Is it possible to calculate pressure without a barometer?

[KrimsonTyger]

For an experiment, I need to pump air into a rocket with varying volumes of water, and thus air. However, I want to keep the pressure the same. Given the amount of air a bicycle pump can pump, the volume of air already in the rocket, and the desired pressure, is it possible to calculate how many pumps I need to achieve a certain pressure without a barometer.

One method I can think of is using the ideal (PV=nRT) and doing a string of calculations. This would involve using the universal molar volume of a gas (22.4 L/mol) to find how many moles of air are already there and how much more I need.

 

[jrmichler]

The calculation is only as accurate as the details and assumptions. How much dead volume at the end of the pump stroke, how much dead volume in the hose, how much leakage, temperature of the compressed air, etc, etc.

On the other hand, you could set the pump on a bathroom scale, and measure the pump force directly. If you pump to the same force, you will have the same pressure. This method will also work for partial pump strokes.

 

[Mister T]

Summary: Given a desired pressure, a known volume, and the volume per pump, is it possible to calculate how many pumps are necessary to reach a desired pressure.

I don't understand the question. If the desired pressure is given, then you just get one pump that will supply that given pressure.

 

[DaveE]

I don't understand the question. If the desired pressure is given, then you just get one pump that will supply that given pressure.

Bicycle pumps (OK, actually most pumps) don't work that way.

 

[sophiecentaur]

The calculation is only as accurate as the details and assumptions. How much dead volume at the end of the pump stroke, how much dead volume in the hose, how much leakage, temperature of the compressed air, etc, etc.

On the other hand, you could set the pump on a bathroom scale, and measure the pump force directly. If you pump to the same force, you will have the same pressure. This method will also work for partial pump strokes.

This pretty much says it all.
I would add that there is a partial solution if you calibrate your system back in the Lab or Prep room with a barometer. You could find out how many pump strokes (and fractions) are needed to get a range of volumes to a range of pressures and then, in the field, you would know how many pump strokes to give it.
Moreover, calibrating in this way could possibly reassure you about the accuracy and repeatability of using just the pump to predict the pressure achieved. If you do decide to calibrate this way then make sure you do lots of measurements 'back indoors'. Then the only problem will be measure the quantity of water for each launch.
If your project is intended to be 'educational' then giving a strong message about the Scientific Method would be important for the students.