NBSIR 75-917

A Multiple-Chamber

Humidity Apparatus

Lewis Greenspan

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

December 1975

Final Report

Prepared for Department of the Army

U.S. Army Natick Development Center

Natick, Massachusetts 01760
I
NBSIR 75-917

A MULTIPLE-CHAMBER

HUMIIDITY APPARATUS

Lewis Greenspan

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

December 1975

Final Report

Prepared for Department of the Army

U. S. Army Natick Development Center

Natick, Massachusetts 01760

U.S. DEPARTMENT OF COMMERCE, Rogers C.B. Morton, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science arid Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Acting Director

A Multiple Chamber Humidity Apparatus

Abstract

An apparatus has been developed for studying the growth of

microbiological organisms on food under controlled conditions of

humidity and temperature. This apparatus contains twenty -eight

individual humidity chambers within a temperature controlled bath.

A wide range of humidities can be provided within the individual

chambers by means of saturated salt solutions. These chambers can

be maintained at stable temperatures from 5 to 50 "C with a constancy

and uniformity within ± .02 deg C. Individual chambers may be conveniently

removed or changed without affecting the other chambers.

1
Introduction

This report describes equipment Intended for the study of

microbiological growth as a function of temperature and relative humidity.

In order that these studies may be performed In a reasonable length of

time, a multiplicity of chambers of different humidities is required. A

practical method of providing a number of different humidities

simultaneously is by means of saturated salt solutions.

Any water solution has an equilibrium water vapor pressure which

is a function of the type of solute, the solute concentration and the

temperature of the solution. When a closed isothermal environment,

containing no sources or sinks for water vapor, is exposed to such

a solution, a fixed humidity will be established within the environment

which is in equilibrium with the solution.

The equilibrium water vapor pressure is known for many solutions

as a function of temperature and concentration. Examples of such

solutions are aqueous sulfuric acid solutions and glycerine and water

solutions. For these, a knowledge of the concentration and temperature

provides a knowledge of the surrounding humidity. The problem with such

solutions is that the concentration must be measured and even the process

of equilibrating a chamber can change this concentration unless the

quantity of solution is large with respect to the chamber volume.

Obviously any humidity sources or sinks will alter the concentration.
Water solutions of various salts also provide equilibrium water

vapor pressures as a function of concentration and temperature. Many

of these salts have a finite solubility at a fixed temperature and

therefore a saturated solution of these salts is a solution of fixed

concentration at a fixed temperature.

If the saturated salt solution contains excess salt, the concentration

will remain constant at any fixed temperature, even in the presence of

sources or sinks for water vapor as long as there is any liquid in the

solution and as long as any undissolved S'olute remains. For these saturated

salt solutions, the equilibrium water vapor pressure can be known by merely

knowing the temperature and the type of solute - no measure of concentration

being required. Such saturated salt solutions appear very appropriate for

use in this apparatus, due to their ease of use and their ability to maintain

a constant humidity despite the sorption by or desorption from the test

samples.

3
Description

The Multiple-Chamber Humidity Apparatus consists of a constant

temperature bath, individual humidity chambers and a rack to hold the

chambers within the bath. Also included are the auxiliary equipment

necessary to operate the constant temperature bath at temperatures

from 5 to 50 °C. All equipment is mounted on a three-shelf open cart

122 cm long and 61 cm wide. The maximum height occupied by the equipment

on the cart is 170 cm.

The bath consists of a stainless steel rectangular parallelepiped

with internal dimensions of 89 by 48 cm and a depth of 29 cm. This

provides an internal volume of approximately 0.12 cubic meter. Figure

1 shows an external view of the bath on its cart.

At the bottom of the bath are two U-shaped heating units extending

the length of the bath. Also at the bottom of the bath at one end is a

magnetic agitator consisting of a weighted vane which is alternately

drawn into an unstable position by means of an electromagnet below the

bottom surface of the bath and released. The frequency and amplitude

of the vane oscillation is controllable. The effect of the agitation is

to cause a wave action in the bath fluid , providing additional circulation

over the cooling coils.

4
The humidity chambers are wide mouth glass containers of approximately

450 cm^ capacity with a coarse external screw thread at the open end.

