Introduction of easy conversion of relative pressure to absolute pressure
The most common “vacuum level” in the international vacuum industry is “absolute pressure”, also known as “ultimate vacuum”. However, “relative pressure” (or “gauge pressure”, or “negative pressure”) is also widely used due to the simplicity of the measurement method.
The two pressures can be theoretically converted with this formula: P=Px-Patm (P = Relative Pressure, Px = Absolute Pressure, Patm means the standard atmospheric pressure with value of 101,325 Pa, or 1013.25 mbar, or 760 mm Hg, or 14.696 psi).
If the required max. vacuum is -0.9 bar/-13 psi, you have to check the ultimate pressure of the vacuum pump to find out what kind of pump type that could be. According to the given formula: Px=P+Patm=-13 psi+14.696 psi=1.696 psi=11693.5 Pa
Conclusion: The required pressure level is obviously rough vacuum and the vacuum pump is possibly a dry piston or diaphragm pump for laboratory application.