All Validyne pressure transducers are calibrated at the factory, just before shipment. But how often should a pressure transducer be calibrated after that? This will depend on the application and the importance of accuracy in the measurement. In some research applications the pressure transducer is calibrated just before an experiment begins, and then again when the experiment is finished. In this case the time frame could be less than one day or several weeks, depending on the length of the experiment. The purpose of checking the pressure transducer calibration afterwards is to verify that no significant drift has occurred.
For industrial applications, most transducers are typically put on a yearly calibration schedule. The calibration interval can be as short as three months for critical industries, such as medical manufacturing or pharmaceutical applications. When a pressure transducer is calibrated, a QA sticker with the date is affixed so that it can be verified as ‘in calibration’ while in service.
Calibration consists of recording the transducer output signal when five known pressures are applied: Zero, 50% of full scale, 100% of full scale, 50% again and zero. The zero and span controls on the transducer are then adjusted, if necessary, to bring the accuracy into the specified tolerance. This data is recorded and filed, and the transducer returned to service.
Validyne can provide calibration services and also make any needed repairs or temperature corrections. Included are incoming and final calibration data and a calibration date sticker – all provided on a fast turn-around basis.
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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
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.