Have questions? Need Help?
757.723.6531 | 800.950.2468

Teledyne Hastings Instruments Blog

Wayne Lewey

Recent Posts

FAQ Corner – What is turndown ratio?

Posted by Wayne Lewey on Mon, Jul 27, 2015 @ 04:03 PM

We are occasionally asked for the turndown ratios of our flow meters and flow controllers.  There are varying perceptions as to what this term actually means.

The turndown ratio of a Mass Flow Meter (MFM) or Mass Flow Controller (MFC) defines the usable range for which it can operate while maintaining its published accuracy.  It can be expressed using the following formula:

Turndown_ratio

Teledyne Hastings Flow Meter HFM-200-202A flow meter with a large turndown ratio will have a large operating range.  This can also be indicative of the flow meter’s cost.  For example, variable area flow meters (rotameters) typically have lower turndown ratios compared to thermal mass flow meters.

Most analog mass flow meters have an accuracy of ± 1% of Full Scale (FS) and have resolution better than 1%.  The usable range is from 1% to 100%.  They will have a turndown ratio of 100/1 or more commonly expressed as 100:1.  Digital flow meters will have an even greater turndown ratio due to their higher accuracy.

HFC-D-308Most analog mass flow controllers also have an accuracy of ± 1% FS.  However, they typically have an automatic valve shut circuit that closes the valve at flow rates below 2% of FS.  This is to ensure full valve closure in the event of a small zero offset.  The usable range is from 2% to 100%.  Since measurement is not possible below 2%, these will have a turndown ratio of 100/2 = 50/1 or 50:1.

 For more information on Turndown Ratio or our Flow Meters, please contact Wayne Lewey 

 

 

Request A Quote

Tags: Flow Meter

FAQ Corner – Specifying range, pressures on Mass Flow Instruments

Posted by Wayne Lewey on Wed, May 29, 2013 @ 10:01 AM

We often see the label “One Size Fits All”.  This may be fine for some consumer goods.  However, it can be quite problematic when applied to items like shirts, gloves, or even golf clubs.  “One Size Fits All” also does not work for Mass Flow Controllers (MFC).  Not all applications are alike.  Forcing a “One Size Fits All” MFC into an unsuitable application can squander accuracy and induce valve failure.

Why do you need to specify the gas flow range on a MFC?  The Full Scale (FS) Range and Gas on a MFC directly correlates to the transmitted output (analog or digital) of the device.  In an analog device, the maximum output value (such as 5 vdc or 20 mA) will be equivalent to the FS value.  The accuracy of most MFCs is a function of this FS Range.  For analog devices, it is commonly ±1% of FS.  For digital devices, it is commonly published to be ±(0.5% of Reading + 0.2% FS).  Selecting the FS Range close to an application’s maximum flow rate optimizes accuracy for that specific application.  This is a good practice.  In addition, the gas must be specified.  Most MFCs use thermal based sensors.  These sensors actually measure the molecular flow rate rather than the mass flow rate.  Various gas molecules transfer heat differently, and thus the gas must be known.

Why do you need to specify Upstream Pressure and Downstream Pressure on a MFC?  Again, not all applications are the same.  A “One Size Fits All” MFC is typically not set up for applications at high pressure, low pressure, high differential pressure, or low differential pressure.  The definition of high and low will also fluctuate from one user to another.  Teledyne Hastings Instruments selects and tests MFC valve components (orifice, spring, etc.) that optimize valve stability for the exact application pressure conditions.When using a Teledyne Hastings Instruments MFC, you will always find the FS Range / Gas and the Upstream / Downstream Pressures listed on the serial number label.

Sample Calibration Sticker for MFC

 

Wayne Lewey was first exposed to mass flow controllers while an undergraduate at North Carolina State University (Chemical Engineering).  Today, Wayne is the International Sales Manager at Teledyne Hastings Instruments and can be reached at wlewey@teledyne.com.

Tags: Teledyne Hastings Instruments, Flow Controller, Gas Flow Range, pressure, range, differential pressure, mass flow controller, mass flow meter