Each container is sealed by means of a stainless steel lid, 6 cm in

diameter which contains a viton gasket, 0.4 cm wide by 0.08 cm thick

inset on the outer edge. The viton gasket seals against the glass lip

of the container. The lid is held against the glass container by means

of a cap which is screwed against the lid by engaging the threads of the

glass container with internal threads in the cap. Suspended from the

lid by means of four stainless steel chains are two stainless steel

shelves for holding test material. Figure 2 is a view of two chambers,

one sealed and the other open.

The chamber rack consists of two identical horizontal stainless steel plates

of . 6 cm thickness which fill the constant temperature bath. One plate

sits on four 5 3/4 cm long stainless steel legs and the other is joined

to the first plate by four stainless steel studs of 9 1/2 cm length. The

legs of the rack sit on the bottom of the bath with the top surface of the

rack being at a depth of 8 cm into the bath. Figure 3 is a view of the

apparatus showing the rack in place in the bath.

5
Both plates of the rack, have 28 circular holes of 8.1 cm diameter

on 10 cm centers, which act as support for a like number of humidity

chambers. At one comer of the plates is a 12 cm square hole which

provides the space for a centrifugal type stirrer and a resistance

thermometer. A stainless steel spring stretches across the center of

each of the circular holes on the top plate. These springs hold the

humidity chambers down in the rack while permitting easy one-handed

access to individual chambers. The chambers sit on an open grid shelf

above the heaters and magnetic agitator. This shelf is supported by

two longitudinal ledges within the bath.

Auxiliary equipment for operation of the constant temperature

bath includes a temperature controller (with associated resistance

thermometer sensor), heaters, stirrer (centrifugal), chiller (mechanical

refrigerator), magnetic agitator, and water feed (solenoid valve).

These items are wired to a control cabinet located on the second shelf

of the cart. The temperature controller also occupies the second shelf

and the chiller is on the bottom shelf. These six auxiliary items are

removable or replaceable and individually plug into appropriate outlets

on an electrical outlet strip located on the second shelf. Not only are

all plugs and outlets labeled, but they are all distinctive, such that

no plug will go into an inappropriate outlet.

Other auxiliary equipment include a liquid level controller, a

water pressure reducer and a water filter mounted on the end of the

bath as shown in figure 1.

6
The control cabinet contains a master power switch and associated

red signal lamp which controls all power to the apparatus. The control

cabinet also contains 6 individual switches and associated signal lights

for control of each of the auxiliary electrical items as shown in the

circuit diagram (Figure 4). The control cabinet also contains three

15-ampere slow blow fuses to protect the equipment.

Operation

Two connections must be made before the equipment is operational.

The green water feed hose must be connected to a water source and the

power cord must be connected to a 20-ampere 5-wire 208-volt 3-phase power

source. The water feed hose is connected by means of a 1/4-inch hose

shank to which the hose is clamped. When the water source is turned

on nothing will happen until the main power switch and the water feed

switch are turned on. When the water source, main power switch and water

feed switch are all on, the bath will fill with water to the proper

level and this level will be maintained. The bath drain valve should

of course be closed during this operation.

The water feed system has been designed to operate with water

pressures up to 2 MPa (300 psi) , thereby allowing the use of any

conventional water supply. The design is such that temperature perturbations

have been minimized by limiting the water flow into the bath during

operation. This design results in a very slow fill rate through the

automatic system and will require a number of hours to fill the

bath through this system. Of course, more rapid filling can be achieved

by filling the bath through the top by other means.

7
When the water has reached the level of the top plate of the

rack, the temperature control equipment can be put into operation.

Whenever the temperature control equipment is in operation the

stirrer should be on, not only to insure proper temperature control

operation but also to prevent overheating of the heaters. It is for

this reason that the signal light on the stirrer is green in order to

suggest that it be on at all times. The stirrer will not be harmed by

operation without fluid in the bath although there is no benefit to be

derived from such operation.

The chiller should not be used when the bath liquid temperature

exceeds ambient temperature by more than 2 degrees Celsius. Not only is

use of the chiller unnecessary under these conditions, but the chiller

can be harmed by operation with the evaporator coil at high temperature.

