# Pressure vs. HP

1. Sep 24, 2004

### Mike P.

Please forgive what I'm sure is a dumb question, but I'm stumped.

Is there a way to determine the amount of horsepower (from an electric motor for instance) that is req'd to pressurize a tank of a known volume to a specific pressure?

In other words if an electric motor is used to compress air in a tank to the point of motor failure/stall, is there a way to calculate the output horsepower of the motor by using the air pressure present in the tank?

MP

2. Sep 24, 2004

### Chi Meson

Unfortunately, the power rating of a compressor cannot tell you the maximum pressure it can attain. The power can be used to compare how fast a pump will attain a certain pressure, but the maximum pressure itself depends on many other factors.

3. Sep 27, 2004

### Mike P.

Perhaps i need to clarify.

I'm trying to build a sort of "poor mans dynomometer" to test electric motor output, my idea was to couple the motor to a pump and use the pump to force a fluid (water) into a pipe that would compress a piston into another cylinder that contains air ovbviously the air pressure would increase as the piston is compressed. At some point the motor will stall and a reading of the air pressure in cylinder #2 would be taken and HP computed.

I might be making this much more difficult than i need to so if anyone has other ideas, I'm eager to hear them.

MP

4. Sep 27, 2004

### Chi Meson

The primary factor affecting max pressure in a compressor is the seals around the compressing piston and in all other parts of the machine.

Assuming that you have perfect pistons and perfect connections that did not leak, the maximum pressure that a compressor can develop will be ddetermined by the maximum force of the power stroke of the "piston" divided by the cross-sectional area of the cylinder the piston goes through (for rotational and diaphragm compressors, there's no "piston" or "cylinder" but they have equivalent parts).

Again, the HP is only a measure of how quickly an engine or motor can do its job. Compressors will have a rating for HP as well as a rating for max pressure. There is no direct relationship between max pressure and HP.

5. Sep 27, 2004

### Staff: Mentor

You may be able to measure RPM separately and with pressure, get HP...

6. Sep 27, 2004

### krab

HP is rate of energy delivery. That means you need to measure not the energy delivered, but the energy delivered divided by the amount of time needed to deliver it. I think you can see that even a low power motor can get your pressure up to any desired level if you use the correct gearing. If you measure the pressure at stall, the only thing you can get is the amount of torque the motor has at zero rpm. Zero rpm also means zero hp. To find hp, you need a dynamic situation. For example, you could measure the amout of volume per unit time (e.g. CFM) that the motor can suck at a given pressure. Power is the volume rate times the pressure.