Why turndown ratio matters

Why turndown ratio matters

Turndown ratio is a measure of the useable range of an instrument, expressed as the full-scale range divided by the minimum point of measure. It indicates how much of the instrument range can produce accurate readings, which is very important when you want to measure or control a very wide flow range without having to change instruments.

How to calculate turndown ratio

Turndown ratio comparison

Consider a 12″ ruler that is graduated to a quarter of an inch. Since the smallest distance that could be measured is 1/4″, the turndown ratio is 12″/0.25″ = 48 to 1 (48:1). Now, what happens if we noticed that on the same ruler all the graduated lines below 2″ are smudged? The accuracy has not changed, because the distance between marks is the same, but the useable range is now much less. In this case, the turndown ratio would now be 12″/2.0″ = 6:1.Using this smudged ruler, if we needed to measure an object that was probably an inch and a half in length, we could not produce an accurate reading. Instead, we would have to use either a smaller ruler with the same turndown (say, a 6″ ruler with lines smudged below 1″), or the same size ruler with a larger turndown ratio, in this case at least 12:1.

The turndown ratio of a flow meter or flow controller is most affected by the device’s signal-to-noise ratio, which results from both the fluid dynamics of the flow measurement technology and the type and quality of the sensor employed. For example, most orifice-plate differential pressure-based flow measurement devices have turndown ratios of 4:1, which indicates that accuracy is not well maintained below 25% of the full-scale flow. By Bernoulli’s Principle, differential pressure is related to the square of the flow velocity within turbulent (non-laminar) flows. Thus, reducing the full-scale flow rate by two reduces the differential pressure by four, and reducing the full-scale flow rate by four reduces the differential pressure by 16. Smaller differential pressures translate to smaller sensor signal strength, and eventually the signals become lost in the signal noise.

By contrast, laminar differential pressure-based flow measurement devices maintain a direct linear relationship between differential pressure and flow velocity. This is made possible by wrangling the turbulent flow into laminar channels, within which differential pressure and flow velocity are linearly related, according to Poiseuille’s Equation. Thus, in an Alicat laminar mass flow instrument, reducing the full-scale flow rate by four only reduces the differential pressure by four. This, combined with our sensor technology, allows us to guarantee accuracy in most of our mass flow products to at least 0.5% of the full-scale flow rate, a turndown ratio of 200:1.

Alicat’s turndown test

The following 1-minute video shows the results of Alicat’s recent turndown ratio test. In this test, we use unedited, real-time video to monitor the actual flow rate coming from a 5-slpm mass flow controller. However, instead of pairing this controller with a 5-slpm meter, we paired it with a 50-sccm mass flow meter to see just how low our controller could perform. At full scale, this meter’s range represents only the bottom 1% of the flow controller’s range, a 100:1 turndown. We start the test at a flow rate of 50 sccm, which represents 100:1 turndown for the controller, and then proceed to 5 sccm (1000:1) and 1 sccm (5000:1).

For each commanded flow rate in the video, the mass flow controller was able to control the flows to within 0.13 sccm, which is a tremendous feat for a 5000-sccm controller. If you were using a flow controller with a 50:1 turndown ratio, you would need one device to cover 100-5000 sccm, another one to cover 2-100 sccm, and yet another to get the last 1-2 sccm. For the budget conscious, higher turndown ratios mean spending less money to cover wide flow ranges. Higher turndown ratios also allow for greater flexibility when an experiment unexpectedly requires a broader range. When you discover mid-project that you need to go lower than you had anticipated, a high turndown ratio can be a project lifesaver.

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