US3634651A

US3634651A - Serological incubator

Info

Publication number: US3634651A
Application number: US95203A
Authority: US
Inventors: Maxwell E Siegel; Edward J Rapoza; Roger A Chevalaz
Original assignee: Becton Dickinson and Co
Current assignee: Becton Dickinson and Co
Priority date: 1970-12-04
Filing date: 1970-12-04
Publication date: 1972-01-11
Anticipated expiration: 1989-01-11
Also published as: DK135385C; SE390067B; DK135385B; JPS4924022B1; CH543894A; GB1304819A; IT942301B; FR2117351A5; DE2157449A1

Abstract

A hollow heat resistant plastic body has openings in a top surface thereof for receiving carriers containing objects to be heated, each opening has a hinged hood formed to enclose parts of the carrier that extend above the top surface of the body. Each opening has associated therewith a spring-loaded elevator platform for sealing the opening to prevent air leakage when the platform is not being used and for lowering into the body when a carrier is placed thereon. A blower is positioned within the body and adjacent to a thermostatically controlled heater for circulating air over the entire heater. A calibrated adjustable air inlet and baffles are provided to optimize the air circulation within the body and to maintain a uniform temperature.

Description

United States Patent [72] lnventors Maxwell E. Siegel 2,715,898 8/I955 Michaelis et al 219/400 Sussex; 2,932,718 4/1960 Marsters 219/530 X Edward J. Rapoza, Butler; Roger A. 3,070,086 12/1962 Smith et al. 12S/l B Chevalaz, Rockaway, all of N.J. 3,109,084 IO/l 963 Walsh 2 I 9/433 [2l] Appl. No. 95,203 3,22I,729 l2/l965 Beasley et aI.... 219/400 X [22] Filed Dec. 4, 1970 3,376,405 4/1968 Gower 219/385 [45] Patented Jan. l1, 1972 3,454,3I8 7/1969 Kemp et al. 219/399 [73] Assignee Becton, Dickinson and Company 3,461,274 8/l969 Williams 2l9/52i X East Rutherford, N.J. FOREIGN PATENTS 725,728 9/1942 Germany 219/400 [54] SEROLOGICAL INCUBATOR Primary Examiner- Volodymyr Y. Mayewsky 10 Claims, 7 Drawing Figs. Attorney- Kane, Dalsimer, Kane, Sullivan and Kurucz [52] U.S. Cl 219/400, H

12g/l B 21g/386 21g/4131 21g/4331219521 ABSTRACT: A hollow heat resistant plastic body has 21g/530 236/3 237/3 23/253 openings in a top surface thereof for receiving carriers con- [5 l] lill. Cl F27d lll/02 taining objects to be heated` each opening has a hinged hood [50] Fleld 0f Search 21g/400, formed to enclose parts of the carrier that extend above the 413 521 523 530' 540 378 380-381 386399 to surface of the body. Each opening has associated 3 37/ 3 /2 5* /2 3 therewith a spring-loaded elevator platform for sealing the opening to prevent air leakage when the platform is not being [56] References cned used and for lowering into the body when a carrier is placed UNITED STATES PATENTS thereon. A blower is positioned within the body and adjacent 1,719,527 7/1929 Bartlett et al. 237/3 to a thermostatically controlled heater for circulating air over 1,865,472 7/1932 LamSein--m 219/433 X the entire heater. A calibrated adjustable air inlet and baffles 2,600,240 6/1952 Grieb 12S/l B are provided to optimize the air circulation within the body v and to maintain a uniform temperature.

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snnorocrcsr. rNcuaAron BACKGROUND OF THE INVENTION l Field of the Invention This invention relates to serological incubators and more particularly to an incubator that utilizes warm air circulation to maintain a prescribed temperature.

2. Description of the Prior Art Incubators are widely used in chemical procedures to aid reactions by maintaining the reactants at a specified temperature. In the Blood Bank of the clinical laboratory, for instance, many tests require a temperature of 37 C., or body temperature, while in SerOlOgy, 56 C. is a commonly used tempera- UI'C.

In the prior art there were two types of incubators in general use, the water bath and the dry bath or heat block type of incubator. The water bath type of incubator maintained a tank of water at a prescribed temperature. There were several advantages to this type of incubator, one being that it held a relatively stable temperature with little variation throughout the tank if a circulating pump was used. The water bath warmed objects quickly and accepted entire test tube racks or other odd-shaped containers. However, the water bath incubator had disadvantages such as loss of water due to evaporation, mold growth, rusting and messiness all of which required maintenance time. The water bath incubator also required extremely long warmup time prior to being brought to the prescribed temperature; therefore, they were usually left in the heated condition continuously to be immediately available when needed. This consumed excessive power and also created a dangerous condition in the laboratory.