There is a separate switch on the chiller itself, which for convenience

should be left on at all times. This retains control of the chiller at

the control cabinet. Likewise, there are switches on the temperature

controller and at the end of the bath for the magnetic agitator which

should be left on at all times in order to maintain all control at the

control cabinet.

8
In all controlled-temperature operation, the temperature controller

has to be on. The controller has a ten-position coarse setting, a tenposition medium setting and a continuous ten-turn fine setting scaled

in 1000 units. With use, it will be appropriate to determine a calibration

curve for the settings. Roughly, there is I.5 deg C between adjacent medium

setting points, 20 deg C between coarse setting points 4 and 5 and 22 deg C

between coarse setting points 5 and 6. There is between .005 and .01 deg C ] unit of the fine setting. A setting of 4-5-000 gives approximately

5 "C and 6-6-150 gives approximately 50 °C.

Although the chiller will operate without the temperature controller

turned on the heater circuit has been intentionally wired so that it can

not be activated without the temperature controller. This will be apparent,

since the heater signal light will not be on without the controller

signal light being on. The separate heater switch is retained to allow

adjustment of the temperature controller without the heaters coming on

when this is desired.

When the chiller is in use, it is advisable to have the magnetic

agitator in operation as well as the centrifugal stirrer. Minor magnetic

agitation suffices to provide the additional mixing desireable and

adjustment can be made by means of the pulse duration and pulse frequency

controls at the end of the constant temperature bath. Maximum pulse

frequency is beneficial but the pulse duration can be adjusted to give a

just audible sound. The magnetic agitator signal light on the control

cabinet indicates when power is available to the magnetic agitator system.

The indicator light on the end of the constant temperature bath indicates

the actual pulsing of the agitator. Similarly, the heater signal

light on the control cabinet indicates the availability

9
of power for the heater while the light on the temperature controller

indicates the actual application of this power to the heater.

A cover is provided for the constant temperature bath, but is not

required for good temperature control. Its main function is to reduce

evaporation from and contamination of the bath fluid.

Salt solutions should be added to the humidity chambers to a

depth of between one and two cm. These salt solutions should contain

considerable excess salt, resulting in a solution with a consistency

that approximates a slush. The test samples are held above the salt

solution on the two shelves provided in the lid assembly. A convenient

method of holding the test samples would be in crystallizing dishes of

approximately 55 mm diameter and 15 mm depth. The dishes are firmly

retained on the shelves by the chain supports while their flexibility

allows easy placement or removal of the dishes.

When not in use, the humidity chambers should not be tightly sealed.

This will prevent the seal from adhering strongly to the glass. Where

not objectionable, a silicone stopcock grease can be used on the gasket

to help sealing while reducing adhesion to the glass. The glass lip and

the gasket should have no extraneous material on them when used. Although

every effort has been made to insure leaktightness,any noticeable increase

in level of the salt solution should be taken as an indication of leakage.

10
When the chambers are sealed for long periods, especially at elevated

temperatures, there may be a tendency for the gasket to adhere to the

glass. The lid must then be gently raised so as not to upset the test

samples. A suction cup with attached handle is provided for this purpose.

The cup is attached to the lid and the seal is broken with a tilting

action which breaks the seal.

Figure 5 shows some of the saturated salt solutions that are available

for use in this type of apparatus along with their approximate relative

humidity as a function of temperature. The uncertainty in

relative humidity involves three factors, namely:

1) the uncertainty in the equilibrium relative humidity of the

salt solutions,

2) the uncertainty in the temperature of the s*lt solution and

3) the uncertainty in the temperature of the test sample.

Further analysis will be required to establish definitive estimates

of uncertainties in the equilibrium relative humidities for the various

possible saturated salt solutions. Our present estimates are that

the relative humidities shown for the saturated solutions in figure 4

are uncertain to less than 1%.

11
The constant temperature bath was operated with 28 humidity chambers

in position at 5, 25 and 53 **C. Temperature measurements were made in

what would have been salt solutions under normal operation at two extreme

locations in the bath at ten minute intervals over long periods of time.

