How to Choose the Right Instrument for Measuring Humidity and Dew Point

```How to Choose the Right Instrument for
Measuring Humidity and Dew Point
What is Humidity?
An Introduction
to Humidity
Parameters
Partial Pressure of
Water Vapor
Humidity is simply water in its
gaseous phase, properly called water
vapor. Because water vapor is a gas,
most of the common gas laws apply
to it, including Dalton's law of partial
pressures. Dalton's law says that the
total pressure of a gas is equal to the
sum of the partial pressures of each
of the component gases:
Ptotal = P1 + P2 + P3…
Humidity measurement and control is called for in a wide variety
of industrial applications. Each application has a different set of
requirements for humidity instruments, such as required measurement
range, tolerance to extreme temperature and pressure conditions,
ability to recover from condensation, ability to operate in hazardous
environments, and options for installation and calibration. There is
no single device that is suitable for all needs. In fact, the range of
available equipment is quite large, varying both in cost and quality.
This paper discusses the following
topics in order to help in selecting
the right humidity instrument:
• Different humidity parameters
• Environmental conditions that
influence the choice of humidity
instrument
• Sensor properties that influence
the choice of humidity instrument
• Practical guidelines for selecting a
humidity instrument
If we consider air, the equation means
that the total atmospheric pressure
of 1.013 bar (14.7psia) is the sum of
the partial pressures of nitrogen,
oxygen, water vapor, argon, carbon
dioxide, and various other gases in
trace amounts.
Definition of Water Vapor
Pressure
Water vapor pressure (P w) is the
pressure exerted by the water vapor
present in air or a gas. Temperature
dictates the maximum partial
pressure of water vapor. This
maximum pressure is known as
saturation vapor pressure (P ws).
The higher the temperature, the
higher the saturation vapor pressure
and the more water vapor the air
can hold. Thus, warm air has a
greater capacity for water vapor
than cold air.
If saturation vapor pressure is
reached in air or in a gas mixture,
vapor requires that an equal amount
condenses out of the gas as a liquid
or a solid. A psychrometric chart
shows graphically the relation
between saturation vapor pressure
pressure tables can be used to see
the saturation vapor pressure at
any temperature, and there are
also a number of computer-based
calculation programs available.
Effect of Pressure
on Humidity
Dalton's law states that a change in
the total pressure of a gas must have
an effect on the partial pressures of
all of the component gases, including
water vapor. If, for example, the
total pressure is doubled, the partial
pressures of all component gases are
doubled as well. In air compressors,
a pressure increase "squeezes" water
out of the air as it is compressed.
Some humidity measurement instruments allow an individual humidity parameter
to be chosen which can then be followed on a graphical display.This picture shows
different views of the Vaisala HUMICAP Hand-Held Humidity and Temperature Meter
HM70 display.
Relative Humidity
When thinking conceptually of water
vapor as a gas, it's easy to define
relative humidity. Relative humidity
(RH) can be defined as the ratio of
the partial water vapor pressure
(Pw) to the water vapor saturation
pressure (Pws) at a particular
temperature:
%RH = 100% × Pw / Pws
Relative humidity is strongly
temperature dependent as the
denominator in the definition (Pws)
is a function of temperature. For
example, in a room with an RH of 50%
and a temperature of 20°C, increasing
the temperature of the room to 25°C
will decrease the RH to about 37%,
even though the partial pressure of
the water vapor remains the same.
Pressure will also change relative
humidity. For example, if a process
is kept at a constant temperature,
relative humidity will increase by a
factor of two if the process pressure
is doubled.
Dew Point Temperature
Humidity calculators are also available
for mobile phones.
This happens because the partial
pressure of water vapor (Pw) is
increased, but the saturation vapor
pressure is still only a function of
temperature. As pressure builds in
a receiver tank and Pw reaches Pws,
water condenses into liquid and must
ultimately be drained from the tank.
If a gas is cooled and gaseous water
vapor begins to condense in the
liquid phase, the temperature at
which condensation occurs is defined
as the dew point temperature (Td ).
At 100%RH the ambient temperature
equals the dew point temperature.
The further negative the dew point
is from the ambient temperature, the
smaller the risk for condensation and
the drier the air.
Dew point directly correlates with
saturation vapor pressure (Pws ).
The partial pressure of water vapor
associated with any dew point can
be easily calculated. Unlike RH, dew
point is not temperature dependent
but it is affected by pressure.
