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Digital Rotameter Flow Meter: Variable Area Tubes with 4-20mA Output Loops
Quick Answer: A digital rotameter flow meter combines a variable area tube with an electronic transmitter. It sends a 4-20 mA signal to a PLC or DCS. Silver Automation Instruments is a flow meter manufacturer and supplier for this type.
Silver Automation Instruments supplies variable area tube flow meters with digital transmitters to customers in Southeast Asia, Oceania, Latin America, Africa, and the Middle East. The product line is often called a digital rotameter. Some engineers call it a metal tube rotameter with transmitter. The operating principle stays simple. A float moves inside a tapered tube. The float position matches the flow rate. An electronic sensor reads that position. The transmitter converts the reading to a 4-20 mA output loop.
Where a 4-20 mA Rotameter Fits in Your Plant
Many process panels still use 4-20 mA analog loops. The signal travels over a twisted pair. It is easy to troubleshoot with a multimeter. A digital rotameter does not require frequency input cards or special communication modules. You can connect it directly to a PLC, DCS, or loop display.
We have seen this on customer sites many times. A small utility line carries cooling water. The flow rate is stable. The plant needs local display and a remote signal for the control room. A digital rotameter solves both needs in one instrument. You do not need a separate flow transmitter.
Model Range and Technical Parameters
For most process applications we specify the Silver Instruments RAMC series metal tube rotameter. The RAMC covers DN15 to DN100. Standard accuracy is 1.5 percent of full scale. Repeatability is better than 0.5 percent. The 4-20 mA output is loop powered from 24 VDC. You can request HART as an option for remote configuration.
Typical water ranges for reference:
DN15 covers 2.5 to 25 liters per hour up to 25 to 250 liters per hour. DN25 covers 16 to 160 liters per hour up to 40 to 400 liters per hour. DN50 covers 0.16 to 1.6 cubic meters per hour up to 0.25 to 2.5 cubic meters per hour. DN80 covers 0.4 to 4 cubic meters per hour up to 1 to 10 cubic meters per hour. Gas ranges depend on pressure and molecular weight.
Standard wetted materials are stainless steel 304 or 316. The float can be stainless steel or PTFE coated. The meter body can handle pressures to 40 bar and temperatures to 200 degrees Celsius. For aggressive chemicals we add a PTFE liner and a magnetic float with corrosion resistant materials.
Why This Meter Works Without Conductivity
Electromagnetic flow meters need a minimum liquid conductivity. Variable area tubes do not. Because the float is mechanical, the reading does not depend on electrical conductivity. This makes the digital rotameter useful for demineralized water, condensate recovery, fuel, and solvents. It also works for low conductivity liquids that would drift on a magnetic flow meter.
But you need to check the viscosity limit. High viscosity changes the float position. For liquids above 30 cP we recommend testing the range with our engineers. A paint manufacturer in Southeast Asia used a DN25 metal tube rotameter for solvent batching. The 4-20 mA signal went to a batch controller. The
Installation Tips for the 4-20 mA Loop
Mount the meter vertically with flow from bottom to top. Install a straight run of 5 pipe diameters upstream and 3 pipe diameters downstream when possible. Add a filter or strainer for dirty liquids. The float can stick if solids build up.
For the electrical side use a 24 VDC supply. A two wire loop needs a load resistor within the transmitter limit. Most RAMC units accept 250 to 500 ohm loop resistance. Use shielded twisted pair cable. Ground the shield at the panel side only. This avoids ground loops in the 4-20 mA signal.
Common Applications
Water and wastewater plants use digital rotameters for chemical dosing lines. The output goes to a SCADA system. Food and beverage plants use them for clean water and CIP chemical monitoring. Marine operators use them for cooling water and fuel transfer. Oil and gas service companies use them for nitrogen purge lines and test skids.
A desalination plant in the Middle East asked us for a seawater flow meter on a chemical feed line. We supplied a DN15 PTFE lined rotameter with a 4-20 mA output. The flow range was 5 to 50 liters per hour for antiscalant dosing. The loop drove a local chart recorder and a remote PLC input.
How to Order and What to Send
Send us your pressure in bar, temperature in degrees Celsius, pipe size in DN, and flow range. Tell us the media name and specific gravity if you know it. We will select the tube material, float material, and calibration range. Standard delivery is 5 to 10 working days for metal tube rotameters with 4-20 mA output.
Contact Silver Automation Instruments. Tel: +86-135-1213-1526. WhatsApp: 86-135-12131526. WeChat: +86 13512131526. You can also visit flow-meter.com.au for the full variable area flow meter range.
FAQ
1. Can I use a digital rotameter for gas flow measurement?
Yes. The meter can measure air, nitrogen, carbon dioxide, and other clean gases. You must provide gas pressure and temperature for a correct scale. The 4-20 mA output reflects the calibrated flow range.
2. What is the turndown ratio of the RAMC series?
Standard turndown is 10 to 1. So a meter with an upper range of 100 liters per hour can read down to 10 liters per hour with useful accuracy. Do not expect a 50 to 1 turndown from a variable area tube.
3. Does the 4-20 mA output need an external power supply?
Yes. A two wire loop uses 24 VDC from the PLC, DCS, or a separate supply. The current loop carries both power and signal.
4. Can the digital rotameter measure liquids with suspended solids?
Not directly. The float and tapered tube need a clean fluid. For dirty liquids install a Y strainer or filter upstream. Solids can block the tube or wear the float.
5. What signal options are available besides 4-20 mA?
You can add HART on top of the 4-20 mA signal. Some models also support a pulse output or two alarm switches. Tell us your control system type before ordering.
Request a quote today. Give Silver Automation Instruments your flow range, media, pressure, temperature, and pipe size. We will reply with a specific model and price within 24 hours.
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