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Turndown Ratio For Low Flow Meter Span Adjustments
Quick Answer: Turndown ratio is the range from maximum to minimum flow where a meter meets its accuracy spec. For low flow meters, a high turndown ratio (like 100:1 or 200:1) lets you adjust the span down to a tiny fraction of full scale without losing repeatability. We help engineers set spans on Coriolis and thermal mass meters to catch flows below 0.1 kg/h.
Why Turndown Ratio Decides Your Low Flow Span
When a plant runs a distillation column in Thailand, the reflux flow swings from 12 m³/h during peak production down to 0.2 m³/h at idle. A meter with a 10:1 turndown clips the signal below 1.2 m³/h. The DCS sees zero flow. The operator assumes a valve failed. In practice, the meter simply cannot track the low end because its turndown is too tight. A turndown of 100:1 or greater fixes this. The span you set in the 4-20 mA HART output must sit inside the usable range of the sensor. If you stretch the span beyond the turndown ratio, the meter flatlines at 4 mA or drifts at the bottom. We see this pattern often on cooling water loops and chemical dosing lines where low flow span adjustments are safety critical.
How Turndown Works for Different Low Flow Technologies
Coriolis meters on viscous fluids hold a turndown of up to 200:1. A DN15 Silver Instruments Coriolis mass flow meter can measure a span from 0.05 kg/h to 10 kg/h for edible oil at 40 cP. The sensor still delivers ±0.1% of rate in the lower 1% of range because the tube amplitude stays stable. Electromagnetic flow meters on water or wastewater see a realistic turndown of 50:1 to 100:1. A DN25 magmeter on a desalination permeate line in Saudi Arabia operates with a span of 0.1 m/s to 10 m/s. Far below 0.1 m/s, electrode noise takes over. Thermal mass flow meters for low gas flows give a turndown of 100:1. A Silver Instruments thermal mass meter measures nitrogen blanket gas from 0.01 Nm³/h to 1.0 Nm³/h. The span adjustment sets analog output scaling so that 20 mA equals 1.0 Nm³/h and 4 mA equals 0.01 Nm³/h. Vortex meters perform worse at the bottom end. A vortex meter on steam often loses its vortex shedding below a Reynolds number of 10,000. The usable turndown shrinks to 15:1 or 20:1. For low flow steam metering, we suggest a Coriolis meter instead.
Adjusting the Span on a Low Flow Meter Without Losing Accuracy
A paint manufacturer in Vietnam runs a solvent dosing line at 0.3 L/min normally but needs to measure down to 0.02 L/min during flushing. The existing oval gear meter with 10:1 turndown cannot register the flushing flow. The engineer calls us and asks for a replacement. We supply a DN6 Coriolis meter with a 200:1 turndown. The span is configured from 0.01 L/min to 2.0 L/min directly on the transmitter keypad. The 4-20 mA loop maps the LRV at 0.01 L/min and the URV at 2.0 L/min. You do not need an external signal conditioner. On Silver Instruments flow meters with HART, you can enter the lower range value and upper range value in the span adjustment menu. The transmitter automatically checks if the requested span stays within the turndown limit. If you exceed it, the display shows a span error. This prevents commissioning mistakes. For explosion proof installations in ATEX Zone 1, we ship the span already set per the customer datasheet. The technician only has to verify the loop current at zero flow.
Real Numbers That Prevent Span R
Most engineers skip this part but the math is simple. A flow meter with a full scale of 100 kg/h and a turndown of 50:1 can measure reliably down to 2 kg/h. If your process minimum is 1 kg/h, the span will be rejected at the low end. You need a meter with at least 100:1 turndown or a smaller full scale. We often recommend lowering the full scale of the sensor. A DN10 Coriolis meter with a full scale of 20 kg/h and 100:1 turndown covers 0.2 kg/h to 20 kg/h. That fits a dosing line for surfactants in a Philippine detergent plant. The same logic applies to volumetric flow. A DN20 electromagnetic meter with a full scale of 5 m/s and turndown 80:1 goes down to 0.0625 m/s. If the minimum flow is 0.05 m/s, we shift to a DN15 meter to stay safely above the cut off velocity. Always match the minimum flow number to the turndown ratio times the full scale. Do not rely on the maximum flow alone. We have seen customer sites where the meter spent 90% of the time below 3% of span and the readings became useless. After resizing the meter based on turndown, the low flow span adjustment held accuracy within ±0.5%.
How Silver Instruments Supports Your Span Adjustment Process
Send us your fluid name, pressure in bar, temperature in °C, pipe size in DN, and min and max flow range. We return a datasheet with the recommended flow meter model, turndown ratio, and a sample span configuration table. You get the LRV and URV values already calculated for 4-20 mA HART output. If you need ATEX, SIL 2, or marine type approval, we handle the documentation. For urgent deliveries to the Middle East or Latin America, we ship within five business days from stock. Our application engineers also double check if your low flow span stays above the cut off point of the technology you chose. This avoids field rejection and costly rework.
FAQ: Turndown Ratio for Low Flow Meter Span Adjustments
Q: What turndown ratio do I need for a water dosing line that runs from 0.05 m³/h to 1.5 m³/h?
A: You need a turndown of at least 30:1. A DN10 electromagnetic flow meter with turndown 80:1 covers 0.01875 m³/h to 1.5 m³/h comfortably.
Q: Can I adjust the span of a Coriolis meter below the published minimum flow?
A: No. The meter continues to send a signal but the accuracy rating falls off. Stay within the turndown ratio to keep the specified error band.
Q: How does turndown affect the 4-20 mA scaling on a thermal mass flow meter for biogas?
A: The turndown sets the lower limit for the 4 mA point. If the meter has 100:1 turndown and the full scale is 100 Nm³/h, the 4 mA point cannot be lower than 1 Nm³/h. Set the span accordingly.
Q: Which flow meter technology gives the best turndown for low flow liquid CO₂ at high pressure?
A: Coriolis meters. A Silver Instruments Coriolis meter on liquid CO₂ at 50 bar delivers a turndown of 200:1 even at cryogenic temperatures.
Q: Why does my vortex meter lose signal when I adjust the span below 10% of full scale?
A: Vortex shedding frequency drops below the sensor threshold at low velocity. The real turndown is often 15:1 on steam or gas. Switch to a thermal mass or Coriolis meter for wider span adjustments.
Specify your low flow span now. Send your pressure (bar), temperature (°C), pipe size (DN), min and max flow range to Silver Instruments. Our engineers will match the turndown to your process.