Humidity Testing Cabinet and Improved Humidity Regulator JOHN R . VAN WAZER' Rumford Division, Heyden Cherrrical Corporation, Rumford, R . I . Close humidity regulation was achieved by letting steam into a cabinet through a solenoid valve controlled by a humidity-sensitive resistor. A description and wiring diagram for a sensitive relay activated by resistance variations are given.
I
S TESTING the keeping ability of packaged food products, it
has become customary to store the packages in a humidity cabinet kept a t 100" F. and 90% relative humidity ( 1 , d ) . These values of temperature and humidity are apparently used becausr they represent the worst conditions under which food products are kept in the temperate zone. Recently it was found necessary to enlarge the testing facilities in this laboratory and information about various commercially available testing cabinets was esaniined. As the previous twting cabinet had not controlled satisfactorily, and the commercially available cabinets not only were expensive (about $2000 apiece) but probably would not afford adequate cont'rol a t 90% relative humidity, it wits decided to build u t,hermohumidist,at. The case from a second-hand 20-cubic foot refrigerator was used as a cabinet, and an air-circulating system consisting of a blower and the appropriate duct work was constructed. The temperature was controlled by means of a de Khotinsky thermoregulator (placed in the cabinet), to which was connected a relay suitable for carrying the current to the heating elements. The amount of moisture in the air was adjusted by adding steam, which was introduced through a solenoid valve and orifice from a low pressure steam line. The orifice was chosen so that, when the system was in operation, the solenoid valve would be open for about one third of the time. Control of the humidity was effected by the use of a moisturesensitive resistor and an electrical system in which a variation in the resistance of the resistor caused a change in the deflection of a microammeter. The microammeter then acted as a switch to activate the solenoid valve on the steam line.
was set to give a full scale reading when the humidity-sensitive element was shorted by switch S . Then the position on the microammeter a t which contact was made was adjusted so that the humidity, as measured by wet and dry, bulb thermometers in the cabinet, remained a t the correct value. In order to achieve good control, it is necessary to have an input of heat and steam sufficiently large to overcome quickly the effects of opening the cabinet doors. This means that the system may be flooded with heat or steam in case something goes wrong, and, therefore, it is advisable to use a safety device. Because both excess heat and escess steam raise the temperature in the cabinet, a therniorelay was arranged to activate the solenoid of a latching relay if the temperature were to rise about 105" F. This relay would then turn off the supply of electricity to the entire cabinet and activate an alarm. No provision was made t o take care of the type of breakdown in which the heat or steam goes off, as tliir type of trouble is equivalent to removing the samples from the testing chamber and results only in lost time without dwtruction of the samples. The entire cost of building this equipment, including labor, v a s
When the apparatus was first constructed, platinum contacts were attached t80the indicating needle of a standard 0-100 microammeter to form a switch, and this switch controlled the grid bias of a vacuum tube relay. However, after the system had been in operation for a while, trouble was caused by sticking of the platinum contacts. -4 Symplytrol meter relay with a full scale deflection of 100 microamperes, manufactured by the Assembly Products Corp., Chagrin Falls, Ohio, was then used in the system, as shown in Figure 1. This meter relay is a very interesting piece of apparatus in which a contact is made between the indicating needle of the meter and an adjustable pointer that can be set to any point on the meter scale by a control knob. Once the contact is made, an electromagnet locks the contacts together until the current in the locking circuit is interrupted. When the locking current is interrupted, a spring snaps the contacts apart so that rticking cannot occur. A circuit for automatically interrupting the locking current once every 2 seconds is shown in Figure 1. \'Vit,h u 200-nlfd. condenser across the cont,acts of relay R,, the steam valve did not open with every interruption of the locking circuit. Although it is possible to construct humidity-sensitive resistors from thin films of hydrophilic polymers cont,aining a sniall amount of inorganic salt, an uncalibrated Aminco-Dunmore humidity sensing element (American Instrument Co., Silver Springs, Md.) (gray color code) was employed, as it. is very sensitivp in t.he propcr humidity range. In adjusting the hunlidity controller, resistance R,, Figure 1,
I +L--l
- - - - - - - - -- - - BI. Plate current relay (5000 ohms) Bz. Plate current relay (7000 ohms)
CI. 200-mfd. electrolytic condenser Cs.
100-mfd. electrolyti'c condenser
Cs. 100-mfd. electrolytic condenser
M. Rh. RI. Rz.
Symplytrol contact meter Humidity-sensitive resistor 800,000ohms 500,000ohms Rs. 6800ohras R4. 460ohms R I . 47ohma Ra. 3300ohms TI. Constant voltage transformer (1:l ratio) Tz. Isolating transformer (1:l ratio) V. Solenoid valve for steam (normally d o s e d )
.
1
110 v. A.C.
Figure 1. Wiring Diagram for Humidity Controller
Present address, Great Lakes Carbon Corp., Morton Grove, Ill.
