At head, measure of work is expressed in feet, and I'll explain how we divide that out there and how that actually works. The 4th term standing out here looking at us is density. Density of a fluid is it's mass per unit of volume, and it's usually expressed as pounds per gallon, okay? And everybody says, "Okay. The density of my water is 8 point something pounds, you know? You know, they kind of get close to it every time. But in the pumping world, when we're doing troubleshooting, we're actually looking for a cubic foot of water, okay? A unit of measure that's 12 inches by 12 inches by 12 inches, and I'll get into that a little bit later on as well. Now, as we're trying to determine problems with my pump, specific gravity comes into place. Specific gravity is the ratio of the density of that fluid, and everything in the pump world that you see, that gallons per minute, feet of head, what's all out there is based on the specific gravity of water.
Water is what we call number one, okay? Water, the specific gravity of water is one. Other fluids, water and salt water, they're heavier than water. So the specific gravity's much higher. Gasoline, as we all know, if you pour water and gas together, the water goes to the bottom, because gasoline's actually lighter than water, so it tends to float on top of the water. So the specific gravity of gasoline is less. Actually, the specific gravity of gasoline is 0.75. So, keeping that in mind, because I can have a pump that'll pump exactly physically a dimension of 100 feet, and if that pump is pumping brine, which is about 1.22, the pressure gauge is going to read about 57 PSI. If that pump is pumping water exactly 100 feet, the pressure at the discharge gauge is going to read 43 PSI. And if I'm pumping gasoline at 0.75, the pressure gauge is going to read somewhere around 35-36 PSI. So it really depends when you're trying to troubleshoot a problem in your facility, "What is this pump pumping?" So the specific gravity will change the relationship to the pressure. So I've kind of pointed that out.
And I can't tell you the number of times I always get that question. Now, there might be a chemist hanging out there to tell you, "Well, in a way you're kind of wrong on that one, but..." I get guys that ask me, "Well, what happens if I mix glycol and water together?" Well, glycol's a little bit lighter than water, but it tends to be capable of being absorbed into the water, so it mixes, it doesn't really ever stratify. Now, depending on the type of glycol, I guess, you put it. But the typical glycol that would be put the cooling system to keep it from freezing and be able to transfer the heat better, it typically takes on the gravity of water. So far, so good. I hope everybody's having fun here. I'm having fun talking. I'm pretty good at this talking stuff. I can talk to dead people. I'm just kidding.
So the pressure exerted on the walls of the container is measured in PSI, okay? Pressure equals force over area. That's a pretty simple statement from before. But I go in there and I ask, "What is the pressure of the air at sea level?" Well, that goes back to the olden days when barometric pressure was out there. We used the barometric pressure of the day. So, as we all know, that if the barometric pressure the weather forecaster comes out and says, "The barometric pressure is 29.89." I know when you walk out the door at that kind of barometric pressure; you're actually going to have probably a sunny day, okay? However, if the weather forecaster comes out and says, "The barometric pressure is 28.3," typically that's going to be the fact we got a low pressure area coming in there, and there's going to be clouds, and it might even be precipitating, you know, a little rainfall. So the pressure at sea level is 29.92 inches of mercury or as we like to refer to as 14.7 PSIA. So the air that we're around every day in our day-to-day activity, we got 14.7 pounds per square inch absolute sitting on top of us, so that would be PSIA. But when I pull that gauge out of the box to put it into that system, it reads 0 PSI. Well, that's gauge pressure. So we have to anticipate the fact that most of the time our pump is operating at 14.7 PSIA.
That's what makes the water go into the pump, okay? So, like I said, what causes that pressure to change: weather, altitude, vacuum. Now, for those of you who are listening in on this conversation out there and you might be in a place like Denver, Colorado, we're up there at the Mile-High City, and the atmospheric pressure there is about 13.3, 13.4, if I remember right off the top of my head. So at 5,000 feet, the atmospheric pressure is less. And I've talked to customers and people who've had pumps operating at higher altitudes than that, and it's even lower. So when we ask about that vacuum, that word "vacuum," kind of one of those words that's confusing to people, because vacuum is actually a measurable pressure, but it's typically anything less than atmospherists considered vacuum, going back to science 101, okay? So we have weather, altitude, and vacuum that causes that pressure to change out.