Citation
Psychrometrics and postharvest operations

Material Information

Title:
Psychrometrics and postharvest operations
Series Title:
Florida Cooperative Extension Service circular 1097
Creator:
Talbot, Michael T.
Baird, C. Direlle (Carl Direlle)
Affiliation:
University of Florida -- Florida Cooperative Extension Service -- Institute of Food and Agricultural Sciences
Place of Publication:
Gainesville, Fla.
Publisher:
Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Publication Date:
Language:
English
Physical Description:
8 p. : ill. ; 28 cm.

Subjects

Subjects / Keywords:
Hygrometry. ( LCSH )
Agriculture ( LCSH )
Farm life ( LCSH )
Farming ( LCSH )
University of Florida. ( LCSH )
Vegetables -- Postharvest technology -- Florida ( LCSH )
Agriculture -- Florida ( LCSH )
Farm life -- Florida ( LCSH )
Relative humidity ( jstor )
Thermometers ( jstor )
Water temperature ( jstor )
Spatial Coverage:
North America -- United States of America -- Florida

Notes

Funding:
Florida Historical Agriculture and Rural Life

Record Information

Source Institution:
Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location:
Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management:
All rights reserved, Board of Trustees of the University of Florida
Resource Identifier:
29459265 ( OCLC )
AJU3488 ( NOTIS )
028454054 ( ALEPH )

Downloads

This item has the following downloads:


Full Text





HISTORIC NOTE


The publications in this collection do
not reflect current scientific knowledge
or recommendations. These texts
represent the historic publishing
record of the Institute for Food and
Agricultural Sciences and should be
used only to trace the historic work of
the Institute and its staff. Current IFAS
research may be found on the
Electronic Data Information Source
(EDIS)

site maintained by the Florida
Cooperative Extension Service.






Copyright 2005, Board of Trustees, University
of Florida




/o/



I0// UNIVERSITY OF

FLORIDA

Florida Cooperative Extension Service


Psychrometrics and Postharvest Operations1


Circular 1097
May 1993


Michael T. Talbot and C. Direlle Baird2


INTRODUCTION

The Florida commercial vegetable industry is large
and diverse and the value of vegetable production in
the state of Florida is over 1.5 billion dollars annually.
Most of these vegetable crops are produced for the
fresh market and require proper postharvest control
to maintain quality and reduce spoilage. The ambient
environment to which the freshly harvested vegetables
are exposed has a very significant effect on the
postharvest life of these perishable commodities.

Psychrometrics deals with thermodynamic
properties of moist air and the use of these properties
to analyze conditions and processes involving moist air
[1, 7] (numbers in brackets refer to cited references).
Commonly used psychrometric variables are
temperature, relative humidity, dew point temperature,
and wet bulb temperature. While these may be
familiar, they are often not well understood [2, 3, 4,
5, 8]. A better understanding of psychrometrics will
allow vegetable producers, packinghouse operators,
and commercial cooler operators to improve
postharvest cooling and storage conditions for fresh
vegetables. This publication presents the relationship
of psychrometric variables, considers their effect on
perishable commodities, and reviews how they can be
measured. This publication further suggests how the
psychrometric variables can be used and more
importantly how they should be used by managers.


PSYCHROMETRIC VARIABLES.

Atmospheric air contains many gaseous
components as well as water vapor. Dry air is a
mixture of nitrogen (78%), oxygen (21%), and argon,
carbon dioxide, and other minor constituents (1%).
Moist air is a two-component mixture of dry air and
water vapor. The amount of water vapor in moist air
varies from zero (dry air) to a maximum (saturation)
which depends on temperature and pressure. Even
though water vapor represents only 0.4 to 1.5% of the
weight of the air, water vapor plays a very significant
role in the effect of air conditions on the postharvest
life of perishable commodities.

The physical and thermodynamic properties of
moist air (psychrometric variables) are related by a
number of physical laws. These properties of moist
air can be expressed in terms of many different
variables. Psychrometric properties important to
postharvest horticulture include dry bulb temperature,
wet bulb temperature, dew point temperature, relative
humidity, humidity ratio, enthalpy, and specific
volume.

