In addition, cleaning and recalibration of the moisture probe is recommended.

3.3

Note:

Corrosive Particulates

Sodium chloride and sodium hydroxide particulates are typical of this category.

Since the active sensor element is constructed of aluminum, any material that corrodes aluminum will deleteriously
affect the operation of the moisture probe. Furthermore, a combination of this type of particulate with water willcause pitting or severe corrosion of the sensor element. In such instances, the sensor cannot be cleaned or repaired
and the probe must be replaced.
Obviously, the standard moisture probe cannot be used in such applications unless the complete removal of suchparticulates by adequate filtration is assured. Please contact Panametrics to review your application requirements.

4

Basic Hygrometry Principles Reference Guide

4.

Aluminum Oxide Probe MaintenanceAs part of a routine preventive maintenance program, the moisture probe should be removed and returned to thefactory for recalibration once a year. Between these calibrations, if the aluminum oxide moisture probe becomes
contaminated with an electrically conductive liquid, the moisture measurements will be erroneously high. In such asituation, the probe should be removed from the sample system and cleaned as described in “Cleaning the Moisture
Probe” on page 5.

WARNING! The probe cleaning procedure should be performed only by a qualified technician or
chemist.

IMPORTANT: Moisture probes must be handled carefully and cannot be cleaned in any fluid which will attack its
components. Panametrics recommends using hexane or toluene, as outlined in the cleaning procedureon the next page. Also, the sensor’s aluminum sheet is very fragile and can be easily bent or distorted.
Do not permit anything to touch it!

WARNING! Make sure you reinstall the moisture probe before restarting the system.

4.1

Cleaning the Moisture Probe

To clean the moisture probe, the following items are required:•approximately 600 ml of reagent grade hexane or toluene, divided into two batches of 300 ml each•approximately 300 ml of distilled (NOT deionized) water•

three glass (NOT metal) containers to hold the above liquids

To clean the aluminum oxide moisture probe, complete the following steps:
1.

Record the dew point of the ambient air.

2.

Carefully remove the protective shield covering the sensor without touching the exposed sensor (see Figure 1 on
page 6).

3.

Soak the sensor in one of the containers of hexane or toluene for 10 minutes.

IMPORTANT: To avoid damaging the sensor during the following steps, do not allow the sensor to contact the walls or
the bottom of the containers.

4.

Remove the sensor from the hexane or toluene and soak it in the container of distilled water for 10 minutes.

5.

Remove the sensor from the distilled water and soak it in the second container of (clean) hexane or toluene for10 minutes.

6.

Remove the sensor from the hexane or toluene and place it sensor- side-up in an oven set at
50°C ± 2°C (122°F ± 3.6°F) for 24 hours.

7.

Repeat Steps 3-6 to clean the protective shield. To ensure the removal of any contaminants that may havebecome embedded in the porous walls of the shield, swirl the shield in the solvents during the soaking procedure.

8.

Without touching the exposed sensor, carefully reinstall the protective shield over the sensor.

9.

Connect the probe cable to the cleaned probe and measure the dew point of the same ambient air recordedin Step 1.

10. If the probe is determined to be in proper calibration (±2°C/±3.6°F), it has been successfully cleaned and may bereinstalled in the sample cell. If the probe is not in proper calibration, proceed to Step 11.
11.

Repeat Steps 1-10 using soaking time intervals of 5 times the previous cleaning sequence, until two consecutive
cleanings produce identical probe responses to the ambient dew point.

Basic Hygrometry Principles Reference Guide

5

Figure 1: Panametrics M Series Moisture Probe

6

Basic Hygrometry Principles Reference Guide

Equivalent to Bendix Conn. #PT1H-8-4P

ref. 1.125 (28.58)

2.750 (69.85) Max.

3/4-16 THD

3.625 (92.07) Max.

Dimensions are in inches (millimeters).

O-Ring Size
3/4 ID x 3/32 Dia.
(Viton A)

0.635 (16.13) Max.

R Shield(100 Micron Porosity)

Sensor5.

Corrosive Gases And Liquids

Note:

This category typically includes some acids and halogens.