Analysis of the data, which consisted of over 100 points at each temperature,

indicated that the mean difference between the two positions was less than

0.02 °C for the three temperatures and that three standard deviations of the

temperature at one of the locations was less than 0.02 "C for the three

temperatures.

The mean difference and the standard deviation indicate the uncertainty

in the known temperature of the salt solution during normal operation. It

is appropriate to analyze the effect such an uncertainty will have on the

uncertainty of the relative humidity within the chamber. The effect of

this uncertainty is related only to the slope of the relative humidity verses

temperature curve. The maximum slope of any of the salt solutions shown in

figure 5 is associated with Mg (N02)2 at 50°C where the slope is -.336% rh

per degree Celsius. Taking the extreme condition of .04*C which is the sum

of the mean difference plus three sigma we would have an uncertainty in rh

of .01%. This is the extreme COTidition for uncertainty in the salt solution

temperature.

The most appropriate location in which to measure the temperature has not

been determined. It is likely that a measurement in the bath fluid near the

centrifugal stirrer will give a representative temperature for the salt solutions

Further measurements should be made to verify this assumption.

12
The third uncertainty is related to temperature difference between

the salt solution and the test sample. The amount of water vapor in the

gas above the salt solution is a function of the salt solution temperature

and at least to a first approximation is independent of the temperature of the

gas above the solution. The effective relative humidity to which the test

sample is exposed is a function of the amount of water vapor in the gas

and the temperature of the test sample. Therefore, the relative humidity

to which the sample is exposed has a further uncertainty related to the

temperature difference between the salt solution and the test sample.

Adequate measurements were not made to establish definitively this

uncertainty. Since the entire chamber is immersed in the bath fluid, it

seems reasonable to assume that the variability in the temperature of the

salt solution represents a greater value than the temperature difference

between the salt solution and the test sample. Let us use three sigma (0.2 deg C)

as the maximum difference. From this we can establish the maximum uncertainty

in effective relative humidity to which the sample is exposed due to this

temperature difference. This uncertainty is a function of the absolute

temperature and the relative humidity and varies for a .02 deg C temperature

difference from 1 part per thousand at 53 "C to 14 parts in ten thousand at

5 °C. The maximum uncertainty in effective rh, for this temperature

difference would therefore be .14% for 5 °C and 100% rh.

13
It would appear that the dominating uncertainty in experiments

with this apparatus will be primarily due to the uncertainty in

the knowledge of the equilibrium rh for the saturated salt solutions

and secondarily due to temperature differences occurring between the

salt solutions and the test samples.

USCOMM-NBS-DC 14
MULTIPLE-CHAMBER HUMIDITY APPARATUS

Figure 1
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HUMIDITY CHAMBER

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NBS-lUA (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. FLMiLK A 1 K)N OR RI-.PORI NO.

NBSIR 75-917

^. tlov t Accession

No.

3. Recipient's Accession No.

A Multiple -Chamber Humidity Apparatus

5. Publication Date December 1975

6. Performing Organization Code

221.06

7. AUJH0R(S1 Lewis Greenspan

8. Performing Organ. Report No. NBSIR 75-917

9. PhR 1- OKMINCj ORCjANIZA riON NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2216A62

11. Contract /Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Department of the Army

U. S. Army Natick Development Center

Natick, Massachusetts 01760

13. Type of Report & Period Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

An apparatus has been developed for studying the growth of

microbiological organisms on food under controlled conditions of

humidity and temperature. This apparatus contains twenty eight

individual humidity chambers within a temperature controlled bath.

A wide range of humidities can be provided within the individual

chambers by means of saturated salt solutions. These chambers can be maintained at stable temperatures from 5 to 50 "c with a constancy

and uniformity within ± .02 deg C. Individual chambers may be conveniently

removed or changed without affecting the other chambers.

17. KEY WORDS (six to twelve entries: alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Humidity; humidity chamber; microbiological growth;

relative humidity; salt solutions; saturated salt.

18. AVAILABILITY [^Unlimited

1

' For Official Distribution. Do Not Release to NTIS

1

^ Order From Sup. of Doc, U.S. Government Printing Office Washineton. D.C. 20402. SD Cat. No. C13

! Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19, SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

21. NO. OF PAGES

21

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

$ 3.25

USCOMM-DC 29042-P74