The dry bulb temperature (db) is the actual air
temperature measured with a common thermometer
or thermocouple. The wet bulb temperature (wb) is
measured with a common thermometer or
thermocouple with the bulb or junction covered with
a water-moistened wick and in a moving stream of
ambient air. Evaporation from the wick attains a
steady state, in which sensible heat from the
surroundings provides heat of vaporization. Air flow


1. This document is Circular 1097, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.
Publication date: May 1993.
2. Associate Professor and Professor, Agricultural Engineering Department, Cooperative Extension Service, Institute of Food and Agricultural
Sciences, University of Florida, Gainesville FL 32611.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational
information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national
origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office.
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / John T. Woeste, Dean




/o/



I0// UNIVERSITY OF

FLORIDA

Florida Cooperative Extension Service


Psychrometrics and Postharvest Operations1


Circular 1097
May 1993


Michael T. Talbot and C. Direlle Baird2


INTRODUCTION

The Florida commercial vegetable industry is large
and diverse and the value of vegetable production in
the state of Florida is over 1.5 billion dollars annually.
Most of these vegetable crops are produced for the
fresh market and require proper postharvest control
to maintain quality and reduce spoilage. The ambient
environment to which the freshly harvested vegetables
are exposed has a very significant effect on the
postharvest life of these perishable commodities.

Psychrometrics deals with thermodynamic
properties of moist air and the use of these properties
to analyze conditions and processes involving moist air
[1, 7] (numbers in brackets refer to cited references).
Commonly used psychrometric variables are
temperature, relative humidity, dew point temperature,
and wet bulb temperature. While these may be
familiar, they are often not well understood [2, 3, 4,
5, 8]. A better understanding of psychrometrics will
allow vegetable producers, packinghouse operators,
and commercial cooler operators to improve
postharvest cooling and storage conditions for fresh
vegetables. This publication presents the relationship
of psychrometric variables, considers their effect on
perishable commodities, and reviews how they can be
measured. This publication further suggests how the
psychrometric variables can be used and more
importantly how they should be used by managers.


PSYCHROMETRIC VARIABLES.

Atmospheric air contains many gaseous
components as well as water vapor. Dry air is a
mixture of nitrogen (78%), oxygen (21%), and argon,
carbon dioxide, and other minor constituents (1%).
Moist air is a two-component mixture of dry air and
water vapor. The amount of water vapor in moist air
varies from zero (dry air) to a maximum (saturation)
which depends on temperature and pressure. Even
though water vapor represents only 0.4 to 1.5% of the
weight of the air, water vapor plays a very significant
role in the effect of air conditions on the postharvest
life of perishable commodities.

The physical and thermodynamic properties of
moist air (psychrometric variables) are related by a
number of physical laws. These properties of moist
air can be expressed in terms of many different
variables. Psychrometric properties important to
postharvest horticulture include dry bulb temperature,
wet bulb temperature, dew point temperature, relative
humidity, humidity ratio, enthalpy, and specific
volume.

The dry bulb temperature (db) is the actual air
temperature measured with a common thermometer
or thermocouple. The wet bulb temperature (wb) is
measured with a common thermometer or
thermocouple with the bulb or junction covered with
a water-moistened wick and in a moving stream of
ambient air. Evaporation from the wick attains a
steady state, in which sensible heat from the
surroundings provides heat of vaporization. Air flow


1. This document is Circular 1097, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.
Publication date: May 1993.
2. Associate Professor and Professor, Agricultural Engineering Department, Cooperative Extension Service, Institute of Food and Agricultural
Sciences, University of Florida, Gainesville FL 32611.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational
information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national
origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office.
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / John T. Woeste, Dean





)0 1
r.,3 (, C-
Psychrometrics and Postharvest Operations I c( q 7

past the bulb must be high enough to prevent
significant change in the ambient air temperature.
Evaporation of water cools the bulb. The drier the
surrounding air, the greater the rate of evaporation
and the lower the wet bulb temperature. The wet
bulb temperature is the lowest temperature to which
an air mixture can be cooled solely by the addition of
water with absolutely no heat removed. The process
of cooling an air mixture with the addition of water
and with no removal of heat is called "evaporative
cooling".

If air is cooled without changing its moisture
content, it will lose capacity to hold moisture. If
cooled enough, it will become saturated and if cooled
further, will lose water in the form of dew or frost.
The temperature that causes condensation to form is
called the dewpoint temperature (dp) if it is above 0 C
(32 F) or the frost point temperature if it is below
0 C (32 F).