Typical applications for dew point
measurement include various drying
processes, dry air applications, and
compressed air drying.
Frost Point Temperature
If the dew point temperature is below
freezing – which is the case in dry gas
applications – the term frost point (Tf )
is sometimes used to explicitly state
that the condensing phase is ice. The
frost point is always slightly higher
than the dew point below 0°C as the
water vapor saturation pressure of
ice is different to water. People also
often refer to dew point for subzero
values, even though they mean frost
point. Ask for clarification if you are
not certain.
Parts Per Million
Unit parts per million (ppm) is
sometimes used for low levels of
humidity. It is the ratio of water vapor
to dry gas or total (moist) gas, and is
expressed either by volume/volume
(ppmvol) or mass/weight (ppmw ).
Parts per million (ppmvol) can be
quantitatively expressed as follows:
ppmvol = [Pw /(P - Pws )] × 106
The ppm parameter is typically used
when defining the water vapor content
of pressurized and dry pure gases.
Mixing Ratio
The mixing ratio (x) is the ratio of
water vapor mass to the mass of dry
gas. It is dimensionless but often
expressed in grams per kilogram of
dry air. The mixing ratio is mainly
used in drying processes and HVAC
applications for calculating water
content when the mass flow of air is
known.
Wet Bulb Temperature
temperature (Tw) is the temperature
indicated by a thermometer wrapped
in a wet cotton sheath. The wet bulb
and ambient temperatures can be
used together to calculate relative
humidity or dew point. For example,
the wet bulb temperature is used
in air conditioning applications
where it is compared to the dry bulb
temperature to determine the cooling
capacity of evaporative coolers.
Water Activity
Absolute Humidity
Enthalpy is the amount of energy
required to bring a gas to its current
state from a dry gas at 0°C. It is used
in air conditioning calculations.
Absolute humidity (a) refers to the
mass of water in a unit volume of
moist air at a given temperature and
pressure. It is usually expressed
as grams per cubic meter of air.
Absolute humidity is a typical
parameter in process control and
drying applications.
Water activity (aw) is similar to
equilibrium relative humidity and
uses a scale of 0 to 1, instead of
0% to 100%.
Enthalpy
The Effect of Environmental Conditions
on Humidity Measurement
Select a Representative
Measurement Location
Always choose a measuring
point that is representative of the
environment being measured,
avoiding any hot or cold spots. A
transmitter mounted near a door,
humidifier, heat source, or air
conditioning inlet will be subject to
rapid humidity changes and may
appear unstable.
As relative humidity is strongly
temperature dependent, it is
very important that the humidity
sensor is at the same temperature
as the measured air or gas. When
of two different instruments, the
thermal equilibrium between the
units/probes and the measured gas is
particularly crucial.
Unlike relative humidity, dew point
measurement is independent
of temperature. However, when
measuring dew point, pressure
conditions must be taken into
account.
Seal
Recommended
Mounting for
duct or chamber
Probe
Insulation
Beware of Temperature
Differences
When mounting a humidity probe
into a process, avoid temperature
drops along the probe body.
When there is a large temperature
difference between the probe and
the external environment, the whole
probe should be mounted within the
process and the cable entry point
should be insulated.
When there is a risk of condensation,
the probe should be mounted
horizontally to avoid water dripping
down the probe/cable and saturating
the filter (see figure 1).
Ensure that air is allowed to
flow around the sensor. Free air
flow ensures that the sensor is
in equilibrium with the process
temperature. At 20°C and 50%RH, 1°C
difference between the sensor and
the measurement zone will cause an
error of 3%RH. At 100%RH the error is
6%RH (see figure 2).
Seal
Insulation
Not
recommended
if risk of
condensation
Figure 1: Mounting a humidity probe
in a condensing environment.
10
9
8
7
dRH (%RH)
Environmental conditions can have
a significant effect on humidity and
dew point measurements. Take the
following environmental factors into
consideration to achieve the best
possible measurement result:
6
5
4
3
2
1
0
-40
-20
0
20
40
Temperature ( C)
60
80
100
Figure 2: Measurement error at
100%RH at various temperatures when
the temperature difference between the
ambient air and the sensor is 1°C.
Installation with the ball valve set
The Right Instrument
for High Humidity
In many applications it is advisable
to isolate the probe from the process
with a ball valve to enable the
removal of the probe for maintenance
without shutting down the process
(see figure 3).