Panametrics aluminum oxide moisture probes have been designed to minimize the effect of corrosive gases andliquids. The moisture content of gases containing corrosives, such as H2S, SO2, cyanide containing gases, acetic acidvapors, etc. may be measured directly, depending on the concentration level of corrosives. Please contact
Panametrics for assistance.
IMPORTANT: Since the active sensor is aluminum, any fluid which corrodes aluminum will affect the sensor’s
performance.
By observing the following precautions, the moisture probe may be used successfully and reliably. Please consult
Panametrics for assistance with your particular application.
1.

Generally, the moisture content of the corrosive fluid must be 10 ppmv or less at 1 atmosphere, or theconcentration of the corrosive fluid must be 10 ppmv or less at 1 atmosphere. However, the exact level that can beused depends on the application.

2.

The moisture sensor should be installed in the sample system. The sample system must be pre-dried with a dry
inert gas, such as nitrogen or argon, prior to introduction of the fluid stream. Any adsorbed atmospheric moistureon the sensor will react with the corrosive fluid to cause pitting or corrosion of the sensor.

3.

The sample system and moisture sensor must be purged with a dry inert gas, such as nitrogen or argon, prior to
removal of the moisture probe. Any adsorbed corrosive fluid on the sensor will react with ambient moisture tocause pitting or corrosion of the sensor.

4.

Operate the sample system at the lowest possible gas pressure, to reduce the partial pressure of the corrosive
contaminant.

5.

At the onset of a high moisture upset, the moisture probe must be switched from process gas to a dry, inert
purge gas, such as nitrogen. The moisture probe should remain in the purge gas until the process upset iscorrected.

6.

Prior to removing the moisture sensor from the sample system, the system must be purged with a dry, inert gas,such as nitrogen.

Using the precautions listed above, the moisture probe may be used to successfully measure the moisture content ingases containing hydrochloric acid, sulfur dioxide, chlorine and bromine. Please consult Panametrics for application
assistance and recommendations for a proper sample system design.

6.

Calculations and Useful Formulas in Gas Applications

A knowledge of the dew point, pressure, and temperature of a system enables one to calculate all other moisturemeasurement parameters. The most important fact to recognize is that for a particular dew point there is one andonly one equivalent vapor pressure.
IMPORTANT: The calibration of Panametrics aluminum oxide moisture probes is based on the vapor pressure of liquidwater above 0°C and frost below 0°C. Panametrics moisture probes are never calibrated withsupercooled water.
Caution is advised when comparing dew points measured with a Panametrics aluminum oxide hygrometer to thosemeasured with a Panametrics chilled mirror hygrometer, since such mirror-type hygrometers may provide the dew
points of supercooled water.
As stated above, the dew/frost point of a system defines a unique partial pressure of water vapor in the gas. Table 3
on page 11 , which lists water vapor pressure as a function of dew point, can be used to find either the saturationwater vapor pressure at a known temperature or the water vapor pressure at a specified dew point. In addition, alldefinitions involving humidity can then be expressed in terms of the water vapor pressure.

Basic Hygrometry Principles Reference Guide

7

6.1

Nomenclature•

RH = relative humidity•

TK = temperature (°K = °C + 273)•

TR = temperature (°R = °F + 460)•

ppmv = parts per million by volume•

ppmw = parts per million by weight•

Mw = molecular weight of water (18)•MT = molecular weight of carrier gas•

PS = saturation vapor pressure of water at the prevailing temperature (mm of Hg)•

PW = water vapor pressure at the measured dew point (mm of Hg)•

PT = total system pressure (mm of Hg)

The following symbols and units are used in the equations that are presented in the next few sections:

6.2

Parts per Million by Volume

The water concentration in an ideal gas system, in parts per million by volume, is proportional to the ratio of thewater vapor partial pressure to the total system pressure:PW
6
ppm v = -------  10
PT

(1)

In a closed system, increasing the total pressure of the gas will proportionally increase the partial pressures of thevarious components. The relationship between dew point, total pressure and ppmv is provided in nomographic formin Figure 2 on page 9.

Note:

The nomograph shown in Figure 2 on page 9 is applicable only to gases. Do not apply it to liquids.