Relative Humidity (RH) is the best known and
perhaps the most widely used (and misused) term for
expressing the water vapor condition of moist air. RH
is defined as the ratio of the water vapor pressure in
the air to the saturation vapor pressure at the same
temperature, and is normally expressed as a percent.

The humidity ratio (or mixing ratio or absolute
humidity) is the ratio of the weight of water vapor in
a moist air sample to the weight of dry air contained
in the sample. It is usually expressed in terms of kg
water- per kg dry air (lb water per lb of dry air). This
property is very useful since it allows two conditions
to be .compared in terms of the moisture gradient
between the conditions. Water vapor will move from
a condition with a higher moisture level to a condition
with a lower moisture level.

The enthalpy is the heat energy content of an air-
water vapor mixture. The energy is both sensible
(indicated by dry bulb temperature) and latent heat of
vaporization (energy content of the water vapor). This
variable is important for engineering calculations such
as estimating the tons of refrigeration required to cool
perishable produce. Enthalpy will not be emphasized
since the purpose of this publication is to discuss the
use of psychrometric variables to analyze
environmental conditions and then determination of
required action to optimize the conditions.

The specific volume of a moist air mixture is
defined as the volume of the mixture per unit weight
of dry air and is expressed in terms of m3 per kg dry


Page 2

air (ft3 per lb dry air). It is also more important for
engineering calculations rather than analysis of
environmental conditions.

PSYCHROMETRIC CHART

The psychrometric chart is a graphical
representation that describes the relationships between
these variables (Figure 1). Although complicated in
appearance, this chart can be used to establish a state
point and is easily mastered. The charts in pads like
graph paper can be obtained from the American
Societyof Heating, Refrigerating and Air-Conditioning
Engineers (ASHRAE) [1] and several refrigeration
equipment manufacturers.

The dry bulb temperature is the horizontal axis of
the chart. The vertical axis located on the right side
of the chart is the humidity ratio. Two of the
variables must be known to establish a state point
from which other variables can be readily obtained as
shown in Figures 1 and 2.

The maximum amount of water vapor that air can
hold at a specific temperature is given by the left
most, upward-curved line in Figure 1. It is noted that
air holds increasingly more water vapor at increasing
temperatures. As a rule of thumb, the maximum
amount of water that the air can hold doubles for
every 11 C (200 F) increase in temperature. This line
in Figure 1 is also called the 100% RH line. A
corresponding 50% RH line is approximated by the
points which represent the humidity ratio when the air
contains one-half of its maximum water vapor content.
The other relative humidity lines are formed in a
similar manner.

The relative humidity without some other
psychrometric variable does not determine a specific
moist air condition on the chart and is not very
meaningful. As will be shown, 80 percent relative
humidity at 00 C (320) is a much different air condition
than 80 percent relative humidity at 200C (68F).

Another commonly used psychrometric variable is
wet bulb temperature. On the chart (Figure 1) this is
represented by lines that slope diagonally upward from
right to left. In practice, wet bulb lines are used to
determine the exact point on the psychrometric chart
which represents the air conditions in a given location
as measured by a psychrometer which will be
described below. The intersection of the diagonal wet
bulb temperature line (equal to the temperature of a
wet bulb thermometer) and the vertical dry bulb







Psychrometrics and Postharvest Operations


DRY BULB
TEMPERATURE
Figure 1. Properties of moist air on a psychrometric chart.


temperature line defines the temperature and humidity
conditions of air.

The dew point temperature for a given state point
is found by the intersection of a horizontal line drawn
through the state point and the 100% RH or
saturation line (Figure 1).

Vapor pressure is not shown on all psychrometric
charts, but is an important concept in handling
perishables. At a given barometric pressure [4], a
direct correlation exists between humidity ratio and
vapor pressure regardless of temperature. Vapor
pressure is often used as an expression of humidity
levels, particularly in terms of the difference between
vapor pressures at two points (vapor pressure deficit).
Water vapor will flow from a point of higher pressure
to a point of lower pressure just like water flow
created by pumps during irrigation or air flow created
by fans during forced-air cooling. The vapor pressure
deficit determines the rate of evaporation and
therefore the transpiration from horticultural products,
which is of great importance when handling fresh
commodities.

