Difference Between Flash Point and Auto Ignition Temperature

calendarJan 29, 2023 · 3 min read  ·
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The key difference between flash point and auto ignition temperature is that flash point determines the lowest temperature at which vapour of a material starts ignition in the presence of an ignition source whereas auto ignition temperature is the lowest temperature at which a material can start ignition spontaneously.

Both flash point and auto ignition temperature are related to the ignition of materials at the lowest possible temperature.

CONTENTS

1. Overview and Key Difference
2. What is Flash Point 
3. What is Auto Ignition Temperature
4. Side by Side Comparison – Flash Point vs Auto Ignition Temperature in Tabular Form
5. Summary

What is Flash Point?

The flash point of a particular material is the lowest temperature at which vapour of the material undergoes ignition in the presence of an ignition source. Often, the terms fire point and flash point are confusing because they seem the same. But, fire point gives the lowest temperature at which the vapour of a substance can keep burning when we remove the ignition source, which is completely different from the definition of a flash point.

When considering the ignition of vapour at the flash point, there is enough vapour to induce ignition when we supply an ignition source. A volatile liquid has a unique concentration of flammable vapour, which is necessary for sustaining the combustion in air.

Difference Between Flash Point and Auto Ignition Temperature

If we are to measure the flash point of a substance, there are two methods: open cup measurement and closed cup measurement. Furthermore, the methods of determining the flash point are specified in many standards.

What is Auto-Ignition Temperature?

Auto ignition temperature is the lowest temperature that a material can start ignition spontaneously. Here, the material starts burning without any effect of an external ignition source, and this ignition occurs at normal atmospheric conditions except the temperature. The temperature provides the activation energy required to start the combustion.

Typically, the temperature required to start the spontaneous ignition depends on the pressure on the material. Increment of pressure decreases the auto ignition temperature. Moreover, when oxygen concentration is increased, the auto-ignition temperature decreases since the presence of sufficient amounts of oxygen makes it easy to ignite spontaneously. Some examples are as follows:

  • Barium (550°C)
  • Bismuth (735°C)
  • Butane (405°C)
  • Calcium (790°C)
  • Carbon disulfide (90°C)

    • What is the Difference Between Flash Point and Auto Ignition Temperature?

    Both flash point and auto ignition temperature are related to the ignition of materials at the lowest possible temperature. The key difference between flash point and auto ignition temperature is that flash point determines the lowest temperature at which vapour of a material starts ignition in the presence of an ignition source whereas auto ignition temperature is the lowest temperature at which a material can start ignition spontaneously.

    Moreover, another significant difference between flash point and auto ignition temperature is that flash point requires external ignition sources while auto ignition temperature does not require external ignition sources. Also, pressure has no effect on flash point whereas increment of pressure decreases the atuo ignition temperature.

    The following table summarizes the difference between flash point and auto ignition temperature.

    Difference Between Flash Point vs Auto Ignition Temperature in Tabular Form

    Summary – Flash Point vs Auto Ignition Temperature

    Both flash point and auto ignition temperature are related to the ignition of materials at the lowest possible temperature. The key difference between flash point and auto ignition temperature is that flash point determines the lowest temperature at which vapour of a material starts ignition in the presence of an ignition source whereas auto ignition temperature is the lowest temperature at which a material can start ignition spontaneously.

    Reference:

    1. “Flash Point.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 15 Oct. 2012, Available here.
    2. “Autoignition Temperature.” Wikipedia, Wikimedia Foundation, 31 Jan. 2020, Available here.

    Image Courtesy:

    1. “Fire Black Yellow Free Photo” (CC0) via Needpix.com

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