FTS Bubbler - 4-20 mA Option

700-Bubbler-Man Rev 12 13 Feb 2020
Page 3/51
1.5 4-20mA OPTION
The Bubbler-has an optional 16 bit 4-20mA output with a resolution of 0.37 µA. A measurement
scale is determined by inputting the minimum and maximum water levels which correspond to 4mA
and 20 mA outputs respectively. The measured current signal is translated into a level
measurement based on the linear relationship between the minimum and maximum levels and the
corresponding current output. The process variable that appears on the 4-20 mA output is the
average stage value calculated by the Bubbler. Note that the process variable (level) and 4-20mA
output are only updated when an SDI-12 measurement command is received
Example:
The Minimum level is 10 feet (4mA); the maximum level is 30 feet (20 mA). A signal output of
12mA (half way between 4 mA and 20 mA) would correspond to a level measurement of 20
feet.
Because the relationship between the level variable and the 4-20mA signal is scaled based on the
range of the minimum and maximum levels, the smaller the range the more precise the level
measurement will be.
The 4-20mA output can be connected to a 4-20mA input on a SCADA system, PLC or chart plotter.
The output from the Bubbler is connected to the SCADA equipment as shown in the following
diagram. The output is isolated from the SDI-12 power supply and relies on current loop to provide
power to the output circuitry.
The 4-20mA output can be validated using a DMM or the SCADA equipment to measure the output
current. It is recommended to use a 6 ½ digit multi-meter for best results.
The expected output current can be determined using the following equation:
Analog Output (mA) = (Analog Slope * Stage) + Analog Offset
New values for the analog slope and analog offset can be determined using the following equations:
Analog Slope = 16/(Max Stage – Min Stage)
Analog Offset = 4 – (Analog Slope * Min Stage)
In which:
Max Stage is the maximum stage expected at the site, represented by a 20mA output.
Min Stage is the minimum stage expected at the site, represented by a 4mA output.
Minimum and Maximum stage