Environments with >90%RH are
defined here as high humidity
environments. At 90%RH a
difference of 2°C can cause water to
condense on the sensor, which in an
unventilated space may take hours
to dry. Vaisala humidity sensors
will recover from condensation.
However, if the condensed water
is contaminated, the instrument
accuracy can be affected due to
deposits on the sensor, especially
salt deposits. Even the life of
the sensor may be shortened. In
applications with high humidity
where condensation can occur, a
warmed sensor head probe such as
the Vaisala HUMICAP® Humidity and
Temperature Transmitter HMT337
should be used.
The Right Instrument for
Low Humidity
Environments with <10%RH are
defined here as low humidity
environments. At low humidities, the
calibration accuracy of instruments
measuring relative humidity may not
point will provide a good indication
of humidity. For example, Vaisala
DRYCAP® products are designed for
measuring dew point.
If a dryer fails in a compressed air
system, water condensation may
appear and the instrument will need
to recover. Many dew point sensors
are damaged or destroyed in such
situations, but Vaisala DRYCAP®
dew point sensors withstand high
humidity – and even water spikes.
The Right Instrument for
Extreme Temperature and
Pressure Conditions
Continuous exposure to extreme
temperatures may affect sensor
and probe materials over time. It is
therefore very important to select
a suitable product for demanding
environments. In temperatures above
When is a Sampling
System Needed for
Dew Point Measurement?
Wherever possible, the probe
should be mounted in the actual
process to achieve the most accurate
measurements and a rapid response
time. However, direct installations
are not always feasible. In such
situations, sample cells installed
in-line provide an entry point for a
suitable measurement probe.
Figure 3: A ball valve installation in a
process pipe line.
60°C the transmitter electronics
should be mounted outside the
process and only a suitable high
temperature probe should be
inserted into the high temperature
environment. Moreover, builtin temperature compensation is
required to minimize the errors
caused by large temperature
swings or operation at temperature
extremes.
When measuring humidity in
processes operating at around
ambient pressure, a small leak may
be tolerable and can be reduced by
sealing around the probe or cable.
However, when the process needs
to be isolated, or when there is a
large pressure difference between
the process and the external
with appropriate mounting must be
used. Pressure leaks at the point of
entry will alter the local humidity and
Note that external sampling systems
should not be used to measure
relative humidity because the
change in temperature will affect
the measurement. Sampling systems
can instead be used with dew point
probes. When measuring dew point,
sampling systems are typically used
to lower the temperature of the
process gas, to protect the probe
against particulate contamination,
or to enable easy connection and
disconnection of the instrument
without ramping down the process.
The simplest dew point sampling
setup consists of a dew point
transmitter connected to a sampling
cell. Vaisala has several models
suitable for the most common
applications and sampling needs.
For example, the easy to install
DSC74 sampling cell is designed for
the flow and pressure conditions in
compressed air applications.
In demanding process conditions,
sampling systems must be designed
carefully. As dew point is pressure
dependent, a flow meter, pressure
gauge, special non-porous tubing,
filters, and pump may be needed. As
an example, a flow chart showing the
Vaisala DRYCAP® Portable Sampling
System DSS70A for DM70 is shown in
figure 4.
In a pressurized system a sample
pump isn’t needed as the process
pressure induces a large enough flow
to the sampling cell.
When measuring dew point with
a sampling system, trace heating
should be used when the ambient
temperature around the cooling
coil or connecting tube is within
10°C of the dew point temperature.
This prevents condensation in the
tubing that connects the dew point
instrument to the process.
Hazardous Environments
Only products with appropriate
certification can be used in
potentially explosive areas. For
example, in Europe products must
comply with the ATEX100a directive,
which has been mandatory since
2003. Intrinsically safe products are
designed in such a manner that even
in the event of failure they do not
generate enough energy to ignite
certain classes of gas. The wiring
from the intrinsically safe product
into the safe area must be isolated
via a safety barrier. For example, the
Vaisala HMT360 series of intrinsically
safe humidity transmitters are
specially designed for use in
hazardous environments.
Vaisala HUMICAP ® Humidity and Temperature Transmitter Series HMT360 is
designed for hazardous and explosive environments.
Figure 4: The DSS70A sampling system includes a filter to clean the gas and a
needle valve to control the sample flow rate. A sample pump is needed to generate
flow from an unpressurized process gas.