The dry-'bath-type incubator usually consisted of a block of aluminum, drilled to receive test tubes or other objects of various sizes. Its advantages included convenience, relatively rapid warmup time and elimination of the disadvantages of the water bath type of incubator. However, its major disadvantage was its lack of flexibility in that it could not receive oddshaped objects or entire racks of test tubes. The aluminum block had to be predrilled to the size test tubes that were to be used. Test tubes had to be individually inserted and removed thus increasing the likelihood of identication errors while consuming excessive time.

Heating ovens have been used in the prior art and in these devices air or other gaseous fluids were heated by a heating element and then circulated by a fan. Generally, these devices did not achieve a uniform heat distribution and did not provide optimum heat transfer from the heating element to the fluid. Undesirable temperature variations existed in different areas of the oven in the form of hot and cold pockets of uncirculated air.

SUMMARY OF THE INVENTION The present invention contemplates a warm air incubator which combines the advantages of both the dry bath and water bath incubators while eliminating the difficulties encountered with heating ovens. Such an incubator has the flexibility of the water bath in that it accepts various shaped containers such as test tube carriers while having the cleanliness and rapid warmup ofthe dry-bath-type incubator. The present invention provides continuous temperature control over a range from room temperature to 60 C. for general purpose use and it is factory calibrated at the two most common clinical temperatures, namely 37 C. and 56 C.

A hollow body has a tangential blower which circulates air over the entire length of a heating rod to provide maximum heat transfer to the air from the rod and to uniformally distribute the heated air throughout the hollow body. The heating rod is'controlled by a variable thermostat utilized to select a desired temperature. Openings are provided in a top surface of the hollow body for receiving carriers, each opening having a hinged hood formed to enclose portions of the carriers that extend above the top surface. Associated with each opening is a spring-loaded elevator platform upon which a carrier is placed. The platform is lowered into the body and retained therein against the forces of the spring loading by retaining stops formed on the carrier. When the carrier is removed the platform rises up and seals the associated opening thereby minimizing air leakage from within the body.

When a carrier containing test tubes is placed on the platform and the platform is lowered into the hollowibody and open ends of the test tubes extend labove the `topsurface and are not subjected to the air circulation within the body, thus minimizing evaporative losses of reactants contained within the test tubes. The hoods enclose the portion of the'test tubes extending above the top surface and prevent loss of heated air.

Uniform temperatureismaintained throughout the space within the body by the use of baffles and a calibrated variable air inlet. The baffles establish a circulation'path throughout the body and the variable air inlet is calibrated at 37 C. and 56 C. to provide optimum circulation and to compensate for air lcakages and extraneous heat inputs.

A l-hour mechanical timer` having an audible alarm is provided for setting various incubationtimes and for providing a warning when the selected times have expired.

The present invention provides a serological incubator that has rapid warmup time so that it maybe turned off when not in use. The device is flexible in thatit receives carriers of test tubes or other odd-shaped containers while requiring less maintenance because `it is not subject to rusting, water evaporation or mold growth as were the water-bathftype incubators ofthe prior art.

One object of the invention is to provide a serological incubator having fast warmup time in comparison with those of the prior art.

Another object of the present invention is to provide an incubator that can receive carriers of test tubes or odd-shaped containers.

Another object of the invention is to provide an incubator that can receive a larger quantity of test tubes than most incubators of the prior art.

Another object of the present invention is to provide an incubator that has a selectable temperature over a range from room temperature to 60 C.

Another object of the invention is to provide an incubator utilizing warm air for heating and having a minimum of air loss during operation.

Another object of the invention is to provide a warm air incubator having a uniform temperature distribution.

Another object of the invention is to provide a warm air incubator that reduces reactant evaporation from open test tubes.

Another object of the invention is to provide a warm air incubator that has increased heating e'rciency and uniform heat distribution.

The foregoing other objects and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description which follows,.taken together with the accompanying drawings wherein one embodiment of the invention is illustrated by way of example.

DESCRIPTION OF THE DRAWINGS rio. 1.

FIG.6 is a schematicdiagram showing the electrical connections'withinthe incubator of FIG. .1.

FIG. 7 is a isometric view of a carrier used with the present invention.

