Here’s why:

Imagine you’re cooking and want to detect if something in the pan is changing (like searing a steak). You have:

• Pan A: Contains your sample (e.g., a steak)

• Pan B: Empty (just the pan)

You heat both pans identically on separate but identical stoves.

If You Only Monitor Pan A:

You see its temperature rise, but you can’t tell:

1. Is the pan itself absorbing heat?

2. Is the steak cooking (endothermic reaction)?

3. Or is it just stove power fluctuation?

→ You learn nothing about the steak’s behavior alone!
 

But If You Compare Pan A vs. Pan B:

When Pan A heats up slower (because the steak absorbs heat to cook) while Pan B heats normally → You realize:
"Aha! Something in Pan A is absorbing heat—it’s undergoing a physical change (like melting or glass transition)!"
 

This Is the "Differential" Principle:

You’re not measuring absolute heat in Pan A—you’re tracking the heat flow difference between Pan A and Pan B.

The following figure is a typical DSC curve showing four types of transitions:

Temperature coefficient is →

Ⅰ For a secondary transition, it is a change in the horizontal baseline

Ⅱ For the heat absorption peak, it is caused by the melting or melting transition of the test sample

Ⅲ For the heat absorption peak, it is caused by the decomposition or cleavage reaction of the test sample

Ⅳ is the exothermal peak, which is the result of the sample crystalline phase transition

Interpretation of DSC Graph Axes

X-axis (Horizontal Axis)

• Represents: Temperature

• Unit: Degrees Celsius (°C)

• Explanation: Straightforward – shows the temperature ramp during heating/cooling.

Y-axis (Vertical Axis)

• Represents: Heat Flow (also called Heat Power)

• Unit: Milliwatts (mW)

• Key Explanation:

  • The Y-axis does not show temperature or total energy.

  • It measures the heat flow difference between the sample and the reference pan to maintain the same heating rate.

  • Example:

    • If the DSC reads Heat Flow = 8 mW, it means:

      • The sample is absorbing heat (endothermic).

      • The instrument is supplying 0.008 J/s extra to the sample (vs. reference) to keep both heating at the same rate.

Slope (Rate of Heat Flow Change)

• Definition: How quickly heat flow changes per unit temperature/time.

• Interpretation:

  • Steeper upward slope → Heat absorption is accelerating (e.g., sudden melting).

  • Flatter slope → Heat flow changes gradually.

  • Steeper downward slope → Heat release is increasing (e.g., exothermic reaction starts).