Shock and Vibration
When the probe will be subject to
excessive shock or vibration, the
choice of probe, mounting method,
and installation location needs
careful consideration.
What Makes a Good Humidity Sensor?
Humidity sensor performance is a
critical contributor to the overall
quality of the humidity measurement.
Consider the importance of the
following sensor properties:
Fast Response Time
The sensor response time is the
speed of response when the sensor
is subjected to a step change in
humidity. In addition to the sensor,
factors such as temperature, airflow,
and filter type all have an effect on
response time. A blocked filter will
give a slower response.
Optimal Measurement
Range
The choice of humidity sensor
depends on the application and
operating temperature, especially at
the extremes of humidity.
The majority of Vaisala's humidity
sensors work over the full range
from 0 to 100%RH. Vaisala HUMICAP®
sensors are the optimal choice for
applications with a relative humidity
around 10–100%RH, whereas
DRYCAP® sensors are designed for
measurements in low humidities
around 0–10%RH.
Good Chemical Tolerance
Aggressive chemicals can damage or
contaminate sensors. The instrument
manufacturer should know the effects
of various chemicals on their sensors
and be able to give advice related to
acceptable chemical concentrations.
High Accuracy
Accuracy as a term is well
established, but difficult to define.
Each step in the calibration chain
– from the primary standard in an
internationally recognized calibration
laboratory, to the actual product
manufacture and the measurement
on site – introduces measurement
error. The sum of these potential
errors is the uncertainty of the
measurement.
When selecting a humidity sensor,
consider the following factors
associated with accuracy:
• Linearity over the working range
• Hysteresis and repeatability
• Stability over a period of time
• Temperature dependence of the
sensor
Humidity sensors from Vaisala.
During manufacture, Vaisala
against factory standards that are
directly traceable to internationally
recognized standards. The
calibration chain is detailed in the
certificates that are supplied with the
majority of Vaisala products.
The Right Humidity Instrument for the Job
or connected with a flexible cable.
The transmitter provides the output
signal.
Protective filters for Vaisala’s humidity
instruments.
No matter what the application, the
total range of gas temperatures and
expected water vapor levels must be
known in order to decide the optimal
humidity parameters and the optimal
instrument for the environment.
The process pressure must also be
known when measuring humidity
within the process. In addition, it
must be decided whether to make the
measurement at the process pressure
or at some other pressure. For gases
other than air, the gas composition
must be known.
The terms probe, transmitter, and
sensor describe products that
measure humidity. The probe is the
part of the product that contains
the humidity sensor. The probe may
be rigidly bound to the transmitter
Vaisala designs and manufactures
a range of products for measuring
relative humidity, temperature,
and dew point based on HUMICAP®
and DRYCAP® sensors. All Vaisala
humidity instruments feature
built-in temperature compensation
to minimize the errors caused by
temperature variations and operation
at extremes of temperature. Many
of the products include built-in
calculations for other humidity
parameters.
Protect the Sensor
and Electronics with
the Right Filter
As well as screening the sensor from
any stray electromagnetic fields,
the filter protects the sensor from
dust, dirt, and mechanical stress.
A membrane or netting filter is a
good alternative for the majority of
applications. In temperatures above
80°C, in high pressure, or in rapidly
moving air up to 75 m/sec, a sintered
filter should be used.
A suitable protective enclosure
protects the instrument electronics
from dust, dirt, and excessive
humidity. An enclosure with an IP65
or NEMA 4 classification gives good
protection against dust and sprayed
water. The cable entry points need to
be sealed during installation.
When using the instrument outside,
it should be mounted in a radiation
shield or Stevenson screen to prevent
solar radiation or extremes of weather
from affecting the measurement.
Must the Instrument
Tolerate Condensation?
Making good quality humidity
measurements in near-condensing
conditions is very challenging.
Warmed probe technology ensures
reliable measurements when
measuring relative humidity close
to saturation point. The humidity
level of the warmed probe always
stays below the ambient level, where
condensation occurs.
Must the Instrument
Withstand Exposure to
Chemicals?
A chemical purge feature helps to
maintain measurement accuracy
in environments with a high
concentration of chemicals or
cleaning agents. The chemical purge
heats the sensor at regular intervals
to remove chemicals that may have
accumulated over time.
The Importance of
Electromagnetic
Compatibility (EMC)
There are many standards defining
the ability of products to withstand
external electrical interference.