DESCRIPTION OF THE INVENTION Referring to FIGS. 1 and 2 there is shown a base member 1 formed of a heat resistant plastic and having feet 3 formed therein and a flange S formed around the periphery thereof. A body member 7 also formed of heat resistant plastic has an inwardly extending flange 9 positioned adjacent flange 5 and secured thereto by mounting screws 11 so that body member 7 and base 1 define a chamber 6. Body member 7 has openings 13 formed in a top surface thereof for receiving carriers. Member 7 has formed therein a control panel 15 for mounting control and indicator devices thereon.

A rocker-type on-off switch 17 is mounted on the control panel and adjacent thereto are mounted pilot lights 19 and 21 for indicating when power is being applied to the incubator and heater respectively. A mechanical l-hour timer 23 is mounted below control panel and has a shaft extending upward therethrough with a knob 25 mounted above the control panel on said shaft for setting a selected time. Timer 23 has a bell for sounding an alarm when a selected time period has elapsed. Inscribed on the control panel about knob 25 are numerals 27 indicating the minutes remaining before the timer provides the alarm. A preheat indication is inscribed on control panel 15 at approximately the S-minute position of timer 23, said position indicates the time required to preheat the air within the incubator to a selected temperature.

Within chamber 6 there is positioned a baffle member 3l formed of a sheet metal material and having front and rear downtumed portions 33 and 35 respectively and

flanged portions

37 and 39.

Flanged portions

37 and 39 have selfclinching standoffs 4l attached thereto and extending downward therefrom. Standoffs 41 are tapped to receive mounting screws 45 for mounting baffle member 31 to base member l and within chamber 6. A thermostat 47 is mounted below baffle 31 by a mounting bracket 49 and mounting screws 51. Thermostat 47 has a shaft 53 extending through side portion 33 and through member 7. A knob 55 is mounted to shaft 53 for setting thermostat 47. Referring to FIG. 4 there is shown knob 55 and surrounding said knob is a rotatable indicating plate 57 for indicating thermostat temperature settings. A set screw 59 is adapted to fix plate 57 at a particular position after calibration of thermostat 47 is accomplished at the factory.

Associated with each opening 13 are a pair of posts 61 shown most clearly in FIG. 2. Posts 61 are upstanding from bale 31 and are tapped for receiving mounting screws 63 for attaching posts 61 to baffle 31. The upper ends of posts 6l are received in cylindrical receptacles 65 formed in member 7. An elevator platform 67 is associated with each of the openings 13 and is slidably mounted between posts 61 for vertical motion. Leaf springs 69 located between baffle 3l and platforms 67 are provided for urging platform 67 to its maximum upward position in which position platforms 67 seal openings 13.

Referring to FIG. 7 there is shown a carrier 71 used in the incubator of the invention. The carrier is designed to hold objects for insertion into the incubator, the embodiment shown is designed to hold test tubes and microscope slides. One skilled in the an can easily design other embodiments for holding other objects to be heated. The carrier shown is made of plastic material and has two end members 123 having upward extensions forming handles 125. The end members are slit along lines extending from each side of handles 125 so that the handles may flex. Retaining stops 122 are formed about midway down each handle. Crossmembers 127 and 129 are cemented parallel to each other and between the end members at the top and midpositions respectively of the end members. A lower crossmember 131 is cemented between the end members substantially near the bottom and parallel to the crossmembers 127 and 129. Crossmembers 127 and 129 have notches 133 formed therein adjacent handles 125 to allow the handles to flex inwardly. Also formed in members 127 and 129 are circular openings 135 for receiving test tubes and slots 137 for receiving microscope slides. Each circular opening and slot in member 127 has an associated opening or slot in member 129 so that the test tubes and slides stand straight up.

Referring to FIG. 3 there is shown the incubator of FIG. 2 with a carrier 7l containing test tubes 73 inserted in one of the openings 13 and resting on one of platforms 67. The weight of carrier 71 and test tubes 73 may force platform 67 in a downward direction against the force of springs 69 so that platform 67 is at its lowest vertical position and the lower portions of test tubes 73 are within chamber 6. However, if the weight is not sufficient then the carrier is pressed downward until retaining stops 122 on carrier 71 engage a lip formed around openings 13 of body member 7 to retain the platform in its lowest position. The upper portions of test tubes 73 extend above the top surface of member 7; therefore, hoods 75, made of clear plastic, are provided to enclose the upper portions of test tubes 73 and prevent air leakage when a platform is not sealing an opening. Each hood 75 has a tab 76 formed thereon for engagement with an opening 77 formed in member 7. Said engagement fomling a hinge whereby hood 75 may be opened as shown in FIG. 3. If desired one hood may be used to cover all of the openings. To remove carrier 7l each handle 125 is flexed inwardly until stops 122 disengage the lip and the carrier is lifted out.