To compute the moisture content for any ideal gas at a given pressure, refer to Figure 2 on page 9. Using astraightedge, connect the dew point (as measured with the Panametrics Hygrometer) with the known systempressure. Read the moisture content in ppmv where the straightedge crosses the moisture content scale.

Typical Problems
1.

Find the water content in a nitrogen gas stream, if a dew point of -20°C is measured and the pressure is 60 psig.

Solution: In Figure 2 on page 9, connect 60 psig on the Pressure scale with -20°C on the Dew/Frost Point scale.
Read 200 ppmv, on the Moisture Content scale.
2.

Find the expected dew/frost point for a helium gas stream having a measured moisture content of 1000 ppmv
anda system pressure of 0.52 atm.
Solution: In Figure 2 on page 9, connect 1000 ppmv on the Moisture Content scale with 0.52 atm on thePressure scale. Read the expected frost point of –27°C on the Dew/Frost Point scale.

8

Basic Hygrometry Principles Reference Guide

Figure 2: Moisture Content Nomograph for Gases
Basic Hygrometry Principles Reference Guide

9

6.3

Parts per Million by Weight

The water concentration in the gas phase of a system, in parts per million by weight, can be calculated directly fromthe ppmv and the ratio of the molecular weight of water to that of the carrier gas as follows:MW
ppm w = ppm v  --------MT(2)

6.4 Relative Humidity

Relative humidity is defined as the ratio of the actual water vapor pressure to the saturation water vapor pressure atthe prevailing ambient temperature, expressed as a percentage.
PW
RH = -------  100
PS(3)

Typical Problems
Find the relative humidity in a system, if the measured dew point is 0°C and the ambient temperature is +20°C.
Solution: From Table 3 on page 11, the water vapor pressure at a dew point of 0°C is 4.579 mm of Hg and thesaturation water vapor pressure at an ambient temperature of +20°C is 17.535 mm of Hg. Therefore, the relative
humidity of the system is 100 x 4.579/17.535 = 26.1%.

6.5 Weight of Water per Unit Volume of Carrier Gas

Three units of measure are commonly used in the gas industry to express the weight of water per unit volume ofcarrier gas. They all represent a vapor density and are derivable from the vapor pressure of water and the Perfect
Gas Laws. Referenced to a temperature of 60°F and a pressure of 14.7 psia, the following equations may be used to
calculate these units:
PW
mg of water----------------------------- = 289  ------TK
liter of gas(4)PW
lb of water-------------------------- = 0.0324  ------3
TR
ft of gas

(5)

6

10  P W
ppm
lb of water------------------------------------- = -------------v = ---------------------21.1  P T
21.1
MMSCF of gas

Note:

(6)

MMSCF is an abbreviation for a “million standard cubic feet” of carrier gas.

6.6 Weight of Water per Unit Weight of Carrier Gas

Occasionally, the moisture content of a gas is expressed in terms of the weight of water per unit weight of carrier gas.
In such a case, the unit of measure defined by the following equation is the most commonly used:
M W  PW
grains of water------------------------------------ = 7000  -----------------------MT  PT
lb of gas

(7)

For ambient air at 1 atm of pressure, the above equation reduces to the following:grains of water =-----------------------------------5.72  P W
lb of gas

10

(8)

Basic Hygrometry Principles Reference Guide

Note:

Table 3: Vapor Pressure of Water
If the dew/frost point is known, the table will yield the partial water vapor pressure (PW) in mm of Hg. If theambient or actual gas temperature is known, the table will yield the saturated water vapor pressure (PS) in
mm of Hg.
Water Vapor Pressure Over Ice

Temp. (°C)

0

2

4

6

8

-90

0.000070

0.000048

0.000033

0.000022

0.000015-80

0.000400

0.000290

0.000200

0.000140

0.000100-70

0.001940

0.001430

0.001050

0.000770

0.000560-60

0.008080

0.006140

0.004640

0.003490

0.002610-50

0.029550

0.023000

0.017800

0.013800

0.010600-40

0.096600

0.076800

0.060900

0.0481000.037800-300.285900

0.231800

0.187300

0.150700

0.120900

Temp. (°C)