In addition, the product must not
generate emissions that can cause
interference to sensitive equipment.
Industrial applications have more
demanding EMC requirements
than HVAC installations – the CE
marking used in Europe guarantees
compliance.
Consider Wiring
and Earthing
Except for short cable runs, a
screened cable is recommended.
Proximity to high-voltage cables or
RF sources should be avoided. It is
good practice to earth the connection
cable screen at a common point and
avoid multiple earth points. Galvanic
isolation is also available on some
Vaisala products.
Vaisala HUMICAP ® Humidity and Temperature Transmitter Series HMT330 is a
flexible product family designed for demanding industrial applications.
Which Power Supply
and Output Signals
are Needed?
The majority of measurement
instruments are powered using a low
voltage supply. If a low voltage AC
supply is used, an isolated supply is
recommended for each transmitter
to avoid earth loops or interference
Analog output instruments usually
have an option for both voltage and
current outputs. The choice depends
on the length of the signal path and
on the interfacing equipment. Some
products have a 4–20mA loop power
connection, which is a 2-wire system
where the output signal current is
measured in the supply line.
In addition to analog outputs, some
Vaisala products feature digital
communication via RS-232, RS-485,
or LAN/WLAN interfaces. Selected
commercial protocols (Modbus,
BACnet) are also available.
Instruments typically need to be
calibrated every year or every second
year. Calibration requirements
depend on the application and the
stability of the instrument, with wide
variations in how easy it is to carry
out field checking and calibration.
Some instruments need to be sent
to a laboratory for calibration,
for example. Understanding the
calibration needs is therefore
an important part of instrument
selection.
Calibration Frequency
An individual calibration certificate
for a particular instrument indicates
the accuracy and linearity at the time
of calibration. However, it does not
reflect the stability of the instrument
in the long run. Calibration at routine
intervals is essential to understand
the instrument’s long-term stability.
Calibration frequency depends on
the operating environment. A rule
of thumb for Vaisala instruments
is that yearly calibration is enough
for HUMICAP® products, whereas
in most applications a two-year
calibration interval is suitable for
DRYCAP® products. When measuring
in constant high humidity (>85%RH),
high temperature (>120°C), or
chemically aggressive atmospheres,
more frequent checks may be
needed.
On-site calibration of a HMW90
humidity transmitter with a HM70
hand-held meter.
Humidity Instrument
Calibration
of an instrument is compared against
a portable reference. The reference
should be regularly calibrated and
provided with a valid certificate.
When selecting one of the many
calibration methods, time, cost,
technical requirements, expertise,
and the unique needs of the
organization must be balanced.
Portable meters and products that
can be removed from the installation
can be calibrated in an approved
laboratory or returned to the
instrument provider for calibration.
Vaisala has four Service Centers
around the world available for
calibration.
Instruments installed in processes
that operate within narrow
boundaries can be calibrated using
on-site one-point calibration that can
be performed without disconnecting
the instrument. One-point calibration
can also be used to identify the
need for further calibration and
Some portables such as the Vaisala
HUMICAP® Hand-Held Humidity
and Temperature Meter HM70 or
the Vaisala DRYCAP® Hand-Held
Dewpoint Meter DM70 can be directly
connected to the installed product
and the readings compared to those
on the portable meter’s display.
In environments with large
variations in humidity, multi-point
calibration is recommended. Twopoint or three-point calibrations
Vaisala Humidity Calibrator HMK15 for multi-point on-site calibrations.
can be accomplished in the
field with the help of humiditygenerating equipment, as long as
the local environment is at a stable
temperature. The advantage of multipoint calibration compared to onepoint calibration is higher accuracy
over the entire measurement range.
Multiple humidity levels can be
created with the Vaisala Humidity
Calibrator HMK15, for example.
customers perform calibrations
on Vaisala DRYCAP® products.
Instead, they should be calibrated in
professional calibration laboratories,
such as Vaisala Service Centers.
However, it is possible to perform a
field check on a dew point instrument
needed, using the Vaisala DRYCAP®
Hand-Held Dewpoint Meter DM70.
Calibration of Dew Point
Instruments
humidity instruments,
visit www.vaisala.com/humidity.
It is a demanding task to perform
high-quality calibrations on low
dew point instruments. For this
reason, Vaisala doesn't recommend
www.vaisala.com or contact
us at [email protected]