Is mV reading the same as ORP reading?

Generally yes. Technically a millivolt reading is a measure of electron voltage potential and a pH/mV or ORP meter with ORP electrode is just working as a Digital Voltage Meter, DVM. ORP/Redox reactions are an ionic balance ratio measurement measured as a small millivolt potential and the ORP/Redox measurement is indicating the chemical reactions as a result of the imbalance in the positive and negative charges of the electrons of the ions in solution as an excess of either positive or negative ions.

ORP is generally not affected by pH unless Hydrogen Ions are present and affects either the oxidizing agent or the reducing agent or both. ORP electrodes are typically made with a Nobel metal, such as Platinum although Gold is used if Platinum acts as a catalyst and interacts with the solution being measured.

Temperature is generally not compensated for in most ORP/Redox reactions. While it is true temperature does affect ORP reactions and the rate of chemical reactions in any solution are usually temperature dependent in general ORP measurements are not temperature compensated since each solution has its’ own unique point where it is independent of temperature. That point is called the thermal isopotential point of that solution. As many solutions have lots of complex chemical reactions occurring simultaneously the isopotential point is often unknown. The ORP electrodes output to the meter and the meters millivolt reading will reflect the change in temperature as it affects the Redox of the chemical reactions. As an example a solution of mixed chemicals frozen solid obviously has very few chemical reactions occurring and a low or nonexistent millivolt output but if heated up that will quickly change and a millivolt potential will occur and be measured.