0.0

0.2

0.4

0.6

0.8

-29

0.317

0.311

0.304

0.298

0.292

-28

0.351

0.344

0.337

0.330

0.324

-27

0.389

0.381

0.374

0.366

0.359

-26

0.430

0.422

0.414

0.405

0.397

-25

0.476

0.467

0.457

0.448

0.439

-24

0.526

0.515

0.505

0.495

0.486

-23

0.580

0.569

0.558

0.547

0.536

-22

0.640

0.627

0.615

0.603

0.592

-21

0.705

0.691

0.678

0.665

0.652

-20

0.776

0.761

0.747

0.733

0.719

-19

0.854

0.838

0.822

0.806

0.791

-18

0.939

0.921

0.904

0.887

0.870

-17

1.031

1.012

0.993

0.975

0.956

-16

1.132

1.111

1.091

1.070

1.051

-15

1.241

1.219

1.196

1.175

1.153

-14

1.361

1.336

1.312

1.288

1.264

-13

1.490

1.464

1.437

1.411

1.386

-12

1.632

1.602

1.574

1.546

1.518

-11

1.785

1.753

1.722

1.691

1.661

-10

1.950

1.916

1.883

1.849

1.817

-9

2.131

2.093

2.057

2.021

1.985

-8

2.326

2.285

2.246

2.207

2.168

-7

2.537

2.493

2.450

2.408

2.367

-6

2.765

2.718

2.672

2.626

2.581

-5

3.013

2.962

2.912

2.862

2.813

-4

3.280

3.225

3.171

3.117

3.065

-3

3.568

3.509

3.451

3.393

3.336

-2

3.880

3.816

3.753

3.691

3.630

-1

4.217

4.147

4.079

4.012

3.946

Basic Hygrometry Principles Reference Guide

11

0

4.579

Table 3: Vapor Pressure of Water
4.504
4.431

4.359

4.287

0.4

0.6

0.8

Aqueous Vapor Pressure Over Water
Temp. (°C)

12

0.0

0.2

0

4.579

4.647

4.715

4.785

4.855

1

4.926

4.998

5.070

5.144

5.219

2

5.294

5.370

5.447

5.525

5.605

3

5.685

5.766

5.848

5.931

6.015

4

6.101

6.187

6.274

6.363

6.453

5

6.543

6.635

6.728

6.822

6.917

6

7.013

7.111

7.209

7.309

7.411

7

7.513

7.617

7.722

7.828

7.936

8

8.045

8.155

8.267

8.380

8.494

9

8.609

8.727

8.845

8.965

9.086

10

9.209

9.333

9.458

9.585

9.714

11

9.844

9.976

10.10910.24410.3801210.51810.65810.79910.94111.08513

11.23111.37911.52811.68011.83314

11.98712.14412.302

12.462

12.62415

12.78812.95313.12113.29013.461

16

13.63413.80913.98714.16614.34717

14.53014.71514.90315.09215.28418

15.47715.67315.87116.071

16.27219

16.47716.68516.89417.10517.31920

17.53517.75317.97418.19718.42221

18.65018.88019.11319.34919.5872219.82720.07020.316

20.56520.81523

21.06821.32421.58321.84522.1102422.37722.64822.92223.19823.47625

23.75624.03924.32624.61724.91226

25.20925.50925.81226.11726.4262726.73927.05527.37427.69628.02128

28.34928.68029.01529.35429.69729

30.04330.39230.74531.10231.46130

31.82432.19132.56132.93433.31231

33.69534.08234.47134.86435.26132

35.66336.06836.47736.89137.30833

37.72938.15538.58439.01839.45734

39.89840.34440.79641.25141.7103542.17542.64443.11743.59544.07836

44.56345.05445.54946.05046.5563747.06747.58248.10248.62749.15738

49.69250.23150.77451.32351.879Basic Hygrometry Principles Reference GuideTable 3: Vapor Pressure of Water
Aqueous Vapor Pressure Over Water (cont.)
Temp. (°C)

