Convert To Display Actual Units

Written by: Roselius, Herb

Convert to display of actual units measured. Eg. 12 mA or 3 V input can be set to display 250 gallons (946 liters).

Transmitter outputs are typically set for 4 to 20 mA with 4mA representing the low end of the range and 20 mA representing the high end of the range. For example in an application measuring the volume in a tank, a level transducer transmits 4mA when the tank is empty and transmits 20 mA when the tank is at full capacity of 500 gallons.

This type of transmitter has several benefits in industrial applications such as this one. The ?live zero? of 4 mA indicates the system and ?loop? is functioning. If the signal falls below 4 mA it indicates an open loop or other failure. A current signal is much more immune to electrical noise than a voltage signal, since only a parasitic or leakage current can cause an error. A current signal can be transmitted over longer distances than a voltage as long as the loop supply voltage can handle the voltage drop due to lead resistance.

The DPL-3 Series panel meters can be easily adjusted to read 000 with a 4 mA input and 500 with a 20mA input. Other intermediate inputs such as 8 mA would display a reading of 125 gallons, and 14 mA would display 313 gallons.

If a hose-proof & splash-proof display with larger digits is desired, the DPL-807 can be substituted for the DPL-3 Series . All these LCD meters are powered directly from the 4 to 20 mA signal eliminating the need for additional power supplies and simplifying the connection without requiring supply isolation. If backlighting is desired it can be powered directly from the loop supply and is isolated from the meter.

If the volume is to be displayed in other units, such as liters, the meters can be simply readjusted to display the level in liters. In this example 500 gallons is equivalent to 1893 liters, so the meter would be readjusted to display 1893 with a 20 mA input.

In the event that the converted value was greater than 2000, e.g. 1800 gallons is 6814 liters, a 4 ½ digit 20,000 count meter, the DPL-4, could be substituted for the DPL-3, and adjusted for the higher readings.