Calorie (IT) per Minute per Square Centimeter [cal(IT)/(min·cm²)]

The unit calorie (IT) per minute per square centimeter, written as cal(IT)/(min·cm²), measures heat flux density or thermal power per unit area using the International Table calorie (1 cal(IT) = 4.1868 J). It represents the amount of heat energy transferred through 1 cm² of surface every minute.

This unit is commonly used in laboratory calorimetry, solar energy experiments, and material testing where small surface areas and the calorie unit are convenient. Typical applications include:

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  Measuring solar radiation intensity on small surfaces

  
  
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  Calorimetry experiments with samples

  
  
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  Thermal testing of materials under concentrated energy flux

  
  

Mathematically, the heat flux is expressed as:

where q is heat flux in cal(IT)/(min·cm²), Q is energy in calories, A is area in cm², and t is time in minutes.

Conversion to SI units:

While SI units like W/m² are standard, cal(IT)/(min·cm²) remains convenient in small-scale experiments and historical calorimetric studies, providing an intuitive measure of energy transfer per minute per small surface area.

Btu (th) per Second per Square Foot [Btu(th)/(s·ft²)]

The unit Btu (th) per second per square foot, written as Btu(th)/(s·ft²), measures heat flux density or thermal power per unit area using the thermochemical British Thermal Unit (1 Btu(th) = 1054.35 J). It represents the amount of heat energy transferred through 1 square foot of surface every second.

This unit is commonly used in high-intensity heat transfer applications, industrial processes, solar energy studies, and material testing, especially where energy flux over relatively large surfaces is significant. Applications include:

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  Heat flux from furnaces, radiators, or heaters

  
  
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  Solar radiation received by panels or flat surfaces

  
  
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  Thermal testing of building or industrial materials

  
  

Mathematically, heat flux is expressed as:

where q is heat flux in Btu(th)/(s·ft²), Q is energy in Btu(th), A is area in ft², and t is time in seconds.

Conversion to SI units:

Although W/m² is the SI standard, Btu(th)/(s·ft²) remains useful in U.S. engineering, thermal studies, and historical literature, providing an intuitive measure of high-intensity heat flux per square foot per second.