Hot air oven - Hot air oven principles PDF

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Hot air oven principles...

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HOT AIR OVEN: Hot air ovens are e lectrical devices which use d  ry heat to s terilize . They were [1] originally developed by Pasteur.  Generally, they use a t hermostat  to control the temperature. Their double walled insulation keeps the heat in and conserves e nergy, the inner layer being a poor conductor and outer layer being metallic. There is also an air filled space in between to aid i nsulation . An air circulating fan helps in uniform distribution of the heat. These are fitted with the adjustable wire mesh plated trays or a  luminium  trays and may have an on/off rocker switch, as well as indicators and controls for temperature and holding time. The capacities of these ovens vary. Power supply needs vary from country  ertz) used. Temperature to country, depending on the v oltage and f requency (h sensitive tapes or biological indicators using b  acterial spores can be used as controls, to test for the e fficacy of the device during use. Dry Heat Sterilization – Principle and Uses What is dry heat sterilization? It is the process of killing bacterial spores and microorganisms using a high temperature. This type of sterilization method is used on items that cannot get wet such as powders, oils, and the likes.

Picture 1: An example of a dry heat sterilizer.

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Commonly used instruments for dry heat sterilization are the following: Hot air oven Microwave Radiation Flaming Incineration/burning Glass bead sterilizer Bunsen burner

Picture 3: Surgical instruments are sterilized using the dry heat method.

The principle of hot air oven dry heat sterilization Sterilization is achieved by means of conduction. The heat in the oven is absorbed by the item inside it and passes towards the center of the item layer by layer. For the item to be fully sterilized, it needs to reach the required temperature.

What dry heat sterilization does is it inflicts damage by oxidizing molecules leading to the organism’s death. Resistant spores are killed by exposing them at a higher temperature for a long period of time. A hot air oven is a type of dry heat sterilization. Dry heat sterilization is used on equipment that cannot be wet and on material that will not melt, catch fire, or change form when exposed to high temperatures. Moist heat sterilization uses water to boil items or steam them to sterilize and doesn't take as long as dry heat sterilization. Examples of items that aren't sterilized in a hot air oven are surgical dressings, rubber items, or plastic material. Items that are sterilized in a hot air oven include: ● ● ● ●

Glassware (like petri dishes, flasks, pipettes, and test tubes) Powders (like starch, zinc oxide, and sulfadiazine) Materials that contain oils Metal equipment (like scalpels, scissors, and blades)

Hot air ovens use extremely high temperatures over several hours to destroy microorganisms and bacterial spores. The ovens use conduction to sterilize items by heating the outside surfaces of the item, which then absorbs the heat and moves it towards the center of the item. The commonly-used temperatures and time that hot air ovens need to sterilize materials is 170 degrees Celsius for 30 minutes, 160 degrees Celsius for 60 minutes, and 150 degrees Celsius for 150 minutes. Bacillus atrophaeus spores should be used to monitor the sterilization process for dry heat because they are more resistant to dry heat than the spores of Geobacillus stearothermophilus. The primary lethal process is considered to be o  xidation of cell constituents.

Types of Hot Air Ovens: There are two types of hot air ovens. One is a forced air hot air oven and the other is a static air hot air oven. The forced air hot air oven is more effective than the static air hot air oven. There are two types of dry-heat sterilizers: 1. the static-air type and 2. the forced-air type.

The s tatic-air type is referred to as the oven-type sterilizer as heating coils in the bottom of the unit cause the hot air to rise inside the chamber via gravity convection. This type of dry-heat sterilizer is much slower in heating, requires longer time to reach sterilizing temperature, and is less uniform in temperature control throughout the chamber than is the forced-air type. The f orced-air or mechanical convection sterilizer is equipped with a motor-driven blower that circulates heated air throughout the chamber at a high velocity, permitting a more rapid transfer of energy from the air to the instruments. Advantages of dry heat sterilization: 1. 2. 3. 4.

A dry heat cabinet is easy to install and has relatively low operating costs; It penetrates materials It is nontoxic and does not harm the environment; And it is n  oncorrosive for metal and sharp instruments.

Disadvantages for dry heat sterilization 1. Time consuming method because of s low rate of heat penetration and microbial killing. 2. High temperatures are not suitable for most materials e.g. plastic and rubber items cannot be dry-heat sterilized because temperatures used (160–170°C) are too high for these materials. 3. The time and temperature required will vary for different substances and overexposure may ruin some substances.] 4. They do not require water and there is not much pressure build up within

 utoclave , making them safer to work with. This also the oven, unlike an a makes them more suitable to be used in a l aboratory  environment. They are much smaller than autoclaves but can still be as effective. They can be more rapid than an autoclave and higher temperatures can be reached compared  ry heat  instead of m  oist heat, some organisms to other means. As they use d like p  rions, may not be killed by them every time, based on the principle of thermal inactivation by oxidation.

INCUBATOR:

 icrobiological cultures or c ell Incubator is a device used to grow and maintain m cultures. The incubator maintains optimal t emperature, h  umidity and other    conditions such as the CO (CO 2) and oxygen content of the atmosphere inside. Incubators are essential for a lot of experimental work in c ell  icrobiology and m  olecular biology and are used to culture biology , m both b  acterial  as well as e ukaryotic cells. Louis Pasteur  used the small opening underneath his staircase as an incubator. Incubators are also used in the p  oultry industry to act as a substitute for hens. This often results in higher hatch rates due to the ability to control both temperature and humidity. Various brands of incubators are commercially available to breeders. The simplest incubators are insulated boxes with an adjustable heater, typically going up to 60 to 65 °C (140 to 150 °F), though some can go slightly higher (generally to no more than 100 °C). The most commonly used temperature both  . coli as well as for mammalian cells is for bacteria such as the frequently used E