Referring to FIGS. 4 and 5 a blower assembly 79 has

end brackets

81 and 83 mounted to

flanged portions

85 and 87 respectively of baffle member 3l. Mounting screws 89

secure end brackets

81 and 83 to the associated flanges of baffle 3l. A heating rod is mounted between

end brackets

81 and 83 by nuts 85 threaded on a threaded rod 91 passing through heater 93. Heating rod 93 is of sufficient length to extend substantially across chamber 6. Heater 93 is of open wire construction and has an electrical rating of about 200 watts. A tangential blower member 97 is rotatably mounted adjacent and parallel to heating rod 93 and between end brackets 8l and 83. Fan 97 has a shaft 99 drivably connected to a motor 101 which is mounted to end bracket 81. Mounted between

end brackets

81 and 83 is a semicircular baffle member 103 that partially encloses fan 9.7 as shown most clearly in FIG. 4. Fan 97 rotates in a counterclockwise direction as indicated by arrow 104 in FIG. 4 for circulating air in a direction as indicated by arrows 105 around baille 31. The blades of fan 97 are substantially equal in length to heating rod 93 so that the air circulates over the entire length of rod 93 for maximum heat exchange while fan 97 is a tangential blower it functions like a paddle wheel.

An adjustable air inlet is shown generally at 107 and comprises a raised segmental area 111 formed on the top surface of member 7. A segmental opening 112 is provided in area l 1 1 for allowing air to enter chamber 6. A screen member 109 is mounted on the inner surface of member 7 below opening 112. A shutter member 113 is slidably mounted between screen 109 and raised area l l1 for varying the effective opening formed in raised area 111. Indices 115 are inscribed on raised area lll to indicate the proper shutter position for optimum air circulation within the incubator for the two most commonly used temperatures. These indices are inscribed at the factory during calibration of the incubator. The adjustable air inlet functions to provide uniform heat distribution at various temperatures by providing the optimum air circulation within the incubator and for compensating for air leaks out of the incubator. Opening 112 is located above fan 97 so that the fan draws air into the chamber to compensate for air leakages out of the chamber and to prevent overheating from extraneous heat sources such as the blower motor.

Referring to FIG. 4 baffle screens 139 made of perforated sheet metal are mounted to baffle 31 to provide a constant resistance to air circulation. Screens 139 extend across chamber 6 to assure a uniform circulation of air in the chamber when test tubes are loaded only in one end of carrier 71. Without the screens the air tends to follow the path of least resistance and bypasses the test tubes when they are unevenly loaded.

Screens 139 equalize the airflow to maintain a uniform heat distribution.

A power cord and plug assembly 116 is provided for connecting to any convenient power source. Cord 116 enters member 7 through a rubber grommet 117 which prevents air leaks and protects the power cord.

Referring to FIG. 6 there is shown plug 116 electrically connected to switch 17 which is a double-pole throw-type switch having two outputs each connected to fan motor 101. Pilot light 19 is connected across the outputs of switch 17 to indicate when switch 17 is in the on position and providing power to the incubator. Series connected between the outputs of switch 17 are a safety control 119, thermostat 47 and heater 93. Safety control 119 is of a type that opens a circuit when the temperature is increased above a specified limit. A device such as the Dual Mite Model No. 206 available from Thenno Engineering Company, Akron, Ohio, may be used for the present invention. Such a device is calibrated to open a circuit at a temperature of about 170 F. Pilot light 21 is connected in parallel with heater 93 to indicate when power is being provided to the heater.

In operation, prior to inserting a carrier knob 25 is turned to the preheat position so that the air within the incubator is heated to a desired temperature which is selected with knob 55. After the preheat period the temperature of the air within the incubator should be as indicated by knob 55 and then a carrier is placed on one of the platforms 67 and pressed down until the stops 122 engage the lip of the opening to retain the platform in space 6 within the incubator. The associated hood 75 is placed over the carrier to seal the opening and prevent warmed air from escaping. If the other openings 13 are not being utilized the associated platforms 67 are in their uppermost position and seal openings 13 to prevent air leakage therethrough. Knob 25 is then turned to the desired incubation time after which period the timer 23 sounds an audible alarm notifying the operator that the incubation period has elapsed.

