The Flow Sensor
Two physical components make up the sensing element of a flow controller, (1) a primary flow channel through which the bulk of the gas flows (2) a secondary flow channel (sensor tube) through a thin-walled small diameter metal tube with multiple high temperature coefficient windings applied to the external diameter.
The “bypass” (think of it as a partial blockage) is located in the primary flow channel and causes a small pressure buildup upstream of the bypass. The inlet of the sensor tube is located just upstream of the bypass and the outlet just downstream of the bypass. Since the inlet and outlet pressure of both components are coincident the pressure drop across each is equivalent. In theory, the flow split ratio between the two channels remains constant through the entire flow range of the instrument. In order to make the split ratio as equivalent as possible across as much of the flow range as possible it is necessary reduce turbulence. The bypass element is designed to produce turbulence free flow (laminar flow) to reduce pressure fluctuation that would disrupt the split ratio and cause erratic flow through the channels. When the flow through the bypass reaches a critical level (either too low or too high) such that the bypass design can no longer maintain laminar flow, the ensuing turbulence causes the split ratio to become non linear.
As gas flows through the sensor tube the gas molecules pick up heat from one winding and transfer it to the other downstream windings. The temperature difference of the windings is a direct correlation to the amount of mass that has passed through the sensor tube. Since the flow split ratio between the two flow channels remains constant (as long a laminar flow conditions exist) the heat transfer of the sensor at any given flow rate can be used to indicate the total measured flow. If the flow rate through the sensor is too low the gas molecules move through the windings so slow that the temperature difference between windings becomes too low to measure. If the flow is too high sufficient heat transfer does not have time to occur and can actually cause an indication of reverse flow.
The optimum gas flow through the sensor tube is dependent on the sensor design and does not change for the total desired flow rate of the device. For higher or lower total flows the bypass is adjusted to produce different back pressures that result in the optimum flow ranges for proper thermal transfer measurement to flow through the sensor tube while allowing the remaining flow to go through the bypass. This is a delicate balance of maintaining laminar flow and constant split ratio, staying within the thermal transfer window of the sensor tube, and achieving total flow. It is pretty remarkable that it works at all. This also explains the number of calibration labs who claim they can check the calibration of a device but they cannot change it to another gas or range. The ability to make that change requires a total understanding of the design characteristics and functional parameters of the specific device and can be quite different from one manufacturer to another, or even one model to another.
Because the flow sensing technique is thermal in nature, all heat transfer modes affect the flow indication. The heat transfer modes that exist in the sensor are free convection, both to the sensor environment and to the process gas at zero flow, conduction along the tube, and to insulating media and radiation heat transfer.
Changes in any of the above quantities can have an effect on the performance of the MFC. The largest detrimental effect is from a change in free convection to or from the environment. This can be caused in several ways, one of which is an internal (to the MFC sensor cover) change in temperature or temperature gradient caused by the heating of the valve or electrical components. This temperature change can cause the natural zero to shift resulting in corresponding shift in calibration.
Another limitation of this sensing technique, inherent in flow controller design is aging of the sensor. Fine resistance wire is wound on the tube in a specific pattern and with a specific tension. Through expansion and contraction caused by heating and cooling of the sensor tube and wire elements these patterns and tensions change. These changes can cause a zero drift or even a change in sensitivity of the sensor due to the change in heat transfer quantities from the moving or shifting of the heating element/temperature sensing wire.
Free convection to the process gas at zero flow (thermal siphoning) can be a significant problem. This problem is reported exclusively in flow controllers that are mounted vertically in a system. A zero shift is observed due to heated gas rising in the sensor tube and cooling upon exit. This sets up a real flow through the sensor and the bypass and is not an indication that the natural zero has changed. The danger here is that if this non-natural zero output is adjusted, a calibration error will result. See the tutorial on Mounting Position for further discussion.
Zero Flow Output
The indicated output of the device with no gas flow through the sensor (Zero Flow Output) is adjusted while the device is sufficiently warmed up, with no flow through the device. This does not mean that the device is hooked up to a gas supply and given a zero set point, which would result in the indication of the amount of gas that is leaking through the valve. That leakage can be substantial for some metal seal devices (see the tutorial on Valve Closure). Rather, the zero is set with no (zero, nada) gas flow through the sensor. This value is adjustable and is usually set slightly positive, usually to +5 to +15 mv to make sure any small drift does not take it negative since many control and data acquisition systems will not recognize a negative number for a voltage output from a system device. The rest of calibration curve is based on this being zero flow. Adjusting this value will offset the entire calibration curve on most devices. A zero value that is not stable or greatly different from the initial setup usually indicates something is wrong or the device is not being used per the initial configuration. Any metal seal device without a positive shutoff valve upstream will almost never read the true zero flow value when at zero set point, and can read as much as 5% of full scale (250mv) in some cases.
Zero flow is almost always expressed in millivolts. This is the indicated electrical flow output of the device. The flow controller is very inaccurate in measuring actual flow on the extreme low end of the scale (zero is about as extremely low as you can get). Any indication of actual flow expressed as volume per minute would only be a calculation and a meaningless number since it is actually zero flow by definition, but not really zero voltage for the reason discussed above.
The mass flow controller valve is made up of:
- a motive force (electromagnetic, thermal, piezo-electric, motor-driven etc.)
- a resistive force (spring, diaphragm, etc.)
- a variable flow restriction (orifice-poppet, orifice-ball,etc.)
The motive force is activated by the control circuitry to act against the resistive force to vary the flow restriction. Most of the pressure-drop across the flow controller takes place across the control valve.
Proper performance of the valve is determined by all of these components being sized properly for the intended gas, range and differential pressure of the system. Problems can occur in several ways. First, the valve restriction or orifice must be sized correctly. An orifice that will pass 100 sccm of Nitrogen when fully opened at 30 PSID may not pass 100 sccm of Nitrogen at 5 PSID. Further, this orifice may not pass 100 sccm of WF6 at either condition. This is because the orifice is too restrictive for the heavier gas. Oscillation can also occur if the resistive force or valve spring is too heavy or too light for the application. A spring with an appropriate spring rate for nitrogen may work fine if used for BCl3 or Argon but can cause oscillation in Hydrogen or Helium.
The differential pressure across the flow controller can be created by elevated upstream pressure or by the creation of a vacuum downstream. This can have a substantial effect upon system performance. As discussed, the exposure of the sensor to vacuum can have performance effects. Further, system time constants such as sensor speed of response change radically under these conditions. The control circuitry may not have the margin to provide optimum response under both conditions and possibly must be tuned to the correct application or oscillation or long settling times may result.
Also see the tutorial on What You Should Know About Valve Closure.