An oxy-hydrogen flame is created by burning a mixture of hydrogen and oxygen gases. The temperature of this flame is around 2800 °C (5072 °F) which is still high, but lower than the temperature of an oxy-acetylene flame.

On the other hand, an oxy-acetylene flame is created by burning a mixture of acetylene and oxygen gases. The temperature of this flame is around 3100 °C (5600 °F). Acetylene is a hydrocarbon gas with a triple bond between its carbon atoms, which makes it more reactive and burns hotter than hydrogen.

In summary, the temperature of an oxy-acetylene flame is higher than that of an oxy-hydrogen flame because acetylene is a more reactive gas that burns at a higher temperature than hydrogen.

In the context of geometric dimensioning and tolerancing (GD&T), the lower deviation of a hole is defined as the difference between the minimum limit of the hole size and its actual size.

It means the algebraic difference between the minimum limit and the basic size is called Lower deviation.

It's important to note that these terms are used in the context of geometric dimensioning and tolerancing (GD&T) and should be used in conjunction with the other GD&T symbols and concepts to ensure that parts are manufactured and assembled to the desired specifications.

Direct current with straight polarity (DCEN) is used when TIG welding copper alloys, the electrode is positive and the workpiece is negative. This polarity is preferred when welding copper alloys because it creates a stable arc with a deep and concentrated heat at the point of contact between the electrode and the workpiece, which helps to achieve a consistent weld.