The Importance of Turndown Ratio and Measuring the Useable Range of an Instrument

The useable range of an instrument can be measured by turndown ratio, which is defined as the ratio between the full-scale range and the minimum point of measure. It denotes how much of the instrument range is capable of producing accurate readings, which is crucial in the measurement or control of a broad flow range without the requirement to change devices.

Turndown Ratio Calculation

For a 12" ruler graduated to a quarter of an inch, the smallest measurable distance is 1/4". The turndown ratio for this ruler is 12"/0.25" = 48 to 1 (48:1) (Figure 1). If all the graduated lines less than 2" are smudged, then the ruler’s useable range is reduced but there is no change in the accuracy as the distance between marks is the same.

Now, the turndown ratio is 12"/2.0" = 6:1 (Figure 1). This smudged ruler could not make an accurate reading of an object measuring an inch and a half in length. In this case, a smaller ruler with the same turndown ratio, i.e., a 6" ruler with lines smudged less than 1" or the same size ruler but with a larger turndown ratio of at least 12:1 would be needed.

Figure 1. Turndown Ratio Calculation

Turndown Ratio of a Flow Meter

A flow meter or flow controller’s turndown ratio is greatly affected by its signal-to-noise ratio caused by both the type and quality of the sensor applied and the fluid dynamics of the flow measurement technology.

For instance, most orifice-plate differential pressure-based flow measurement instruments have a turndown ratio of 4:1, showing that the device cannot produce accurate readings below 25% of the full-scale flow.

According to Bernoulli’s Principle, there is a correlation between the differential pressure and the square of the flow velocity within turbulent (non- laminar) flows. Hence, decreasing the full-scale flow rate by two lowers the differential pressure by four.

Similarly, decreasing the full-scale flow rate by four lowers the differential pressure by 16. Smaller differential pressures convert to smaller sensor signal strength, and ultimately the signals will be lost in the signal noise.

Conversely, laminar differential pressure-based flow measurement instruments have a direct linear relationship between flow velocity and differential pressure. This is achievable by wrangling the turbulent flow into laminar channels, where there is a linear relation between flow velocity and differential pressure in accordance with Poiseuille’s Equation.

Hence, in an Alicat laminar mass flow instrument, lowering the full-scale flow rate by four only decreases the differential pressure by four. This in conjunction with Alicat sensor technology ensures accuracy in most of Alicat’s mass flow products to at least 0.5% of the full-scale flow rate with a turndown ratio of 200:1.

Alicat’s Turndown Test

The results of Alicat’s recent turndown ratio test are illustrated in the following one-minute video. The unedited, real-time video monitors the actual flow rate of a 5-slpm mass flow controller that is paired with a 50-sccm mass flow meter rather than with a 5-slpm meter to observe just how low the Alicat controller could perform.

At full scale, the range of this meter represents the bottom 1% of the range of the flow controller, i.e., a turndown ratio of 100:1. The test began at a flow rate of 50 sccm, representing 100:1 turndown for the controller, and then continued to 5 sccm (1000:1) and 1 sccm (5000:1).

Alicat’s Turndown Test

Conclusion

The mass flow controller controlled the flows to within 0.13 sccm for each commanded flow rate in the video, representing a remarkable feat for a 5000-sccm controller.

Conversely, if a flow controller with a 50:1 turndown ratio was used, then one device is required to get 100-5000 sccm, second one to get 2-100 sccm, and third one to cover the last 1-2 sccm.

For the cost conscious, higher turndown ratios represent investing less money to get wide flow ranges. Higher turndown ratios also facilitate greater flexibility when a test suddenly needs a broader range.

This information has been sourced, reviewed and adapted from materials provided by Alicat Scientific.

For more information on this source, please visit Alicat Scientific.