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100:1 Turndown Flow Meter Span Adjustments and Accuracy
Quick Answer: A flow meter with a 100:1 turndown can measure from 1% to 100% of its maximum flow rate. Span adjustment lets you re-range the 4-20 mA output so your minimum flow corresponds to 4 mA and maximum to 20 mA. To maintain accuracy, avoid setting the lower range value below 2-3% of the sensor’s full scale limit.
What Is a 100:1 Turndown Ratio?
A 100:1 turndown means the meter can accurately measure flow down to 1% of its maximum calibrated range. On a DN50 electromagnetic flow meter with a full scale of 100 m³/h, the unit reads down to 1 m³/h and stays within its stated accuracy class. This wide range is standard on electromagnetic, Coriolis, and many thermal mass flow meters. Silver Automation Instruments supplies electromagnetic flow meters with a true 100:1 turndown as standard, often removing the need for dual-range setups.
Span Adjustments Without Losing Precision
Most process engineers ask how to set the 4-20 mA span on a high turndown meter. You can freely adjust the upper range value (URV) and lower range value (LRV) via the local display or a HART communicator. In a wastewater treatment plant in Indonesia, our technicians set the LRV to 2 m³/h and URV to 200 m³/h on a DN80 meter. The analog output mapped this full range, and the meter delivered ±0.5% of rate accuracy because the sensor signal remained strong above 2% of full scale.
A common mistake is setting the LRV at exactly 0.0 m³/h. At very low flows, vortex meters and some differential pressure devices become nonlinear. With electromagnetic and Coriolis meters from Silver Instruments, you can set the LRV down to 0.1 m/s velocity, but the accuracy specification often applies only above a minimum Reynolds number or a defined low-flow cut-off. Check the datasheet for your specific model.
Accuracy Over the Full 100:1 Range
Here’s the thing: high turndown does not mean uniform accuracy at every point. For a Silver electromagnetic flow meter, the accuracy is typically ±0.5% of reading from 10% to 100% of flow. From 1% to 10% of flow, the error may increase to ±1% of reading or a fixed offset in mm/s. If your batch process mostly runs between 50 and 150 L/min, avoid spanning a meter for 0-500 L/min. Instead, use a smaller meter or restrict the span to 0-300 L/min so the normal flow sits above 15% of the adjusted range. We see this often in chemical dosing applications in the Philippines where a meter sized for pipe diameter had to be rescaled to match actual 24-ho
How to Set Up Span on a Silver Instruments Electromagnetic Flowmeter
Using the local keypad or a HART handheld, navigate to “Output Setup.” Enter the LRV in engineering units (m³/h, L/min, etc.) and then the URV. The output circuit will auto-scale. Do not forget to set the damping time to 2-5 seconds if the flow pulses at low rates. On our MFE series flow meters, the 100:1 turndown is guaranteed with a 0.2 m/s low-flow velocity limit. Below that, you can still measure, but the uncertainty goes up.
FAQ on 100:1 Turndown Span Adjustment and Accuracy
Can I adjust the span without recalibrating the flow meter? Yes, span adjustment changes the 4-20 mA scaling only. The factory calibration of the sensor and internal digital signal processing stays intact. For custody transfer, however, some local regulations may require re-verification.
What is the lowest measurable flow with a 100:1 turndown meter? Theoretically 1% of full scale, but practical accuracy limits typically start at 2-3% for electromagnetic meters. Silver Instruments guarantees 0.5% accuracy down to 10% of full scale and 1% down to 2% of full scale for the MFE series.
Does the turndown ratio affect the analog output resolution? Slightly. If you set a 100:1 span, the current loop sensitivity becomes 0.16 mA per 1% flow. This is still acceptable for most PLC and DCS inputs. Use a HART digital signal if you need higher resolution at the low end.
Which industrial flow meter technologies support a true 100:1 turndown? Full-bore electromagnetic meters, Coriolis mass flow meters, and thermal mass flow meters for gas typically offer 100:1 or higher. Vortex meters usually max out at 20:1 for liquids. Ultrasonic transit-time meters can reach 100:1 in clean liquids with enough straight pipe.
Can I get a 100:1 turndown in a DN15 flow meter for chemical dosing? Yes. Silver Automation Instruments provides mini electromagnetic flow meters in DN15 with a 100:1 turndown and a span adjustable from 0-20 L/h up to 0-2000 L/h. Just specify the dosing rate, fluid viscosity in cP, and connection type when sending us a request.
Need a quote? Send us your pipe size (DN), fluid, temperature, pressure, and the minimum and maximum flow rates you need to cover. Our engineers will propose a 100:1 turndown flow meter with the correct span and liner material. Email [email protected] or see our electromagnetic flow meter range at https://flow-meter.com.au.