Many measurement devices can measure and generate current. To measure or generate current with a DAQ device, you need a resistor.

You can measure current through an analog input connector. You can generate current through an analog output connector. Place the resistor in parallel with the connector and the current source. To measure voltage dropped across the resistor and convert it to current, use Ohm's Law.

I(A) = V(V)/ R(Ω)

where I is the current, V is the voltage, and R is the resistance.

4 to 20 mA Loops

4-20 mA loops are commonly used in measurement systems. 4-20 mA loops couple a dynamic range with a live zero of 4 mA for open circuit detection in a system that does not produce sparks. Other advantages include a variety of compatible hardware, a long operating range, and low cost. 4-20 mA loops have a variety of uses, including digital communications, control applications, and reading remote sensors.

The 4-20 mA current loop allows the sensor to transmit a signal in the form of a current. In the following figure, a single unit typically contains the Level Sensor and Remote Sensor Electronics. An external 24 VDC supply powers the sensor. The sensor regulates the current, which represents the value of what the sensor measures. In this case, the sensor measures the fluid level in a tank.

Current Loop Wiring

The DAQ device reads the voltage drop across the 249 Ω resistor Rp using Ohm's Law.

Since the current is 4-20 mA and Rp is 249 Ω, V ranges from 0.996 V to 4.98 V, which DAQ devices can read. Although the equation is useful for calculating the current, the current typically represents a physical quantity you want to measure. The figure below shows the following:
  • The tank level measures 0 to 50 feet.
  • 4 mA represents 0 feet.
  • 20 mA represents 50 feet.
  • L is the tank level.
  • I is the current.

Graph showing tank level L versus current I with a linear relationship.

Linear Relationship Between Tank Level and Current

Using Ohm's Law and substituting 0.249 for the value of Rp, you can derive L in terms of measured voltage:

L = 25 × V 8 × 0.249 - 25 2