Don’t overlook a vital component that affects 4-20 mA measurement success: Measurement resolution.
Measurements from sensors that have a 4-20 mA output are very common. Customers always ask how to connect them to a data logger or data acquisition system, which we describe in detail here. However, they often overlook a vital measurement component that affects their 4-20 mA measurement success: Measurement resolution. This application note presents a table of popular product implementations from DATAQ Instruments and associated resolution values as a quick reference guide.
So, What’s 4-20 mA Measurement Resolution, Anyhow?
Good question! The term “measurement resolution” refers to the smallest change in an applied signal that an instrument can detect. Although this change can be described in terms of applied mA, it is more constructive to describe it in terms of measured units. For example, telling you a given instrument has a measurement resolution of 80 µA is much less descriptive than saying that your measurement resolution is 0.50 psi. If the smallest measured change is too coarse, you need an instrument with higher resolution.
4-20 mA Measurement Resolution for Seletced Instruments
In the table that follows, just divide the full scale output of your 4-20 mA source by the indicated ADC Counts for a given Instrument to determine resolution in measured units. For example, a DI-145 Starter Kits applied to a 4-20 mA pressure sensor with a 0-1000 psi range provides 4.9 psi of resolution (1000 ÷ 204). A load cell with a full scale range of 100 lbs. applied to a DI-155 yields 0.020 lbs. of resolution (100 ÷ 5320.)
The table makes the following assumptions:
- You use either an external shunt resistor with a value of 250 Ohms, or an 8B32 or 8B42 process current amplifier (if the instrument supports them.)
- You have selected the optimum instrument voltage range for the measurement. For example, the 5-V range for the DI-155.
- Your 4-20 mA output is zero engineering units at 4 mA and full scale EU at 20 mA.
- The instrument supports only bipolar inputs.
|Instrument||ADC Counts||Example (psi)*||Comments|
|DI-145||204||4.9||External 250Ω shunt|
|DI-149||204||4.9||External 250Ω shunt|
|DI-155||~5320||0.19||External 250Ω shunt|
|DI-245||6553||0.15||External 250Ω shunt|
|DI-710-xHx||6553||0.15||External 250Ω shunt|
|DI-710-xLx||3276||0.31||External 250Ω shunt|
|DI-718B||8192||0.12||DI-8B32 or -8B42 amplifier|
|DI-718Bx||8192||0.12||DI-8B32 or -8B42 amplifier|
|DI-788||8192||0.12||DI-8B32 or -8B42 amplifier|
|DI-720**||8192||0.12||DI-8B32 or -8B42 amplifier|
|DI-730**||8192||0.12||DI-8B32 or -8B42 amplifier|
* Assumes a full scale measurement range of 1000 psi.
** With the DI-78B Backpack option.