Figure 2 illustrates the properties of air that can
be determined when the dry bulb and wet bulb
temperature are known (73 F db and 52 F wb), which
for this case are 20% RH, 300F dp, 0.0035 lb water


per lb of dry air humidity ratio, 21.3 Btu/lb dry air
enthalpy and 13.5 ft3 per lb dry air specific volume.

Figures 3 and 4 are psychrometric charts in
English and metric units, respectively, which will help
to illustrate the meaning of various terms.

Psychrometric charts and calculators are based on
a specific atmospheric pressure, usually a typical sea
level condition. Precise calculations of psychrometric
variables will require adjustment for barometric
pressures different from those listed on a particular
chart. The ASHRAE Handbook [1] provides more
information. Most field measurements will not
require adjustment for pressure.

EFFECT OF PSYCHROMETRIC VARIABLES
ON PERISHABLE COMMODITIES

Temperature

Air temperature is the most important variable
because it tends to control the flesh temperature of
perishable commodities. All perishables have an
optimum range of storage temperatures [8, 9]. Above
the optimum, they respire at unacceptably high rates
and are more susceptible to ethylene and disease
damage. In fact, horticultural commodities respire at
rates which double, triple, or even quadruple for every


HUMIDITY
RATIO


Page 3







Psychrometrics and Postharvest Operations


Figure 2. Properties of moist air at 73F db and 52F wb.
Figure 2. Properties of moist air at 730F db and 52*F wb.


100C (18 F) increase in temperature [9].
Temperatures below the optimum will result in
freezing or chilling damage. Accurate control of
temperature during precooling and storage is vitally
important in maintaining maximum shelf-life and
quality.

Humidity Ratio/Vapor Pressure

The rate of moisture loss from a perishable is
primarily controlled by the difference in vapor
pressure between the air in the intercellular spaces of
plant material and the air surrounding it. As indicated
above vapor pressure increases as the air moisture
content (humidity ratio) increases. The air in fresh
plant material is nearly saturated or, in other words,
is close to 100% RH. Therefore, the humidity ratio
of this air is determined solely by the temperature of
the plant material. From the psychrometric chart it
is apparent that low temperatures result in low
humidity ratios and high temperatures cause high
humidity ratios.

Consider several examples of how the drying of
perishables is influenced by vapor pressure (humidity
ratio) differences. If sweet corn were precooled to
0C (320 F) [Point A, Figures 3 and 4] and placed in
a refrigerated room with saturated air at 0C (32 F)
[also Point A, Figures 3 and 4], the sweet corn would
not lose moisture because the humidity ratio and
temperature of the air in the sweet corn and the
surrounding air are the same. However, if the sweet
corn were at 200C (68 F) [Point B, Figures 3 and 4]


because it was not precooled before being placed in
the same refrigerated room, the air in the sweet corn
would have a high vapor pressure (high temperature
and humidity ratio) compared to the refrigerated air,
causing the sweet corn to dry. If the sweet corn were
precooled to 0C (32 F) [again Point A, Figures 3 and
4] but the refrigerated air were at 70 % RH [Point C,
Figures 3 and 4], drying would also occur because the
refrigerated air is at a lower humidity ratio than the
saturated air in the sweet corn. However, the rate of
moisture loss is much greater when the sweet corn is
not precooled than when the sweet corn is at the
storage temperature but the storage room air is not
saturated. For this example, the difference in
humidity ratio between the air in the sweet corn and
the storage air is over nine times more when the
sweet corn is not precooled than when it is cooled and
put in unsaturated storage air.

Drying of perishables in refrigerated storage is
reduced by decreasing the difference in humidity ratio
(vapor pressure) between air in the perishable
commodity and air surrounding it. Total moisture loss
is reduced by reducing the time of exposure to this
difference in humidity ratio by cooling the product
close to the surrounding air temperature as rapidly as
possible and by maintaining the condition of the
surrounding air as close to saturation as possible.
Both the temperature of the commodity and humidity
ratio in the surrounding air must be controlled. It is
important that these variables be known (measured)
so proper control actions can be implemented by
managers.


