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Low Flow Meter With 4-20mA Output Transmitter Settings: Practical Setup for Small Lines
Quick Answer
To set a low flow meter with a 4-20mA output, confirm the span. The 4 mA point must represent zero flow or your minimum flow cutoff. The 20 mA point must match the maximum flow you expect. Use a HART handheld or the onboard push buttons to write the LRV and URV. Check damping and low flow cut-off right after. Most Silver Instruments units ship with a factory scale, but field adjustment is normal.
Low Flow Meter Types That Use a 4-20mA Transmitter
Silver Automation Instruments supplies several low flow meter types with loop-powered 4-20mA outputs. Oval gear positive displacement meters handle flows from 0.5 L/h up to 100 L/h. Our SIL-OG series comes with a built-in 4-20mA HART transmitter for diesel, lube oil, and solvent dosing. Coriolis mass flow meters, like the SIL-COR Micro, measure mass flow as low as 1 kg/h and output a 4-20mA signal proportional to kg/h or g/min. For gas, thermal mass flow meters from the SIL-TMF line cover 0.02 Nm³/h to 50 Nm³/h with a linear 4-20mA output. All variants let you re-scale the analog output via HART, local keys, or the Bluetooth app.
Setting the 4-20mA Range in the Field
Connect your HART communicator across the loop. Most Silver Instruments low flow transmitters use a standard 250-ohm resistor in series for HART. Read the basic setup variables. Identify PV, LRV, and URV. LRV is the flow value at 4 mA. URV is the flow value at 20 mA. For example, a water dosing skid in a Philippine beverage plant needed a range of 0.2 L/h to 8 L/h for enzyme injection. We set LRV to 0 and URV to 8.0 L/h on a SIL-OG DN4 oval gear meter. The transmitter’s DIP switches allow quick span selection on older models. On newer digital boards, you enter the menu, go to “Analog Output,” and type the numbers. Write protect must be off. After the write, verify the loop current with a multimeter. Set the low flow cut-off to 0.05 L/h to prevent signal noise at zero.
Zero Trim and Damping Adjustments
Here is the thing. On real low flow lines, a slight zero drift happens after temperature swings. Perform a zero trim when the pipe is full and flow is completely stopped. Do not force a trim while fluid is moving. For thermal mass flow meters on biogas, let the sensor soak in still gas for five minutes, then apply the zero trim via HART command 43 or the local button. Most engineers skip this part and wonder why the loop shows 4.1 mA with no flow. Damping is another parameter we change often. Silver Instruments default damping is 3 seconds. If the pump creates pulsating flow, increase damping to 8 or 12 seconds. In a glue dosing project in Thailand, we used 15 seconds on a SIL-COR Coriolis meter because the gear pump generated 40 pulses per minute. The 4-20mA output smoothed out and the PLC analog input card locked the reading.
Wiring and Loop Power for Low Flow Transmitters
Two-wire 4-20mA is the most common connection. The same pair carries power and signal. Our SIL-OG oval gear meter and SIL-TMF thermal mass meter work on 12-36 V DC loop power. For SIL-COR micro Coriolis models, we recommend a 24 V DC supply with at least 50 mA capacity. Never share a loop supply with inductive loads. In a water treatment plant in Malaysia, a shared 24 V bus caused a 0.8 mA drift on the low flow meter loop. Fixi
Common Failures on 4-20mA Low Flow Loops
We have seen this on customer sites many times. A small air bubble trapped inside an oval gear meter chamber causes a 4-20mA output to spike. Open the vent plug and bleed the line. On thermal mass meters, condensation on the sensor forces a high mA signal. Install a moisture trap upstream. For Coriolis meters, two-phase flow even 2 percent gas by volume makes the drive gain jump and the output goes into alarm mode. The device sets the loop to 3.6 mA or 22.8 mA per NAMUR NE43. Silver Instruments transmitters default to 3.8 mA low alarm and 20.5 mA high alarm. If your DCS shows 3.8 mA, check for empty pipe, slug flow, or sensor fault.
Recommended Silver Instruments Models for Low Flow
SIL-OG oval gear flow meter with 4-20mA HART handles low viscosity liquids down to 0.5 L/h. Use it for diesel, ink, palm oil, and solvent. SIL-COR Micro Coriolis mass flow meter covers 1 kg/h to 50 kg/h with 0.1 percent accuracy and 4-20mA output plus pulse. It works on high viscosity adhesive up to 20,000 cP. SIL-TMF thermal mass flow meter measures gas micro flow 0.02 Nm³/h and outputs 4-20mA linear without temperature and pressure compensation. All units accept HART configuration. The transmitter settings menu is identical across series, so field technicians do not need to relearn the map.
FAQ
How do I re-span a Silver Instruments oval gear low flow meter from 0-10 L/h to 0-20 L/h? Connect HART communicator, read URV, change it from 10.0 to 20.0, apply, verify loop current at maximum flow. You can also use the front membrane keys: hold Enter, key in password 0416, scroll to URV setting, adjust, and save.
Can a 4-20mA low flow meter drive two PLC cards in series? Yes, if total loop resistance stays below the maximum load limit. For our SIL-OG transmitter, limit is (V supply 12) / 0.022 ohms. With 24 V DC, that is about 545 ohms. Two 250 ohm inputs work. Add a loop isolator if ground loops appear.
What is the smallest flow a thermal mass flow meter can detect with a stable 4-20mA output? The SIL-TMF-015 model reads down to 0.02 Nm³/h air equivalent. The output stays stable above 0.03 Nm³/h. Below that, we set the low flow cut-off to 0.02 and the output freezes at 4 mA.
Do I need a separate flow computer if my low flow meter already outputs 4-20mA? No. Direct 4-20mA goes to your PLC AI card. For custody transfer, you may need a flow computer to compensate density or temperature. But for dosing and batching, the scaled 4-20mA signal is enough.
How do I order a low flow meter with 4-20mA output from Silver Automation Instruments? Send us the fluid name, viscosity in cP, operating pressure in bar, temperature in °C, pipe size in DN, and required flow range. Mention if you need Ex d ATEX Zone 1 certification or 316L wetted parts. We reply with a quotation, data sheet, and typical transmitter settings report within 4 hours.
Tell us your minimum flow cut-off and 20 mA full scale value. Every low flow meter we ship gets a free pre-configuration service. Ask our engineer for the SIL-OG, SIL-COR, or SIL-TMF setting sheet before installation.
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