0.0

0.2

0.4

0.6

0.8

39

52.44253.00953.58054.15654.73740

55.32455.91056.51057.11057.72041

58.34058.96059.58060.22060.8604261.50062.14062.80063.46064.1204364.80065.48066.16066.86067.5604468.26068.97069.69070.41071.1404571.88072.62073.36074.12074.8804675.65076.43077.21078.00078.8004779.60080.41081.23082.05082.87048

83.71084.56085.42086.28087.14049

88.02088.90089.79090.69091.590

50

92.51

93.50

94.40

95.30

96.30

51

97.20

98.20

99.10

100.10101.1052

102.09103.10104.10105.10106.2053

107.20108.20109.30110.40111.4054

112.51113.60114.70115.80116.9055

118.04119.10120.30121.50122.6056

123.80125.00126.20127.40128.6057129.82131.00132.30133.50134.7058

136.08137.30138.50139.90141.2059

142.60143.90145.20146.60148.0060

149.38150.70152.10153.50155.0061

156.43157.80159.30160.80162.3062163.77165.20166.80168.30169.8063

171.38172.90174.50176.10177.7064

179.31180.90182.50184.20185.8065

187.54189.20190.90192.60194.3066

196.09197.80199.50201.30203.1067204.96206.80208.60210.50212.3068

214.17216.00218.00219.90221.8069

223.73225.70227.70229.70231.7070

233.70235.70237.70239.70241.8071243.90246.00248.20250.30252.4072

254.60256.80259.00261.20263.4073

265.70268.00270.20272.60274.8074277.20279.40281.80284.20286.6075

289.10291.50294.00296.40298.8076

301.40303.80306.40308.90311.4077314.10316.60319.20322.00324.6078

327.30330.00332.80335.60338.20

Basic Hygrometry Principles Reference Guide

13

Table 3: Vapor Pressure of Water
Aqueous Vapor Pressure Over Water (cont.)

14

Temp. (°C)

0.0

0.2

0.4

0.6

0.8

79

341.00343.80346.60349.40352.2080355.10358.00361.00363.80366.8081369.70372.60375.60378.80381.8082

384.90388.00391.20394.40397.4083

400.60403.80407.00410.20413.6084416.80420.20423.60426.80430.2085433.60437.00440.40444.00447.5086450.90454.40458.00461.60465.2087468.70472.40476.00479.80483.4088

487.10491.00494.70498.50502.2089506.10510.00513.90517.80521.8090525.76529.77533.80537.86541.9591546.05550.18554.35558.53562.7592566.99571.26575.55579.87584.2293588.60593.00597.43601.89606.3894610.90615.44620.01624.61

629.2495

633.90638.59643.30648.05652.8296657.62662.45667.31

672.20677.1297682.07687.04692.05697.10702.1798707.27712.40717.56722.75727.9899733.24738.53743.85749.20754.58100760.00765.45770.93776.44782.00101

787.57793.18798.82804.50810.21

Basic Hygrometry Principles Reference Guide

6.7

Comparison of ppmv Calculations

There are three basic methods for determining the moisture content of a gas in ppmv:•

the calculations described in this reference guide•calculations performed with a slide-rule device that is available from Panametrics•

values determined from tabulated vapor pressures

For comparison purposes, examples of all three procedures are listed in Table 4 below.
Table 4: Comparative ppmv Values
Calculation Method

Dew Point (°C)

Pressure (psig)

Slide-Rule

This Guide

Tabulated

-80

0

0.5

0.55

0.526

100

0.065

N.A.

0.0675

800

0.009

N.A.

0.0095

1500

0.005

N.A.

0.0051

0

37

40

38.88

100

4.8

5.2

4.98

800

0.65

0.8

0.7016

1500

0.36

0.35

0.37730

N.A.

20,00023,072.36

100

3000

3000

2956.9

800

420

400

416.3105

1500

220

200

223.9

-50

+20

Basic Hygrometry Principles Reference Guide

15

7.