Referring to FIG. 4 it may be noted that baffle 31 extends horizontally to a position adjacent fan 97. Baffle 3l functions to establish an oval flow pattern within the incubator. The counterclockwise rotation of fan 97 establishes a counterclockwise flow of air about baffle 31. Baffle 103 functions to direct the air returning from under baffle 3l back into the blades of fan 97 where it is again forced under baffle 31 and around the counterclockwise oval path. Fan 97 tends to draw a limited amount of air through opening 112 as shown by arrows 121. Air entering opening 112 compensates for air leakage out of chamber 6 and mixes with air that has passed over heating rod 95 to prevent overheating. Opening 112 is located near the motor end of blower 79 to compensate for excess heat radiated by the motor.

It may be noted from viewing FIG. 3 that the upper portions of test tubes 73 extend into hoods 75 and are thus protected from the flow of warm air around baffle 3l thus minimizing evaporative loss of the reactants within test tubes 73. The incubator is not limited to receiving test tubes or microscope slides, the caniers may be designed to hold any odd-shaped objects.

Thus, the present invention provides a serological incubator having a fast warmup time so that the device may be shut off when not in use. The incubator has a continuous temperature range from room temperature to 60 C. and can receive entire test tube carriers or odd-shaped containers thus reducing the possibility of mislabeling and identifying the test tubes. A long maintenance-free life is expected since the incubator is not subject to rust or mold formation as were those of the prior art. Maximum heating efficiency and uniform heat distribution are achieved by the use of a tangential blower, baffles and a calibrated variable air inlet.

What is claimed is:

l. An incubator for heating reactants contained in open ended test tubes to a selected tempcrature,'comprising:

a body member having an inner chamber and openings in a top surface thereof for inserting the test tubes into the chamber;

a spring-loaded platform associated with each opening for 5 sealing the associated openings when the platform has no test tubes placed thereon and for lowering into said chamber when test tubes are placed thereon;

hood means for enclosing the openings and the portions of the test tubes that extend above the top surface;

electrical heating means for heating a gas within the chamber;

thermostat means in thermal contact with the gas for controlling said heating means in response to gas temperature; and

circulating means for circulating the gas throughout the chamber and overthe heating means, whereby the open ends of the test tubes are not subjected to circulating gas and evaporation of the reactants is minimized.

2. An incubator as described in claim l, wherein the support means additionally comprises a carrier for holding the test tubes upright and for retaining the spring-loaded platform in the chamber.

3. An incubator as described in claim l, wherein each spring-loaded platform comprises:

a pair of posts mounted vertically beneath the top surface of the body member;

a level platform slidably mounted to said posts for vertical motion; and

leaf spring positioned beneath the platform forurging the platform in an upward direction to seal the associated opening.

4. An incubator for heating materials contained in containers to selected temperatures, comprising:

a body member having an inner chamber and openings in a surface thereof for inserting the containers into the chamber;

spring-loaded platform means associated with each opening for supporting the inserted containers and for sealing the associated opening when the platform has no containers placed thereon;

hood means for sealing the openings and enclosing any portion ofthe `containers that extend above the surface of the body member;

electrical heating means for heating a gas within the chamber;

thennostat means in thermal contact with the gas for controlling said heating means in response to gas temperature; and

circulating means for circulating the gas throughout the chamber and over the heating means, whereby the openings are sealed by the spring-loaded platform when no containers are placed on the platform.

5. An incubator as described in claim 4, wherein the hood means is constructed of a transparent material.

6. An incubator as described in claim 4, additionally comprising carrier means for supporting containers to be heated.

7. An incubator as described in claim 4, wherein the heating means comprises a heating rod contained within the chamber and extending substantially across the chamber, and the circulating means comprises a tangential blower positioned adjacent said heating rod and having a length substantially equal to that of the heating rod.

8. An incubator as described in claim 4, additionally comprising baffle means for establishing a circulation path throughout the entire chamber, and a screen baffle extending across the circulation path, whereby uniform heat distribution is provided within the chamber when the containers are unevenly inserted in the chamber.

9. An incubator as described in claim 4, additionally comprising a variable gas inlet positioned so that the circulating means draws unheated gas into the chamber and mixes it with heated gas and calibrated to provide uniform heat distribution 75 within the chamber at selected temperatures.

