The key difference between open cup and closed cup flash point is that open cup method gives usually higher values for the flash point than closed cup method.
Flash point is the lowest temperature at which the flammable vapour of a volatile liquid ignites when we provide it with an ignition source. There are two major methods for the measurement of the flash point: open cup and closed cup method.
CONTENTS
1. Overview and Key Difference
2. What is Open Cup Flash Point
3. What is Closed Cup Flash Point
4. Side by Side Comparison – Open Cup vs Closed Cup Flash Point in Tabular Form
5. Summary
What is Open Cup Flash Point?
Open cup flash point is a value we can obtain using a vessel that is exposed to the outside air. Here, the vapour above the liquid is in equilibrium with the liquid. In this method, the volatile liquid is in an open cup. First, we need to heat it. After that, at intervals, we can bring a flame (ignition source) over the surface of the liquid. Then, we can determine the lowest temperature at which the ignition of vapour initiate. Thus, this method gives the flash point, which coincides with the fire point. Usually, this method gives higher values for the flash point, compared to the closed cup method.
What is Closed Cup Flash Point?
Closed cup flash point is a value that we can obtain from a closed vessel. Here, the vapour above the liquid is in not in equilibrium with the liquid. The closed cup method comes in two forms as non-equilibrial method and equilibrial method.
Figure 01: A Closed Cup Tester
In the non-equilibrial method, the vapour above the liquid is not in equilibrium with the liquid. But, in the equilibrial method, the vapour is in temperature equilibrium with the liquid. However, in both methods, the cups are sealed (closed with a lid) and we can provide the ignition source through the lid. Therefore, this method normally gives lower values for the flash point when compared with the open cup method.
What is the Difference Between Open Cup and Closed Cup Flash Point?
Open cup flash point is the flash point we obtain from open cup method where the vapour above the liquid is in equilibriums with the liquid. In contrast, closed cup flash point is the flash point we obtain from the closed cup method where the vapour above the liquid is not in equilibrium with the liquid. Because of this, the key difference between open cup and closed cup flash point is that open cup method gives usually higher values for the flash point than closed cup method.
The below infographic summarizes the difference between open cup and closed cup flash point.
Summary – Open Cup vs Closed Cup Flash Point
Flash point is the lowest temperature at which the vapour of a volatile liquid will ignite when we provide an ignition source. Open cup and closed cup flash point are two methods to measure the flashpoint. In summary, the key difference between open cup and closed cup flash point is that open cup method gives usually higher values for the flash point than closed cup method.
Reference:
1. “Flash Point.” Wikipedia, Wikimedia Foundation, 26 Apr. 2019, Available here.
Image Courtesy:
1. “APM-932hd-1” By Sirflashpoint – Own work (CC BY-SA 3.0) via Commons Wikimedia