USE WITH HEATEC PRODUCTS
Typical uses of 4–20 mA circuits for Heatec products are as follows:

• Vega radar units on asphalt tanks

• Triac valves on fuel preheaters

• Siemens pressure transmitters on fuel tanks

• Honeywell burner controls on heaters

The device that outputs the signal usually does so in response to one or more inputs it receives from other devices outside of the 4-20 mA circuit. These include devices such as sensors, transmitters and thermocouples.

The device that receives the signal may be a readout or display. Or it may be a valve that opens or closes in proportion to a 4–20 mA signal. Or it may be a Honeywell modulation motor on the burner of a heater.

A detailed explanation of how the radar works is given below as an example of how a typical 4–20 mA circuit actually works. Brief explanations of the other uses follow the radar discussion.

HOW RADAR CONTROLS WORK
Figures 2, 3 and 4 show a Heatec asphalt tank, a Vega radar sensor, and a Honeywell controller.

Figure 2. Heatec vertical asphalt tanks equipped with radar sensors for level indications.

Figure 3. The Vega radar sensor outputs a 4–20 mA signal in response to feedback from the radar beam it emits to determine asphalt level.

Figure 4. Honeywell controller displays asphalt levels in feet and tenths of a foot.

The radar sensor uses electromagnetic waves to determine the fluid level in the tank. It converts level information into a 4–20 mA signal and transmits it to the Honeywell controller. The controller converts the 4–20 mA signals into feet (and tenths of a foot) and shows this on its built-in display.

These numbers represent the height or level of asphalt above the bottom of the tank. Operators can use a printed table that converts levels to volume to learn how many gallons of asphalt are in the tank.

The 20 mA signal represents 100 percent of the so-called “full” level of the asphalt tank. The “full” level is an arbitrary level that reserves some empty space to allow for expansion of the asphalt.

A 4 mA signal represents zero percent or the empty level of the tank. Thus, when the sensor outputs a 4 mA signal the display shows 0.0 feet, which means that the tank is empty.

Since 4 mA represents empty and 20 mA represents “full” there is a total of 16 mA (between 4 mA and 20 mA) available to represent intermediate levels from “full” to empty. And each mA is subdivided into smaller values. Consequently, there are many discrete or separate values from 4 mA to 20 mA. These mA values are converted into height values in feet and tenths of a foot and shown on the display as levels of asphalt in the tank.

The height values are in direct proportion to the mA values. Thus, 5.6 mA is 10% of the “full” height. 7.2 mA is 20% of the “full” height. 8 mA is 25% of the “full” height. 12.0 mA is 50% of the “full” height. And so on. See Figure 1.

The operator needs to be aware of the level (in feet) that represents “full”. This is because the display will not show any feet higher than what is shown for a 20 mA signal. Remember, the 20 mA is the maximum signal and represents 100 percent.

Consequently, if the asphalt expands so as to cause the actual level to rise above the number of feet displayed at 20 mA, the display will not change.

Incidentally, the controller cuts off the unloading pump before the asphalt reaches overflow level. And it shuts off heat to the coils when the tank nears empty to prevent coking the coils.

HOW OTHER CONTROLS WORK
Heatec fuel preheaters use a 4–20 mA circuit to transfer signals from a Yokogawa temperature controller to a Triac valve that modulates the flow of hot oil that heats the fuel.

Heatec fuel tanks equipped with Siemens pressure transmitters use a 4–20 mA circuit to transfer signals from the transmitter to a Honeywell controller. The controller has a display that indicates the fuel levels and initiates signals to shutoff certain devices at predetermined levels.

Heatec heaters use a 4–20 mA circuit to transfer signals from a Honeywell modulation controller to the modulation motor that controls the burner. However, please note that the resistor used is not fixed at 250 ohms as in conventional circuits. Consequently the voltage needed to produce the 4–20 mA current is not the same as in circuits with a fixed 250 ohm resistor. Otherwise, the burner control circuit works the same as other 4–20 mA circuits.