Simple Manometer Calibrates Pressure Transducers

Introduction: A simple U-tube manometer made from easily obtained materials can be used to calibrate pressure transducers over the range of a few inches of water to a few psi. This application note describes how to construct a manometer and determine the accuracy that can be expected.

Items Needed:

• 5 to 10 ft Clear Plastic Tubing
• Ruler or Tape Measure
• Water

Constructing the Manometer: A length of clear plastic tubing, formed into a U-tube and partially filled with water should be configured as shown below. Depending on the pressure you need to generate, the tubing should be a few inches to a few feet high. The limiting factor is likely to be the amount of vertical space available to form the water column. Connect one end of the manometer to the transducer pressure port (normally the + port). Secure the ruler or tape measure to the surface behind the U-tube. Raise or lower the free end of the tube to increase or decrease the fluid head applied to the sensor. Note that the sensor need not be filled with liquid; the air inside the transducer has no place to go and will compress until it is at the same pressure as the fluid column. Determine the applied water column head by measuring the distance between the fluid level in each leg of the U-tube (see sketch above). The pressure will be expressed as In H2O, CM H2O, etc. Accuracy: The accuracy of the pressure generated by a simple U-tube manometer depends on the accuracy of the ruler or tape measure used to determine the height of the fluid column. If the tape measure used over 100 CM, for example, is marked in mm, then the fluid level can be determined to within one part in a thousand (0.1% FS). This is better than twice the accuracy of the 0.25% pressure transducer. Enhancement: For smaller fluid pressures, try slanting the U-tube. You will have to figure the trigonometry, but the fluid distance along the slanting tube will be the hypotenuse of a triangle whose opposite leg is the actual fluid head. So the fluid must travel further along the tube to raise the pressure. This increases the accuracy of the fluid pressure determination.