What Is a Turndown Ratio?

A turndown ratio, also commonly known as rangeability, is the ratio between a transmitter's maximum span and the minimum span it can be adjusted to while still measuring accurately. It tells you how far an adjustable range pressure transmitter or level transmitter can be “turned down” from its full-scale range to fit a smaller process range.

Turndown ratio is one of the most important, and most often overlooked, specifications when selecting an intelligent (smart) pressure transmitter. Finding a transmitter with a factory-calibrated range that exactly matches your application is rare. A transmitter with a high turndown ratio can be re-ranged in the field to match the process, so a single model covers many measurement points.

Turndown is written as a ratio such as 4:1, 10:1 or 100:1. The higher the number, the wider the range of spans the instrument can be set to, and the more flexible it is.

How to Calculate Turndown Ratio

The turndown ratio formula is simple:

Turndown Ratio = Maximum Span ÷ Minimum Span

To find the smallest span a transmitter can be set to, divide its maximum span by its turndown ratio.

Example: A pressure transmitter has a maximum calibration range of 0 psi to 300 psi and a 10:1 turndown ratio. The minimum span is 300 psi ÷ 10 = 30 psi. The span can be adjusted anywhere between 0 psi to 30 psi and 0 psi to 300 psi.

Real-world example: The NOSHOK PTI30 Series 145 psig range has a maximum span of 145 psi and a minimum span of 14.5 psi. That is a 10:1 turndown, so one transmitter can be ranged for a 0 psig to 15 psig process, a 0 psig to 145 psig process, or anything in between.

Keep in mind that the requested span, not the upper range value, is what must fit. If operations asks for a 50 psi to 90 psi range, the span is 40 psi. A transmitter whose minimum span is 30 psi can handle it; one whose minimum span is 50 psi cannot.

Turndown Ratio and Accuracy: Why “Adjusted Span” Matters

A turndown ratio is only useful if the transmitter stays accurate after it is turned down. That depends on how the accuracy specification is written.

• Accuracy as a percentage of full scale: The error band is fixed by the sensor's maximum range. Narrow the range and that same error becomes a much larger share of the new span.
• Accuracy as a percentage of adjusted span: The error band scales with the range you set. Accuracy holds as the transmitter is re-ranged.

Consider a 0 psi to 145 psi sensor turned down to 0 psi to 14.5 psi. At ±0.25% of full scale, the error is ±0.36 psi, which is 2.5% of the new 14.5 psi span. At ±0.1% of adjusted span, the error is just ±0.0145 psi.

This is why NOSHOK intelligent transmitters specify accuracy based on adjusted span, not full scale. When comparing pressure transmitter turndown ratios, always check the accuracy stated at the turndown you plan to use.

Benefits of a High Turndown Ratio

• Reduced instrumentation inventory: One adjustable range transmitter can replace several fixed range models, minimizing required spare parts.
• Plant-wide standardization: A common transmitter platform across tanks, lines and skids simplifies training, commissioning and maintenance.
• Field re-ranging without removal: Intelligent transmitters can be re-ranged with on-board display controls or remotely over the 4 mA to 20 mA loop with HART® protocol.
• No reference pressure required: NOSHOK 20, 25 and 30 Series sanitary transmitters allow range adjustment without applying a test pressure.
• Flexibility as the process changes: When product densities, tank heights or operating pressures change, the transmitter can be re-ranged instead of replaced.

How to Select the Right Turndown Ratio

1. Define the normal operating range and calculate the span you need.
2. Choose a transmitter whose maximum span covers your upper limit and whose minimum span is at or below your required span.
3. Confirm the accuracy at that turndown is based on adjusted span and meets your process requirement.
4. Consider future range changes. A higher turndown, such as 10:1 or 100:1, leaves room to grow.
5. If the range will never change, a fixed range pressure transmitter may be the more economical choice.

Turndown ratio is especially valuable in tank and silo level measurement, differential pressure flow and filter monitoring, low pressure and vacuum applications, and food, beverage and dairy plants with many vessel sizes.

NOSHOK Transmitters with Adjustable Range and High Turndown

NOSHOK offers intelligent industrial, differential, sanitary and submersible level transmitters with turndown ratios from 4:1 to 100:1. Most specify accuracy based on adjusted span, and HART® protocol is standard or optional on each series.

When a process range is known and will not change, NOSHOK PT Series fixed range transmitters and transducers provide a cost-effective alternative.

For help selecting the right turndown ratio for your application, contact NOSHOK Customer Support at 440.243.0888 or info@noshok.com.

Related reading: Gauge, Absolute, Sealed & Differential Pressure Defined and Instrumentation Solutions for the Food & Beverage Industry.