SOLVED: Download the datasheet of 1N4733A Zener diode. The manufacturer usually specifies the voltage across the Zener diode VZ at a specified test current, IZT. From the spec sheet of 1N4733A Zener
1n4734 Zener Diode Datasheet, Pinout, Features & Applications - The Engineering Projects
1N4733 datasheet - Discrete, Diodes, Zener
1N4733 Datasheet(PDF) - First Components International