When we turn on the vacuum pump of the marinator, it starts to suck air out of the barrel and after a certain time the pressure stabilizes. “Has it reached vacuum?” ask my customers. No, it has not. Commercial marinator does not have to reach vacuum to marinate effectively.
At sea level, the atmospheric pressure is about 101 kPa (0.101 MPa, or 760 mmHg). The value varies, depending on your geographic location and altitude.
Then what does the gauge tell us about the pressure inside the barrel of the vacuum marinator?
There are two types of pressure readings. One is gauge pressure and the other is absolute pressure. Their relationship is represented by the following formula.
Pabs = Pg + Patm
If the scales of your vacuum gauge (as in Brandon’s vacuum marinator) show negative values, they are gauge pressure. A reading of -0.04 MPa means that the pressure inside the barrel is 0.04 MPa less than that outside of the barrel, which is the atmospheric pressure.
It is easy to convert the gauge pressure to absolute pressure. If the atmospheric pressure is 101 kPa (0.101 MPa), the absolute pressure inside the barrel is:
0.101 MPa – 0.04 MPa
= 0.061 MPa
Strictly speaking, a vacuum means zero MPa so 0.061 MPa is not really the vacuum to what a scientist refers. However, it does not affect the functions of marinator that much because under a negative pressure the meat is already pulled aprt and marinade can penetrate to fill the void.
A few year ago one of my customers suggested a better name to me. He is geeky and believed that “Negative Gauge Pressure Marinator” is a better name than vacuum marinator. I just smiled. Is the new name too tongue twisting?