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How to Read Air Flow Meter: Interpreting Analog 4-20mA Output Loops on System SCADAs
Quick Answer: A 4-20 mA air flow meter output is a current loop. 4 mA equals the low end of your flow range. 20 mA equals full scale. Read the current value on the SCADA analog input, then scale it linearly to air flow in kg/h, Nm3/h, or m3/h.
In practice, many engineers prefer 4-20 mA because the loop is simple to troubleshoot. A live zero at 4 mA means a broken wire shows 0 mA. That is a fast fault check before you climb a tank or open a panel.
What the 4-20 mA Signal Represents
Your air flow meter does not send a flow number to the SCADA. It sends current in mA. The analog input card reads current and turns it into counts. The SCADA software then converts counts to engineering units.
For example a thermal mass flow meter may be ordered with a range of 0 to 500 kg/h for compressed air at 7 bar. At zero flow the meter sends 4.0 mA. At 500 kg/h it sends 20.0 mA. At 12.0 mA the flow is 250 kg/h.
Scaling Formula for Air Flow
Use a linear formula. Flow = (measured mA - 4) divided by 16 then multiplied by the full scale span.
Suppose the full scale is 0 to 500 kg/h. If the SCADA reads 14.4 mA the calculation is (14.4 - 4) / 16 x 500 = 325 kg/h.
Most SCADA and PLC software lets you define the analog input range as 4-20 mA and the engineering unit range as 0-500 kg/h. After that you do not need to calculate by hand.
Reading the Current in the Field
Set a multimeter to mA. Put the meter in series on the loop. If you measure 4.0 mA at zero flow the loop is healthy. If you measure 3.6 mA or 21 mA the transmitter may be in alarm. Some flow meters follow NAMUR NE 43 fault levels. Low alarm sits below 3.8 mA and high alarm sits above 20.5 mA.
Do not read voltage in parallel. That gives false values and can disturb the loop. We have seen this on customer sites many times. A technician reads 24 VDC and thinks the transmitter is fine. Then the SCADA still shows zero because the wiring polarity is reversed.
Wiring and Grounding Problems in SCADA Loops
Use twisted pair shielded cable for the 4-20 mA signal. Keep it away from VFD motor cables. In a wastewater aeration basin in Mexico the air flow signal jumped whenever the blower VFD ramped up. The fix was to separate cable trays and ground the shield at one end only.
If the SCADA reads 0 mA the loop is open. Check terminals, fuses, and input card channels. If the SCADA reads above 20 mA the flow may be above full scale or the loop power supply may
Air Flow Meter Types and 4-20 mA Output
Thermal mass flow meters measure air mass flow directly. They work well on compressed air, blower air, combustion air, and aeration lines. A 4-20 mA signal typically represents kg/h or Nm3/h. Vortex flow meters measure volumetric flow such as m3/h. If you need mass or standard volume you may need a pressure and temperature input or a flow computer.
Silver Automation Instruments supplies thermal mass air flow meters from DN15 to DN200. They offer 4-20 mA output, HART protocol, and optional ATEX Zone 1 approval for hazardous areas. Accuracy is typically 1.5 percent of reading plus 0.5 percent of full scale. This is the same range we quote for real projects.
Recommended Silver Instruments Air Flow Meters
For compressed air flow in a paint manufacturer in Southeast Asia we supplied a thermal mass flow meter on a 2 inch line. The customer wanted to track air flow per batch. They used the 4-20 mA signal into a local PLC and logged the data from the SCADA. The meter paid for itself by exposing a leak during night shifts.
For combustion air to a boiler in the Middle East we quoted a vortex flow meter with a 4-20 mA HART output. The pipe size was DN80 and the flow range was 0 to 800 m3/h. The HART signal allowed the plant to pull diagnostics without opening the panel.
FAQ
Why does a 4-20 mA flow meter use 4 mA as zero?
A live zero lets the system detect a broken wire. If the current drops to 0 mA the loop is open or the transmitter is off.
Can I connect a 4-20 mA air flow meter directly to a SCADA?
Yes. Use an analog input card that accepts 4-20 mA. Configure the tag with the same range as the flow meter data sheet.
What is the formula to convert mA to air flow?
Flow = (mA value - 4) divided by 16 times the full scale span. Add the low range value if it is not zero.
How do I know if the 4-20 mA signal is faulty?
Look for values below 3.8 mA or above 20.5 mA. Also compare a handheld mA meter with the SCADA reading.
Does a thermal mass air flow meter output mass flow or volume?
Most thermal mass meters output mass flow such as kg/h. Many models can be set to display Nm3/h or standard cubic feet per minute.
Need a specific quote? Send your air flow range, pipe size DN, pressure in bar, temperature in °C, and location to Silver Automation Instruments. Contact Tel: +86-25-68650347, WhatsApp: +86-25-52155837, WeChat: +86 15365082610. You can also visit flow-meter.com.au for more air flow meter options.
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