approximately 37 °C (99 °F), as these organisms grow well under such conditions. For other organisms used in biological experiments, such as the budding yeast S accharomyces cerevisiae, a growth temperature of 30 °C (86 °F) is optimal. More elaborate incubators can also include the ability to lower the temperature (via refrigeration), or the ability to control humidity or C  O2  levels. This is  umidity  is important in the cultivation of mammalian cells, where the relative h typically >80% to prevent evaporation and a slightly acidic p  H is achieved by maintaining a CO2  level of 5%. The next innovation in incubator technology came in the 1960s, when the CO2  incubator was introduced to the market. Demand came when doctors to identify and study pathogens realized that they could use CO 2 incubators  found in patients' bodily fluids. To do this, a sample was harvested and placed onto a sterile dish and into the incubator. The air in the incubator was kept at 37 degrees Celsius, the same temperature as the human body, and the incubator maintained the atmospheric carbon dioxide and nitrogen levels necessary to promote cell growth. At this time, incubators also began to be used in genetic engineering. Scientists could create biologically essential proteins, such as insulin, with the use of incubators. Genetic modification could now take place on a molecular level, helping to improve the nutritional content and resistance to pestilence and disease of fruits and vegetables. Today’s role of incubator: Incubators serve a variety of functions in a scientific lab. Incubators generally maintain a constant temperature, however additional features are often built in. Many incubators also control humidity. Shaking incubators incorporate movement to mix cultures. Gas incubators regulate the internal gas composition. Some incubators have a means of circulating the air inside of them to ensure even distribution of temperatures. Many incubators built for laboratory use have a redundant power source, to ensure that power outages do not disrupt experiments. Incubators are made in a variety of sizes, from tabletop models, to warm rooms, which serve as incubators for large numbers of samples.

Laboratory water bath : .

 ater bath is laboratory equipment made from a container filled with heated Aw water. It is used to incubate samples in water at a constant temperature over a  nalogue interface to long period of time. All water baths have a digital or an a allow users to set a desired temperature. Utilisations include warming of r eagents, melting of s ubstrates  or incubation of cell cultures. It is also used to enable certain chemical reactions to occur at high temperature. Water bath is a preferred heat source for heating flammable chemicals instead of an open flame to prevent i gnition . [1]   Different types of water baths are used depending on  [ 3] When application. For all water baths, it can be used up to 99.9 °C. [2] temperature is above 100 °C, alternative methods such as oil bath, s ilicone  bath  or sand bath may be used. [4] Precautions [ e dit] ● ●

Use with caution.  oisture sensitive or It is not recommended to use water bath with m [5] pyrophoric reactions. Do not heat a bath fluid above its flash point. [5]  [ 6]

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Water level should be regularly monitored, and filled with distilled water only.[7]  [ 8] This is required to prevent salts from d  epositing  on the heater.[8]  [6][ 7] Disinfectants can be added to prevent growth of organisms.  Raise the temperature to 90 °C or higher to once a week for half an hour for  econtamination . [6]  the purpose of d Markers tend to come off easily in water baths. Use water resistant ones. If application involves liquids that give off f umes, it is recommended to operate water bath in fume hood or in a well ventilated area. [9]  The cover is closed to prevent evaporation and to help reaching high  temperatures.[9]  Set up on a steady surface away from f lammable  materials.[6]

Types of water bath:

Circulating Water Baths[e dit]

 ) are ideal for applications Circulating the water baths (also called stirrers [10] when temperature uniformity and consistency are critical, such as e nzymatic and s erologic  experiments. Water is thoroughly circulated throughout the bath resulting in a more uniform temperature. Non-Circulating Water Baths[e dit ] This type of water bath relies primarily on c onvection  instead of water being uniformly heated. Therefore, it is less accurate in terms of temperature control. In addition, there are add-ons that provide stirring to non-circulating water  baths to create more uniform heat transfer. [4] Shaking Water Baths[e dit ] This type of water bath has extra control for shaking, which moves liquids around. This shaking feature can be turned on or off.  icrobiological  practices, constant shaking allows liquid-grown c ell In m cultures grown to constantly mix with the air. Some key benefits of shaking water bath are user-friendly operation via keypad , convenient bath drains, adjustable shaking frequencies, bright LED-display, optional lift-up bath cover, power switch integrated in keypad and warning and cut-off protection for low/high temperature.

Laminar flow cabinet: Laminar flow cabinet or t issue culture hood is a carefully enclosed bench designed to prevent contamination of semiconductor wafers, biological samples, or any particle sensitive materials. Air is drawn through a H  EPA  filter and blown in a very smooth, l aminar flow towards the user. Due to the direction of air flow, the sample is protected from the user but the user is not protected from the sample. The cabinet is usually made of s tainless steel with no gaps or joints where spores might collect.[1]  Such hoods exist in both horizontal and vertical configurations, and there are many different types of cabinets with a variety of a  irflow patterns and acceptable uses. Laminar flow cabinets may have a U  V-C g  ermicidal lamp  to sterilize the interior and contents before usage to prevent contamination of experiment. Germicidal lamps are usually kept on for 15 minutes to sterilize the interior and no contact is to be made with a laminar flow hood during this time. During this time, scientists normally prepare other materials to maximize efficiency. (It is important to switch this light off during use, to limit exposure to skin and eyes as stray ultraviolet light emissions can cause cancer and c ataracts . Under normal conditions, all laminar-flow devices will maintain Class 100 air cleanliness conditions for the first operational location downstream from the HEPA filter bank. The vertical laminar flow (downflow) room normally fulfills a Class 100 air cleanliness level throughout the entire room, and a horizontal-laminar-flow (crossflow) room of 50-60 feet in length will maintain a Class 10,000 air cleanliness level within the room.

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