HUMIDITY
RATIO



0.0035 Ib moisture
per Ib dry air


Page 4







Psychrometrics and Postharvest Operations


Figure 3. Psychrometric chart in English units.

Relative Humidity ascertains the direction of potential water vapor
migration.


Relative humidity is a commonly used term for
describing the humidity of the air but is not
particularly meaningful without knowing the dry bulb
temperature of the air. These two variables allow the
determination of humidity ratio which is a better index
of the potential for desiccation. For example as noted
above, the humidity ratio of air at 80% RH and 0C
(320) [Point D, Figures 3 and 4] is much less than the
humidity ratio of air at 80% RH at 200C (68F)
[Point E, Figures 3 and 4]. In the example above, if
the sweet corn were cooled to 100 C (500 F) [Point F,
Figures 3 and 4] and the refrigerated air was at 0C
(32F) and 100% RH [again Point A, Figures 3 and
4] drying would also occur because the refrigerated
air is at a lower humidity ratio than the saturated air
in the sweet corn. Therefore 100% RH alone does
not mean there is no moisture loss potential. To
further illustrate that use of relative humidity alone
can create confusion, consider a cold storage running
at 20C (350F) and 100% RH [Point G, Figures 3 and
4] exposed to an outside air condition of 230 C (72 F)
and 50% RH [Point H, Figures 3 and 4]. Considering
% RH only, there is an apparent 2 to 1 moisture
gradient from the storage room atmosphere outward
toward the ambient conditions, while considering the
humidity ratio, the actual moisture gradient is 2 to 1
from the ambient conditions inward toward the storage
room atmosphere. Use of the psychrometric chart


Dew Point Temperature

Condensation of liquid water on perishables and
on container surfaces can be a factor in causing
disease problems and degradation of container
strength. If a commodity is cooled to a temperature
below the dew point temperature of the outside air
and brought out of the cold room, condensation will
form. This can occur when the product is exposed to
ambient conditions between the precooler and the
cold storage and the cold storage and refrigerated
trucks.

Condensation on the perishables, containers, and
walls of the storage room can also occur in storage if
air temperatures fluctuate too greatly. Another form
of condensation occurs in the storage room as the air
in the room is circulated over the evaporator cooling
coils of the refrigeration system. The temperature of
the cooling coil is usually lower than the return air
and the air is cooled below the dew point
temperature; moisture condenses and is removed from
the cold storage (drain pan). Unless moisture is
added by a humidification system, the moisture
condensed on the coils will be replaced by moisture
from the product in storage. To reduce the moisture
loss due to condensation on the cooling coils, the
temperature difference between the return air and the


Page 5







Psychrometrics and Postharvest Operations


0 5 10 15 20 25 30 35 40 45
Dry Bulb Temperature (C)
Figure 4. Psychrometric chart in metric units.


coil must be reduced. This can be accomplished by
using sufficiently large coil surface area. This will
increase the cooling system cost but is the best way to
maintain high humidity levels.

MEASUREMENT OF PSYCHROMETRIC
VARIABLES

All psychrometric properties of air can be
determined by measuring two psychrometric variables
(three, if barometric pressure is considered). For
example, if wet and dry bulb temperatures are
measured, then relative humidity, humidity ratio
(vapor pressure), dew point, and so on, can be
determined with the aid of a psychrometric chart.
While many variables can be measured to determine
the psychrometric state of air, the most commonly
measured are dry bulb temperature, wet bulb
temperature, dew point temperature, and relative
humidity.

Dry Bulb Temperature

Dry bulb, temperature can be simply and
inexpensively measured by a mercury-in-glass
thermometer. The thermometer should be marked in
divisions of at most 0.20C (0.5F) divisions if the
thermometer is used in conjunction with a wet bulb
thermometer for determining cold storage air
conditions. The thermometer should be shielded from
radiant heat sources such as motors, lights, external


walls, and people. The shielding can be accomplished
by placing the thermometer so it cannot "see" the
warm object or by protecting it with a radiant heat
shield assembly.