Liquid Applications

7.1

Theory of Operation

The direct measurement of water vapor pressure in organic liquids is accomplished easily and effectively withPanametrics aluminum oxide moisture probes. Since the moisture probe pore openings are small in relation to thesize of most organic molecules, admission into the sensor cavity is limited to much smaller molecules, such as water.
Thus, the surface of the aluminum oxide sensor, which acts as a semi-permeable membrane, permits themeasurement of water vapor pressure in organic liquids just as easily as it does in gaseous media.
In fact, under equilibrium conditions, an accurate sensor electrical output will be registered whether the sensor is
directly immersed in the organic liquid or it is placed in the gas space above the liquid surface. As with gases, theelectrical output of the aluminum oxide probe is a function of the measured water vapor pressure.

7.2

Moisture Content Measurement in Organic Liquids

When using the aluminum oxide sensor in non-polar liquids having water concentrations 1% by weight, Henry’s Law
is generally applicable.

Henry’s Law Type Analysis

Henry’s Law states that, at constant temperature, the mass of a gas dissolved in a given volume of liquid is
proportional to the partial pressure of the gas in the system. Stated in terms pertinent to this discussion, it can besaid that the ppmw of water in hydrocarbon liquids is equal to the partial pressure of water vapor in the system times
a constant which is specific to the fluid and its temperature.
As discussed above, a Panametrics aluminum oxide probe can be directly immersed in a hydrocarbon liquid tomeasure the equivalent dew point. Since the dew point is functionally related to the vapor pressure of the water, adetermination of the dew point will allow one to calculate the ppmw of water in the liquid by a Henry’s Law typeanalysis. A specific example of such an analysis is shown below.
For liquids in which a Henry’s Law type analysis is applicable, the parts per million by weight of water in the organic
liquid is equal to the partial pressure of water vapor times a constant:(9)

ppm w = K  P W
where, “K” is the Henry’s Law constant in the appropriate units, and the other variables are as defined in“Nomenclature” on page 8.
Also, the value of “K” is determined from the known water saturation concentration of the organic liquid at themeasurement temperature:
Saturation ppm
K = ----------------------------------------wPS(10)

As the temperature of the liquid varies, its saturation concentration and the saturation vapor pressure will also vary.
Thus, it is important to know the temperature of the liquid at the moisture sensor for an accurate determination of
PPMw. For a mixture of organic liquids, an average saturation value can be calculated from the weight fractions andsaturation values of the pure components as follows:nAve. C S = Xi  CS i

(11)

i=1

where, Xi is the weight fraction of the ith component, (CS)i is the saturation concentration (ppmw) of the ith
component, and n is the total number of components.
In conclusion, the Henry’s Law constant (K) is a constant of proportionality between the saturation concentration (CS)and the saturation vapor pressure (PS) of water, at the measurement temperature. In the General Case, the Henry’s
Law constant varies with the measurement temperature, but there is a Special Case in which the Henry’s Law

16

Basic Hygrometry Principles Reference Guideconstant does not vary appreciably with the measurement temperature. This special case applies to saturated,straight-chain hydrocarbons such as pentane, hexane, heptane, etc.

General Case

Determination of Moisture Content in PPMw if CS is Known:
The nomograph for liquids in Figure 3 on page 18 can be used to determine the moisture content in an organic liquid,
if the following values are known:•

the temperature of the liquid at the time of measurement•

the saturation water concentration at the measurement temperature•

the dew point, as measured with the Panametrics hygrometer

Complete the following steps to determine the moisture content from the nomograph:1.

Using a straightedge on the two scales on the right of Figure 3 on page 18, connect the known saturation
concentration (ppmw) with the measurement temperature (°C).

2.

Read the Henry’s Law constant (K) on the center scale.

3.

Using a straightedge, connect the above K value with the dew/frost point, as measured with the Panametrics
hygrometer.

4.

Read the moisture content (ppmw) where the straight edge crosses the moisture content scale.

Determination of K and CS
If the values of K and CS are not known, the Panametrics hygrometer can be used to determine these values. In fact,
only one of the values is required to determine ppmw from the nomograph in Figure 3 on page 18. To perform suchan analysis, proceed as follows:1.

Obtain a sample of the test solution with a known water content; or perform a Karl Fischer titration on a sample ofthe test stream to determine the ppmw of water.

Note:
2.