10. An incubator for heating objects such as reactants contained in open ended test tubes to predetermined temperatures, comprising:

a body member having an inner chamber and openings in a top surface thereof for inserting test tubes into the chamber;

spring-loaded platform means associated with each opening for sealing the associated opening when there are no test tubes placed thereon and for supporting said test tubes within the chamber so that the open end of the test tubes extend above the top surface of the incubator and out of the openings;

hood means for closing the openings and enclosing the portions of the test tubes that extend above the top surface;

an electric heating rod contained within the chamber and extending substantially across the chamber;

a tangential blower positioned adjacent said heating rod and having a length substantially equal to that of the heating rod so that the fan circulates air over the entire length of the heating rod;

a baffle for establishing a path to circulate the air throughout the chamber;

a screen baffle extending across the circulation path, to provide uniform heat distribution within the chamber when loaded with odd-shaped or unsymmetrically loaded test tube rocks;

a variable air inlet positioned relative to said fan so that air is aspirated into the chamberl said air inlet being calibrated at selected temperatures to provide uniform heat distribution within the chamber; and

thermostat means in thermal contact with the air within the chamber for controlling the electrical heating rod in response to the air temperature, whereby the air within the chamber is maintained at a selected temperature.

Claims

1. An incubator for heating reactants contained in open ended test tubes to a selected temperature, comprising: a body member having an inner chamber and openings in a top surface thereof for inserting the test tubes into the chamber; a spring-loaded platform associated with each opening for sealing the associated openings when the platform has no test tubes placed thereon and for lowering into said chamber when test tubes are placed thereon; hood means for enclosing the openings and the portions of the test tubes that extend above the top surface; electrical heating means for heating a gas within the chamber; thermostat means in thermal contact with the gas for controlling said heating means in response to gas temperature; and circulating means for circulating the gas throughout the chamber and over the heating means, whereby the open ends of the test tubes are not subjected to circulating gas and evaporation of the reactants is minimized.

2. An incubator as described in claim 1, wherein the support means additionally comprises a carrier for holding the test tubes upright and for retaining the spring-loaded platform in the chamber.

3. An incubator as described in claim 1, wherein each spring-loaded platform comprises: a pair of posts mounted vertically beneath the top surface of the body member; a level platform slidably mounted to said posts for vertical motion; and leaf springs positioned beneath the platform for urging the platform in an upward direction to seal the associated opening.

4. An incubator for heating materials contained in containers to selected temperatures, comprising: a body member having an inner chamber and openings in a surface thereof for inserting the containers into the chamber; spring-loaded platform means associated with each opening for supporTing the inserted containers and for sealing the associated opening when the platform has no containers placed thereon; hood means for sealing the openings and enclosing any portion of the containers that extend above the surface of the body member; electrical heating means for heating a gas within the chamber; thermostat means in thermal contact with the gas for controlling said heating means in response to gas temperature; and circulating means for circulating the gas throughout the chamber and over the heating means, whereby the openings are sealed by the spring-loaded platform when no containers are placed on the platform.

9. An incubator as described in claim 4, additionally comprising a variable gas inlet positioned so that the circulating means draws unheated gas into the chamber and mixes it with heated gas and calibrated to provide uniform heat distribution within the chamber at selected temperatures.

10. An incubator for heating objects such as reactants contained in open ended test tubes to predetermined temperatures, comprising: a body member having an inner chamber and openings in a top surface thereof for inserting test tubes into the chamber; spring-loaded platform means associated with each opening for sealing the associated opening when there are no test tubes placed thereon and for supporting said test tubes within the chamber so that the open end of the test tubes extend above the top surface of the incubator and out of the openings; hood means for closing the openings and enclosing the portions of the test tubes that extend above the top surface; an electric heating rod contained within the chamber and extending substantially across the chamber; a tangential blower positioned adjacent said heating rod and having a length substantially equal to that of the heating rod so that the fan circulates air over the entire length of the heating rod; a baffle for establishing a path to circulate the air throughout the chamber; a screen baffle extending across the circulation path, to provide uniform heat distribution within the chamber when loaded with odd-shaped or unsymmetrically loaded test tube rocks; a variable air inlet positioned relative to said fan so that air is aspirated into the chamber, said air inlet being calibrated at selected temperatures to provide uniform heat distribution within the chamber; and thermostat means in thermal contact with the air within the chamber for controlling the electrical heating rod in response to the air temperature, whereby the air within the chamber is maintained at a selected temperature.