Datalogging in Science

Pressure in a liquid
How the pressure change with depth

Background
Divers experience increased pressure as they dive deep. Changing depth to quickly can give rise to a painful experience called the bends. The pressure is due to the weight of water above the diver acting on his/her body.
The Pressure sensor used in this experiment is a differential pressure sensor and is not therefore affected by atmospheric conditions, it measures pressure change from the reference point (normally the air).

Apparatus

Datalogger.

Differential Pressure sensor (±10 kPa)

Length of tubing (3 mm inside diameter).

Piece of metal rod (optional).

Large measuring cylinder or transparent tube marked in cm divisions.

Procedure

1. Set up the apparatus as shown in the diagram. Connect the tubing to the Pressure sensor. Tape the end of the tubing to a metal rod (or something similar) to make it easier to position the open end of the tube).


2. Launch the graph software and set it to record in Snapshot mode

3. Check that the pressure is zero when the tubing is not in the water, if it is not reading zero make a note of the difference from zero.


4. Place the tubing in the water column at a depth of 10 cm and record the pressure, slowly lower the tubing to 20 cm, 30 cm etc and record the pressure at each depth. Make sure the open end of the tube is on the depth mark.


5. Plot a graph of pressure against depth making the axis big enough to extend the line to give pressure depth readings beyond the data you collected. If the sensor was not reading zero in air you may want to correct all the pressure readings by the difference from zero noted in step 2.

6. Calculate the gradient of the line, this will be the conversion factor that you can use to change pressure to depth.


Notes

Tubing to fit directly over the nipples on the pressure sensor should be 3 mm inside diameter and have a little flexibility.
To protect the sensor from any water moving up the tube a small loose plug of cotton wool can be used in the end of the tubing, close to the pressure sensor, that will connect to the pressure sensor.

Results and analysis
A graph of pressure against depth should be plotted and the results analysed.

Questions

1. Does the pressure increase with depth in predictable way?

2. What will be the pressure at the bottom of a 2m deep swimming pool?

3. What sort of pressure will a diver experience if they are working at a depth of 30 metres?

4. Why would a diver be interested in the pressure around them?


Extension

Investigate if the pressure at all points at the same depth below the surface, are equal. For example, is the pressure one meter below the surface at the shallow end of a swimming pool is the same as the pressure one meter below the surface at the deep end?

Data Harvest users

1. Connect the sensor to the datalogger and the logger to the computer
2. Then click the Set Up icon to the right of this message.
3. When the software opens, click the Play button.