3.

The Karl Fischer analysis involves titrating the test sample against a special Karl Fischer reagent until an
endpoint is reached.

Measure the dew point of the known sample with the Panametrics hygrometer.
Measure the temperature (°C) of the sample.

4.

Using a straightedge, connect the moisture content (ppmw) with the measured dew point, and read the K valueon the center scale.

5.

Using a straightedge, connect the above K value with the measured temperature (°C) of the sample, and readthe saturation concentration (ppmw).

IMPORTANT: Since the values of K and CS vary with temperature, the hygrometer measurement and the test sample
analysis must be done at the same temperature. If the moisture probe temperature is expected to vary,
the test should be performed at more than one temperature.
Determination of moisture content if the Henry’s Law constant (K) is known:
1.

In the nomograph for liquids in Figure 3 on page 18, use a straightedge, connect the known K value on the centerscale with the dew/frost point, as measured with the Panametrics hygrometer.

2.

Read moisture content (ppmw) where the straightedge crosses the scale on the left.

Basic Hygrometry Principles Reference Guide

17

Figure 3: Moisture Content Nomograph for Liquids
18

Basic Hygrometry Principles Reference Guide

Typical Problems
1.

Find the moisture content in benzene, at an ambient temperature of 30°C, if a dew point of 0°C is measured with
the Panametrics hygrometer.
a.
b.
c.

2.

b.
c.

Using the straightedge, connect the above K value of 27.4 with the measured dew point of 0°C, and read thecorrect moisture content of 125 ppmw where the straightedge crosses the moisture content scale.

From the literature, it is found that CS for heptane at a temperature of 50°C is 480 ppmw.

Using a straightedge on Figure 3 on page 18, connect the 480 ppmw saturation concentration with the 50°C
ambient temperature and read the Henry’s Law Constant of 5.2 on the center scale.
Using the straightedge, connect the above K value of 5.2 with the measured dew point of 3°C, and read thecorrect moisture content of 29 ppmw where the straightedge crosses the moisture content scale.

Find the moisture content in hexane, at an ambient temperature of 10°C, if a dew point of 0°C is measured with
the Panametrics hygrometer.
a.
b.
c.

Note:

7.3

Using a straightedge on Figure 3 on page 18, connect the 870 ppmw saturation concentration with the 30°C
ambient temperature and read the Henry’s Law Constant of 27.4 on the center scale.

Find the moisture content in heptane, at an ambient temperature of 50°C, if a dew point of 3°C is measured with
the Panametrics hygrometer.
a.

3.

From the literature, it is found that CS for benzene at a temperature of 30°C is 870 ppmw.

From the literature, it is found that CS for hexane at a temperature of 20°C is 101 ppmw.

Using a straightedge on Figure 3 on page 18, connect the 101 ppmw saturation concentration with the 20°C
ambient temperature and read the Henry’s Law Constant of 5.75 on the center scale.

Using the straightedge, connect the above K value of 5.75 with the measured dew point of 0°C, and read thecorrect moisture content of 26 ppmw where the straightedge crosses the moisture content scale.

If the saturation concentration at the desired ambient temperature cannot be found for a straight-chainhydrocarbon, the value at any other temperature may be used, because K is nearly constant over a large
temperature range.

Additional Notes for Liquid Applications

In addition to the topics already discussed, the following general application notes pertain to the use of Panametrics
moisture probes in liquid applications:
1.

2.
3.

4.

8.

All Panametrics aluminum oxide moisture probes can be used in either gas phase or liquid phase applications.
The calibration data supplied with Panametrics moisture probes is applicable to both liquid phase (for thoseliquids in which a Henry’s Law analysis is applicable) and gas phase applications.
As indicated in Table 2 on page 3, the linear velocity of the liquid is limited to a maximum of 10 cm/sec.
Possible probe malfunctions and their remedies are discussed in previous sections of this reference guide.

Solids Applications

Panametrics offers a line of humidity analyzers capable of measuring the moisture content in certain solid materials.
Please contact Panametrics for more information.

Basic Hygrometry Principles Reference Guide19

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20

Basic Hygrometry Principles Reference Guide

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