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Chapter 4
Example Measurements
The purpose of this 4-20 mA current loop is for the sensor to transmit a
signal in the form of a current. In the diagram, the Level Sensor and
Remote Sensor Electronics are typically built into a single unit. An external
24 VDC supply powers the sensor. The current is regulated by the sensor
and represents the value of whatever parameter the sensor might measure,
in this case the fluid level in a tank. The DAQ device reads the voltage drop
across the 249Ω resistor Rp. Then Ohm’s Law is used to derive the current:
V ( Volts )
I ( mA ) = --------------------R p ( Kohms )
Because the current is 4-20 mA, and Rp is 249Ω, V ranges from 0.996 V to
4.98 V. This is within the range that DAQ devices can read. While the above
equation is useful for calculating the current, the current typically
represents a physical quantity you want to measure. In this example, the
level sensor measures 0 to 50 feet. This means 4 mA represents 0 feet and
20 mA represents 50 feet. Assuming this to be a linear relationship, it can
be described by the graph and equation shown in Figure 4-16 where L is the
tank level and I is the current.
Figure 4-16. Linear Relationship between Tank Level and Current
Using the Ohm’s Law equation and substituting 0.249 for the value of Rp,
you can derive L in terms of our measured voltage:
25 ⋅ V
25
L = --------------------- – -----8 ⋅ 0.249 2
LabVIEW Measurements Manual
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