A syringe is connected to a dial pressure gauge. Compressing the syringe causes an increase in pressure. Works well under the document camera.
Start with the syringe disconnected from the pressure gauge. Set the syringe to some arbitrary volume and reconnect to the pressure gauge. Push or pull on the syringe to change the volume and the corresponding pressure will be shown on the gauge.
1) EME Corporation - Boyle's Law and Absolute Zero Study Guide.