# Understanding Digital Displays

Digital display specifications can be confusing. Here is a short application note that describes the terminology and capabilities of a digital display..

Digital displays are specified as being either 3-1/2 digits or 4-1/2 digits. In a 3-1/2 digit display the three rightmost digits may read any value between 1 and 9. The left-most digit is limited to 1 or a blank. It is often easier to think in terms of counts: a 3-1/2 digit display can read up to 1999 counts, so it is suitable for displaying readings that have a maximum value of 20, 200 or 2000. Note that the decimal point may be placed within the right most three digits: 19.99, 199.9 or 1999.

Similarly a 4-1/2 digit display may read 19.999, 199.99, 1999.9 or 19999.

To specify the correct number of digits in a display, keep the following in mind: The resolution of the display should be five to twenty times better than the accuracy.

For example, if a 20 psi transducer with 0.25% FS accuracy is used, a 3-1/2 digit display configured to read 19.99 psi will provide a resolution of 0.01 psi when the accuracy of the transducer is 0.05 psi, and this is a good balance between resolution of the display and accuracy of the sensor.

A 4-1/2 digit display specified for the same transducer would read 19.999 psig at full scale, and the last digit would be unusable since it represents 50 times more resolution than the transducer accuracy.

However, if the transducer accuracy is 0.1% FS, then the 4-1/2 digit display would be appropriate because the accuracy of the transducer is now 0.02 psi and the 4-1/2 digit display would have a resolution 20 times greater than the accuracy. A 3-1/2 digit display, by contrast, would only read to 0.01 psi – just twice the accuracy of the sensor.

The diagram below shows how the 3-1/2 and 4-1/2 digital displays are used.

Check out Validyne products with a Digital Display

PS309 Portable Digital Manometer

PS409 Digital Panel Mount Manometer

P532 Ultra-Low Pressure Transmitter

DR800 Draft Range Transmitter

CD23/223 Carrier Demodulator with LED Display

CD379 Portable Carrier Demodulator with Digital Display

# 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.