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Desired Characteristics of a Thermal Mass Flow Sensor - Part 1 of 2

Posted by Will Harrison on Fri, Mar 20, 2015 @ 10:59 AM

ThermalMassFlowSensorCutawayThis is part 1 of a 2 part blog on Thermal Mass Flow Sensor.  We will describe the desired characteristics of a thermal mass sensor in Part 1 and Part 2 will discuss the operation of the 300 Series flow sensor (Patent #6,125,695) from Teledyne Hastings.

A thermal mass flow meter consists of the following:

  • Electronic Circuit Card

  • Flow Sensor

  • Bypass Shunt

  • Base

A cutaway is shown in the image on the right.

In a typical thermal mass meter, gas enters the flow meter via the upstream port which is attached to the process with a fitting (VCR, Swagelok…). Most of the gas will move through the bypass shunt; however, a certain fraction will flow through the thermal mass flow sensor. Note that the shunt is selected such that amount of gas moving through the flow sensor is approximately the same at full scale flow. The gas then exits the flow meter via the downstream port.

Ideally, the thermal mass flow sensor would exhibit the following characteristics: first, it would be linear. What we mean by linear is that the sensor’s electronic output should be directly proportional to the flow rate moving through the sensor throughout its range. Linearity of the flow sensor leads to the second desired characteristic: accuracy. An accurate flow sensor can give the users the benefit of better gas flow measurement, control, and understanding of their system parameters.

FlowSensorOutput

Before we move on with our desired characteristic list, we need to discuss a little about how linearity can factor into calibration. Typically, a thermal mass flow meter is calibrated in nitrogen (or in the case of very large flows, it may be calibrated in air). The output of the flow meter can then be scaled for use in other process gases. (In other words, the flow meter technician can calibrate a flow meter for use in a corrosive process gas like silane (SiH4) – without having to use silane). A linear flow sensor will retain its linear behavior as the gas is switched from the calibration gas (N2) to the process gas.

Our next desired characteristic is fast response. Ideally, the flow sensor would respond instantaneously to a change in the flow rate. Aside from the obvious benefit of instant real-time vision of the flow in a process, fast response becomes critical when the flow meter is coupled with a proportional control valve to create a thermal mass flow controller. Finally, we would like the thermal mass flow sensor to have a low pressure drop. A low differential pressure drop across the flow meter is ideal for leak detection and gas sampling applications.

Teledyne Hastings' Thermal Mass Flow Sensors are used worldwide.  For more information on on Best Practices for Mass Flow Controllers and Mass Flow Meters download our whitepaper.

Download Whitepaper

Be sure to visit our website for additional information on Teledyne Hastings Mass Flow Controllers and Mass Flow Meters

In our next blog, we will describe the sensor that is core to the Teledyne 300 Series of mass flow meters. We will also look at how the 300 Series thermal mass flow sensor addresses each of the desired characteristics described above.

 

 

Tags: Thermal Flow

FAQ Corner – What is the Importance of STP Conditions on Mass Flow

Posted by Brandon Hafer on Thu, Mar 07, 2013 @ 03:19 PM

As I go through the day looking at various mass flow applications, I often notice that it is very easy for users to overlook one of the crucial items required for calculating mass flow. Looking at an application with its established requirements, we often jump right to determining “what flow rate is required?” However, it is important to remember that mass flow applications using volumetric units must reference a standard temperature and pressure. But why is this the case?

When examining liquid flow instruments, we know that liquids are incompressible and thus the amount of a substance present is determined by the volume being used. This leads to a simple calculation using density with the already determined volume to find the mass present in the volume or the volumetric flow.

GAs molecules @ STP Gases, however, ARE compressible and so the volume is only one factor in determining the amount of material being measured. If we look at the ideal gas law that you may remember from a chemistry class school (PV = nRT), we understand that temperature (T) and pressure (P) must also be considered in the equation. Otherwise it is impossible for us to know “how much” of the substance (n) there is in the space (V) or flowing through the system.

But given all of this information do we actually end up with the mass flow? The actual quantification of this “how much” calculation is expressed in moles (n), which is an extremely large number of molecules of a gas stated as Avogadro’s number, equal to 6.02x1023 (Don’t be scared by this value, though. A mole is a number, just like one dozen is 12, so one mole is 6.02x1023 molecules). Since the number of molecules of a gas and the mass are directly related for each gas type (i.e. molar mass), we are able to calculate the mass of the volume or volumetric flow based on the number of moles present. This is based on the assumption that the measured gas is pure and not contaminated with any other gases.

We’ll look at an example of the difference of STP conditions in a mass flow meter.  Teledyne Hastings Instruments assumes STP of 0°C and 760 Torr, but would prefer the customer to specify their STP conditions for the application. We will use the frequently referenced STP of 20°C and 760 Torr for the second part of this example. Suppose that we are looking to  measure 1 SLM (Standard Liter per Minute) of Nitrogen gas. As I’ve discussed earlier, the 1 SLM must be referenced to an STP value, so we will use our assumed conditions of 0°C and 760 Torr. If we were to change to the second set of conditions, the number of moles present in the flow (Molar Flow Rate) would change, and our mass flow rate would thus change (based on the direct relationship between mass and moles). Our initial mass flow rate of 1 SLM of Nitrogen at 0°C and 760 Torr would now be 1.074 SLM of Nitrogen at 20°C and 760 Torr.

Mass Flow Meter  Mass flow controllerAn important item to note is that the STP conditions are not actually present during the calibration of mass flow meters and mass flow controllers. Gas conditions are not brought to 0°C and 760 Torr prior to running calibration of equipment. The substance may not even be in gas phase at 0°C. The STP conditions are simply stated to define the standard volumetric flow rates of a substance IF it were an ideal gas at standard conditions.

This is also the reasoning for the addition of the “S” or “Standard” at the start of the stated volumetric flow rate (e.g. Standard Liters per Minute (SLM) or Standard Cubic Centimeters per Minute (SCCM)). We are stating the volumetric flow that would be present using standard conditions. So, using the information that we learned earlier, by stating the units in Standard Volumetric Flow Rate we are actually stating the Molar Flow Rate. This information changes based on the standards we are referencing and emphasizes the importance of stating the required STP conditions.

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Brandon Hafer is an Application Engineer with Teledyne Hastings Instruments. He completed his undergraduate degree studying meteorology at the Pennsylvania State University before serving as an officer in the United States Navy. He received his master’s degree in Systems Engineering from George Washington University and has been with Teledyne Hastings Instruments for two years. If you would like to contact him, he can be reached at brandon.hafer@teledyne.com.

Tags: Teledyne Hastings Instruments, Flow Controller, Flow Meter, STP, Thermal Flow, Standard Temperature and Pressure