Hand-held thermistor, resistance bulb, or
thermocouple thermometers can also be used. They
are more expensive than a mercury-in-glass
thermometer but are not necessarily more accurate.
A hand-held thermocouple thermometer offers several
advantages including fast response time, durability, and
flexibility. An instrument of adequate accuracy can be
purchased from a number of agricultural and general
supply catalogs for $100 to $250 depending on options
and accessories. These instruments are equipped with
a sharp probe allowing them to be used for pulp
temperature measurement, which is very important in
determining initial and final precooling temperatures.
In addition to portable sensors, thermocouple leads
can be extended to some central location for remote
monitoring, but this would require additional initial
cost for leads and multichannel capability. Inexpensive
alcohol-in-glass and bi-metallic dial thermometers
(meat thermometers) are not recommended due to
limits on accuracy, inadequate calibration, and slow
response time.


PSYCHROMETRIC CHART
101.325 kPa


Page 6







Psychrometrics and Postharvest Operations

Wet Bulb Temperature

The use of a wet bulb thermometer in conjunction
with a dry bulb thermometer is a very common
method of determining the state point on the
psychrometric chart. Such an instrument, called a
psychrometer, consists of a pair of matched
temperature sensors, one of which is maintained in a
wetted condition. The wet bulb thermometer is
basically an ordinary glass thermometer (although
electronic temperature sensing elements can also be
used) with a wetted, cotton wick secured around the
reservoir. Air is forced over the wick causing it to
cool to the wet bulb temperature. The wet and dry
bulb temperatures together determine the state point
of the air on the psychrometric chart allowing all other
variables to be determined.

A psychrometer is a valuable instrument for
evaluating the conditions inside a cold storage room.
Several types of psychrometers are available from a
number of agricultural and general supply catalogs.
A sling psychrometer consists of the dry and wet bulb
thermometers and a handle for rotating the
psychrometer in order to provide the necessary air
flow for adequate evaporation. Prices range from $50
to $200. A portable psychrometer replaces the handle
with a battery powered fan and is available in the price
range of $125 to $200.

An accurate wet bulb temperature reading is
dependent on: (1) sensitivity and accuracy of the
thermometer, (2) maintaining an adequate air speed
past the wick, (3) shielding the thermometer from
radiation, (4) use of distilled or deionized water to wet
the wick, and (5) use of a cotton wick.

The thermometer sensitivity required to determine
an accurate humidity varies according to the
temperature range of the air. At low temperatures
more sensitivity is needed than at high temperatures.
For example, at 650C (149F) a 0.5C (0.90F) error
in wet bulb temperature reading results in a 2.6
percent error in relative humidity determination but
at 0C (320F) a 0.5C (0.90F) error in wet bulb
temperature reading results in a 10.5 percent error in
relative humidity measurements [8]. In most cases,,
absolute 'calibration of the wet and dry bulb
thermometer is not as important as ensuring they
produce the same reading at a given temperature. For
example, if both thermometers read 0.50 C (0.90 F) low
this will result in less than a 1.3 percent error in
relative humidity at dry bulb temperatures between
650C (1490F) and 0C (32F) when the difference


Page 7


between dry and wet bulb temperature readings is 5 C
(90F) [8]. Before wetting the wick of the wet bulb
thermometer, both thermometers should be operated
long enough to determine if there is any difference
between their readings. If there is a difference and
the thermometers must be used, one is assumed
correct and the reading of the other adjusted
accordingly when determining relative humidity.

The rate of evaporation from the wick is a
function of air velocity past it. A minimum air velocity
of about 3 m per sec (500 ft per min) is required for
accurate readings. An air velocity much below this
will result in an erroneously high wet bulb reading.
Wet bulb devices that do not provide a guaranteed air
flow, such as those that sit on a desk, cannot be relied
on to give an accurate reading.

As with the dry bulb thermometer, sources of
radiant heat such as motors, lights, and so on, will
affect the wet bulb thermometer. The reading must
be taken in an area protected from these sources of
radiation or thermometers must be shielded from
radiant energy.

A buildup of salts from impure water or
contaminants in the air will affect the rate of water
evaporation from the wick and result in erroneous
data. Distilled or deionized water should be used to
moisten the wick and the wick should be replaced if
there is any sign of contamination. Care should be
taken to ensure that the wick material has not been
treated with chemicals such as sizing compounds that
would affect the water evaporation rate.

In general, properly designed and operated wet
and dry bulb psychrometers can operate with an
accuracy of less than 2 percent of the actual relative
humidity. Improper operation will greatly increase the
error.

