IOP PUBLISHING Eur. J. Phys. 29 (2008) 1235–1241

EUROPEAN JOURNAL OF PHYSICS

doi:10.1088/0143-0807/29/6/012

High precision pressure measurement with a funnel
´ T Lopez-Arias, L M Gratton and S Oss
Physics Department, University of Trento, 38100 Povo (Trento), Italy E-mail: teresa@science.unitn.it, gratton@science.unitn.it and stefano.oss@unitn.it

Received 21 June 2008, in final form 13 August 2008 Published 22 September 2008 Online at stacks.iop.org/EJP/29/1235
Abstract

A simple experimental device for high precision differential pressure measurements is presented. Its working mechanism recalls that of a hydraulic press, where pressure is supplied by insufflating air under a funnel. As an application, we measure air pressure inside a soap bubble. The soap bubble is inflated and connected to a funnel which is placed, upside down, in a container filled with distilled water, placed on a scale. Our method provides a theoretical precision for the pressure measurement of the order of 0.01 Pa. Beyond this, the advantage of this method relies on the simplicity of the materials used and on the opportunity to discuss, at an undergraduate level, basic concepts regarding all those phenomena in which low or very low differential pressures are relevant. (Some figures in this article are in colour only in the electronic version)

1. Introduction

We present a high precision pressure measurement apparatus made with very simple and available materials. The pressure range accessible to this system is limited to relatively low differential pressures, with an absolute value of 300 Pa. Other apparatuses of comparable precision, such as differential low pressure transmitters, are not only expensive, but their working mechanisms are much more difficult to explain in the classroom. As a simple application, we measure the excess pressure, P , between the pressure inside and outside a soap bubble1 . Soap films and bubbles offer an inexpensive and attractive way to illustrate