5 Gould Road, PO Box 2155 New London, NH 03257 USA Voice: (603) 526-9800 info@canarysystems.com www.canarysystems.com Wiring and Measuring 4-20mA Instruments Sensor Application Note #33 - Revision 1/24/2019 Overview Sensors with 4-20mA current outputs commonly include flowmeters, pressure transducers, displacement transducers and load cells. The primary advantages of a 4-20mA output are that the signal can be transmitted over long cable lengths with minimal degradation, the voltage conversion using a shunt resistor is easily read by most data acquisition systems and the output is usually standardized for the range which simplifies programming, e.g. 20mA corresponds to a rated full-scale output of the transducer, say 100 psi. This Sensor Application Note offers guidance to connect these instruments to several Campbell Scientific dataloggers, including the CR1000, CR6, CR300 and CR200. There are 4 considerations when connecting 4-20mA instruments to a data acquisition system: First, if the instrument requires power, whether it is loop-powered or powered through the same connections as the 4-20mA output, or requires a separate power supply. Related, if the sensor requires a separate power supply, whether it requires 24VDC (typically) or can be powered using the datalogger 12VDC power system. Second, the value of the shunt resistor. A value of 125ohms is recommended as the 2.5VDC maximum output is supported by all the dataloggers. Alternate values include 100ohms or 250ohms. Third, whether the shunt resistor is connected for single-ended or differential measurements. Single-ended measurements are more universally supported (for example the CR200 does not support differential measurements) however differential measurements provide higher measurement resolution. Differential wiring and measurements should be used where supported. Last, how the measurements are converted from a voltage measurement to engineering units. This requires knowing the value of the shunt resistor and the full-scale range of the instrument. Additionally, this generally requires configuration of a proper Zero Reading to provide offsets from the zero output of the instrument. Note - This Sensor Application Note does NOT consider connections using a multiplexer, however all the same principles apply when it comes to making the instrument connections. Multiplexers are configured for either 2-wire switching, typically used for loop-powered instruments, or 4-wire switching, typically used for instruments with separate power supplies. Wiring and Measuring 4-20mA Instruments 1
Instrument Wiring In this section several wiring examples are illustrated, note configurations for different dataloggers. Consult the technical documentation for the 4-20mA instrument to determine which wiring example applies. Note - The examples shown utilize the recommended 125Ω resistor. Loop powered 4-20mA instrument with 12VDC supply from the datalogger: Loop powered 4-20mA instrument with separate 24VDC supply: 2 Wiring and Measuring 4-20mA Instruments
Separate power 4-20mA instrument: The following table describes minimum and maximum voltages for different resistors: Resistor Ω (R) 4mA (I) (E = I * R) 20mA (I) (E = I * R) VDC 100 0.4 VDC 2.0 VDC 1.6 125 0.5 VDC 2.5 VDC 2.0 250 1.0 VDC 5.0 VDC 4.0 Prior to configuring the datalogger the voltage must be checked at the shunt resistor. Use a Digital Voltmeter (DVM) to read the shunt resistor voltage. Voltage measurements should fall in the range shown in the table above. Note - Do not proceed with system configuration if the shunt resistor voltage is not within the range per the shunt resistor being used. Wiring and Measuring 4-20mA Instruments 3
MultiLogger Configuration Within MultiLogger, the Gage Type selections for each datalogger vary, however all the gage types output volts as the standard output type. This section illustrates the configuration for each datalogger type. The shunt resistor as assumed to be 125Ω, the full-scale output of the instrument is assumed to be 250 GPM. Calculations of the Zero Reading and Gage Factor for alternate configurations are explained in the following section. CR1000 (MLDAQ): CR6 (MLDAQ): 4 Wiring and Measuring 4-20mA Instruments
CR300 (MCLOG): CR200 (MCLOG): Zero Reading and Gage Factor Configuration Conversion to engineering units is dependent on the value of the shunt resistor and the full-scale output of the instrument. The Zero Reading corresponds to 4mA output voltage (although an in-situ measurement should be used) and the Gage Factor is calculated by dividing the full-scale range of the instrument output by the voltage delta. For example, if the full-scale output is 500gpm and the shunt resistor is 250Ω the Gage Factor is 125.0 (500/4). MultiLogger is a Registered Trademarks of Canary Systems, Inc. Campbell Scientific is a Registered Trademark of Campbell Scientific, Inc. Wiring and Measuring 4-20mA Instruments 5