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Hot air oven - Hot air oven principles

Hot air oven principles

Course

Analytical Biochemistry

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Bharathiar University

Academic year: 2018/2019

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HOT AIR OVEN:

Hot air ovens are electrical devices which use dry heat to sterilize. They were originally developed by Pasteur.[1]Generally, they use a thermostat to control the temperature. Their double walled insulation keeps the heat in and conserves energy, the inner layer being a poor conductor and outer layer being metallic. There is also an air filled space in between to aid insulation. An air circulating fan helps in uniform distribution of the heat. These are fitted with the adjustable wire mesh plated trays or aluminium 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 to country, depending on the voltage and frequency (hertz) used. Temperature sensitive tapes or biological indicators using bacterial spores can be used as controls, to test for the efficacy 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.

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.

The static-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 forced-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:

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 noncorrosive for metal and sharp instruments.

Disadvantages for dry heat sterilization

Time consuming method because of slow rate of heat penetration and microbial killing.
High temperatures are not suitable for most materials e. plastic and rubber items cannot be dry-heat sterilized because temperatures used (160–170°C) are too high for these materials.
The time and temperature required will vary for different substances and overexposure may ruin some substances.]
They do not require water and there is not much pressure build up within the oven, unlike an autoclave, making them safer to work with. This also makes them more suitable to be used in a laboratory 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 to other means. As they use dry heat instead of moist heat, some organisms like prions, may not be killed by them every time, based on the principle of thermal inactivation by oxidation.

INCUBATOR:

Incubator is a device used to grow and maintain microbiological cultures or cell cultures. The incubator maintains optimal temperature, humidity 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 cell biology, microbiology and molecular biology and are used to culture both bacterial as well as eukaryotic cells.

Louis Pasteur used the small opening underneath his staircase as an incubator. Incubators are also used in the poultry 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 for bacteria such as the frequently used E. coli as well as for mammalian cells is

Laboratory water bath :

A water bath is laboratory equipment made from a container filled with heated water. It is used to incubate samples in water at a constant temperature over a long period of time. All water baths have a digital or an analogue interface to allow users to set a desired temperature. Utilisations include warming of reagents, melting of substrates 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 ignition.[1] Different types of water baths are used depending on application. For all water baths, it can be used up to 99 °C.[2][3] When temperature is above 100 °C, alternative methods such as oil bath, silicone bath or sand bath may be used.[4]

Precautions[edit]

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

● Water level should be regularly monitored, and filled with distilled water only.[7][8] This is required to prevent salts from depositing on the heater.[8] ● Disinfectants can be added to prevent growth of organisms.[6][7] ● Raise the temperature to 90 °C or higher to once a week for half an hour for the purpose of decontamination.[6] ● Markers tend to come off easily in water baths. Use water resistant ones. ● If application involves liquids that give off fumes, 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 flammable materials.[6]

Types of water bath:

Circulating Water Baths[edit]

Circulating the water baths (also called stirrers [10]) are ideal for applications when temperature uniformity and consistency are critical, such as enzymatic and serologic experiments. Water is thoroughly circulated throughout the bath resulting in a more uniform temperature.

Non-Circulating Water Baths[edit]

This type of water bath relies primarily on convection 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[edit]

This type of water bath has extra control for shaking, which moves liquids around. This shaking feature can be turned on or off. In microbiological practices, constant shaking allows liquid-grown cell 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.

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