Relative Humidity

Direct relative humidity measurement usually
employs an electric sensing element or a mechanical
system. Electric hygrometers operate using substances
whose electrical properties change as a function of
their moisture content. As the humidity of the air
surrounding the sensor increases, its moisture
increases proportionally affecting the sensor's
electrical properties. These devices are more
expensive than wet and dry bulb psychrometers, but
their accuracy is not as severely affected by incorrect
operation. An accuracy of less than 2 percent of the







Psychrometrics and Postharvest Operations

actual humidity is often obtainable. Sensors will lose
their calibration if allowed to become contaminated
and some lose calibration if water condenses on them.
Most sensors have a limited life. Relative humidity
instruments are not recommended for use in the harsh
conditions found in commercial packinghouses.
Mechanical hygrometers usually employ human hairs
as a relative humidity sensing element. Hair changes
in length in proportion to the humidity of the air. The
hair element responds slowly to changes in relative
humidity and is not dependable at very high relative
humidities. These devices are acceptable as an
indicator of a general range of humidity but are not
especially dependable for accurate relative humidity
measurement.

Dew Point Indicators

Two types of dew point sensors are in common
use today: a saturated salt system and a condensation
dew point method. The saturated salt system will
operate at dew points between -12 to 37 C (100 to
100 F) with an accuracy of less than 10C (20F). The
system is lower in cost than the condensation system,
is not significantly affected by contaminating ions, and
has a response time of about 4 minutes. The
condensation type is very accurate over a wide range
of dew point temperatures (less than 0.5C (0.90F)
from -730 to 1000 C (-100 to 2120 F). A condensation
dew point hygrometer can be expensive.

There are a variety of other methods for
measuring psychrometric variables [2]. Some are
extremely accurate and have some characteristics
which make them suited to particular sampling
conditions. However, most are not commercially
available and are used primarily as laboratory
instruments.

SUMMARY

The use of the psychrometric chart and the
relationship of psychrometric variables and their effect
on perishable commodities, were presented. This
article further suggests how the psychrometric
variables can be measured and used -- more
importantly how they should be used by vegetable
producers, packinghouse operators, and commercial
cooler managers. A better understanding of
psychrometrics will allow vegetable growers,
packinghouse operators, and commercial cooler
operators to improve postharvest cooling and storage
conditions for fresh vegetables. A $500 investment in
a simple, reliable, and accurate hand-held


Page 8


thermocouple thermometer and portable psychrometer
and use of a psychrometric chart will allow the
determination of all the psychrometric variables
needed to properly control the ambient environment
within precoolers and cold storage rooms. Proper use
of these tools will allow the managers to correct
problems (such as high temperatures and low moisture
levels) and maintain the quality and reduce the
spoilage of their valuable perishable commodities.

LITERATURE CITED

1. ASHRAE. 1989. ASHRAE handbook--
fundamentals volume. Am. Soc. Heating,
Refrigeration and Air Conditioning Engineers.
Atlanta, GA.

2. Gaffney, J.J. 1978. Humidity: basic principles and
measurement techniques. HortScience,
13(5):551-555.

3. Grierson, W. 1964. Grove heating: some
thermodynamic considerations. Proc. Fla.
State Hort. Soc. 77:87-93.

4. Grierson, W. and W.F. Wardowski. 1975.
Humidity in horticulture. HortScience,
10(4):356-360.

5. Grierson, W. and W.F. Wardowski. 1978.
Relative humidity effects on the postharvest
life of fruits and vegetables. HortScience,
13(5):570-573.

6. Hardenburg, R.E., A.E. Watada, and C.Y. Wang.
1986. The Commercial storage of fruits,
vegetables, and florist and nursery stocks.
Agricultural Handbook No. 66.
U.S.D.A./A.R.S. Washington, D.C.


7. Henderson, S.M and R.L. Perry.
Agricultural Process Engineering.
AVI Publishing Co, Westport, CT.


1980.
3rd ed.


8. Kader, A.A., R.F. Kasmire, F.G. Mitchell, M.S.
Reid, N.F. Sommer, and J.F. Thompson.
1985. Postharvest technology of horticultural
crops. California Coop. Ext. Serv. Pub. 3311.