1242
V O L U M E 21, NO. 10, O C T O B E R 1 9 4 9
IC+ than $500, and it has now been operating satisfactorily for wvi.ral months. In operation the temperature and humidity coiitrols go on or off approximately once a minute, and the wet :tiid dry bulb temperatures do not vary mole than 0.2" F. during the cycle. Although the cabinet n a s designed to operate a t 100" F. and 90% relative humidity, it r i m be used at other values
1243 of temperature and humidity by readjusting the controls and using the appropriate humidity-senqitive resistor. LITERATURE CITED
(1) +km. sot. ~~~~i~~Materials, Tentative )lethod D 89547T. (2) Tech. Bssoc. Pulp Paper Ind., Standard T463m. R~~~~~~~~january 29, 1949.
Apparatus for Measurement of the Vapor Pressure lowering of Solutions ROBERT M. MCGILL AND EDWARD S . AMIS, University of Arkansas, Fayetteville, iirk.
A new apparatus is presented for measuring the vapor pressure lowering of solutions. The principle on w-hich the apparatus operates is the vaporization under tension of the solvent from a solution at the boiling point of the pure solvent. The tension is applied by means of a liquid head.
T
HE apparatus described belon 14x5 used to compare, a t the same temperature, the pressures of boiling of a pure solvent and of a solution of nonvolatile solute in the solvent. In this \\a? the vapor pressure loweiing of the solvent by the nonvolatile solute was obtained. Thc apparatus was tried on two solvents and two solutes and found to be satisfactory. EXPERIMENTAL
The apparatus as finally constructed is ahon-n in Figure 1. The solution chamber, A , was made by sealing off the female end of a 45/50 standard-taper joint so that the over-all length was 1 3 cm. The male end of the joint was sealed so that its total length was about 8 cm. At one side of the top of the male joint \vas sealed a 10/30 female standard-taper joint, R, for the insertion of a thermometer fitted with the corresponding male fitting. A 6-mm. stopcock, C, sealed into the top of the male joint, made connection with a Hyvac pump through a vapor trap fitted with ground-glass joints. -41-mm. capillary bore stopcock, D, was sealed through the male joint so that one end extended to the I)ottom of the solution chamber. The other end was fitted with a 1 2 , 1 ground-glass spherical joint for connecting with the vapor c*h:imber. These three seals were symmetrically arranged. The solution chamber was heated with a rheostat-controlled, detach:tble heating jacket which extended nearlv to the bottom of the ground-glass joint. The thermometer h a d :t range of -10" to 250° C. The vapor chamber, V , was not a vhamber in the ordinary sense of the word, but was that end of the tube in which vaporization took place and which was used in messuring the vapor pressure lo\vering. This tube, E, was made o f 1-nun. bore capillary tubing I)rnt in the shape of an inverted U. One end of the U, 45 cm. loiig, was used for measuring the liquid head from which the vapor pressure lowering is calculated. The tube was graduated in millimeters below the level of the vapor chamber stopper and tmlcd in a stopcock, H , which made it possible to close the tube quickly and thus maintain any desired liquid head in the tube. The other end of the U was curved upward to form the vapor c.h:imber, and was closed by a ground-glass mercury-sealed stop1 ) held ~ firmly in place by springs and fitted with a small glass ring at, the top. The inverted C \vas connected a t its apex to the solution chamber through a tube bearing the 12/1 female groundglans spherical joint. The details of the vapor chamber are shown in the insert in Figure 1. boiling jacket, to which a small condenser in Figure 1. T h r jacket was heated with an The vapor trap, G, was 12 cni. long and 2 em. in diameter, f i t t d with a ground-glass stopper and 10/30 female standardtapvr joint for connecting with stopcock C of the solution chamiwr. The trap was filled with glass i,inps t o provide greater con-
(itansation surfarp. .\Ierck's thiophrne-free bciizcwo
\\.:I-
I)urificyl :w*ording to the.
WPILLARY
V DETAIL OF VAPOR CHAMBER AND MERCURY-SEALED STOPPER
Figure 1. Diagram of lpparatus
procedure of brashburn and Read (5). The middle fraction boiling a t 78.50' C. and 720.0-mm. pressure was used. Merck's C.P. carbon tetrachloride was purified according to the instructions of Cameron (1) followed bv distillation over calcium oxide in an all-glass apparatus. The middle fraction boiling a t 74.53" C. and 727.0-mm. pressure was used. Anthracene was Eastman Kodak practical grade, recrystallized three times from 95% ethanol and dried several days in a vacuum desiccator. The product consisted of crcam colored crystals melting at 217.0-217.5" C. Crude benail from a student preparation was recrystallized four times from !)5y0 (Athano1 h r saturating a boiling alcohol yolution aiitl ,ctl(tirig ciiitilli5,l n:itcbr The producat, (lried sevrrd days over