9. Sargent, S.A., M.T. Talbot, and J.K. Brecht. 1991.
Evaluating precooling methods for vegetable
packinghouse operations. Veg. Crop Dept.
Special Series SSVEC-47, Inst. Food and Ag.
Sci., University of Fla, Gainesville, FL.







Psychrometrics and Postharvest Operations

actual humidity is often obtainable. Sensors will lose
their calibration if allowed to become contaminated
and some lose calibration if water condenses on them.
Most sensors have a limited life. Relative humidity
instruments are not recommended for use in the harsh
conditions found in commercial packinghouses.
Mechanical hygrometers usually employ human hairs
as a relative humidity sensing element. Hair changes
in length in proportion to the humidity of the air. The
hair element responds slowly to changes in relative
humidity and is not dependable at very high relative
humidities. These devices are acceptable as an
indicator of a general range of humidity but are not
especially dependable for accurate relative humidity
measurement.

Dew Point Indicators

Two types of dew point sensors are in common
use today: a saturated salt system and a condensation
dew point method. The saturated salt system will
operate at dew points between -12 to 37 C (100 to
100 F) with an accuracy of less than 10C (20F). The
system is lower in cost than the condensation system,
is not significantly affected by contaminating ions, and
has a response time of about 4 minutes. The
condensation type is very accurate over a wide range
of dew point temperatures (less than 0.5C (0.90F)
from -730 to 1000 C (-100 to 2120 F). A condensation
dew point hygrometer can be expensive.

There are a variety of other methods for
measuring psychrometric variables [2]. Some are
extremely accurate and have some characteristics
which make them suited to particular sampling
conditions. However, most are not commercially
available and are used primarily as laboratory
instruments.

SUMMARY

The use of the psychrometric chart and the
relationship of psychrometric variables and their effect
on perishable commodities, were presented. This
article further suggests how the psychrometric
variables can be measured and used -- more
importantly how they should be used by vegetable
producers, packinghouse operators, and commercial
cooler managers. A better understanding of
psychrometrics will allow vegetable growers,
packinghouse operators, and commercial cooler
operators to improve postharvest cooling and storage
conditions for fresh vegetables. A $500 investment in
a simple, reliable, and accurate hand-held


Page 8


thermocouple thermometer and portable psychrometer
and use of a psychrometric chart will allow the
determination of all the psychrometric variables
needed to properly control the ambient environment
within precoolers and cold storage rooms. Proper use
of these tools will allow the managers to correct
problems (such as high temperatures and low moisture
levels) and maintain the quality and reduce the
spoilage of their valuable perishable commodities.

LITERATURE CITED

1. ASHRAE. 1989. ASHRAE handbook--
fundamentals volume. Am. Soc. Heating,
Refrigeration and Air Conditioning Engineers.
Atlanta, GA.

2. Gaffney, J.J. 1978. Humidity: basic principles and
measurement techniques. HortScience,
13(5):551-555.

3. Grierson, W. 1964. Grove heating: some
thermodynamic considerations. Proc. Fla.
State Hort. Soc. 77:87-93.

4. Grierson, W. and W.F. Wardowski. 1975.
Humidity in horticulture. HortScience,
10(4):356-360.

5. Grierson, W. and W.F. Wardowski. 1978.
Relative humidity effects on the postharvest
life of fruits and vegetables. HortScience,
13(5):570-573.

6. Hardenburg, R.E., A.E. Watada, and C.Y. Wang.
1986. The Commercial storage of fruits,
vegetables, and florist and nursery stocks.
Agricultural Handbook No. 66.
U.S.D.A./A.R.S. Washington, D.C.


7. Henderson, S.M and R.L. Perry.
Agricultural Process Engineering.
AVI Publishing Co, Westport, CT.


1980.
3rd ed.


8. Kader, A.A., R.F. Kasmire, F.G. Mitchell, M.S.
Reid, N.F. Sommer, and J.F. Thompson.
1985. Postharvest technology of horticultural
crops. California Coop. Ext. Serv. Pub. 3311.

9. Sargent, S.A., M.T. Talbot, and J.K. Brecht. 1991.
Evaluating precooling methods for vegetable
packinghouse operations. Veg. Crop Dept.
Special Series SSVEC-47, Inst. Food and Ag.
Sci., University of Fla, Gainesville, FL.