Making a flow system work best
Alicat devices—mass flow control, pressure control, and metering—enable custom solutions for a variety of conditions and materials. Optimizing flow requires understanding and accounting for:
- Flow Range
- Valve Sizing
- PID Tuning
- Response Time
- Environmental conditions
- Pressure differentials
This article provides an explanation of, and offers ways of adjusting for, these issues. Talking with one of our Application Engineers while designing your system helps ensure everything will work properly and is optimized while using an Alicat device.
Flow range: the speed limit
Alicat mass flow devices offer different available flow ranges depending on your application. Knowing the correct flow range for your application will help you determine the best meter or controller for your system. This applies to all gas and liquid units, but also pressure controllers for proper valve sizing.
Ideally, the device’s full scale should match the largest flow rate that you need to be able to measure. Alicat offers custom flow ranging when you purchase your instrument, to give you the best possible accuracy and control over the device’s full range. If a device has a smaller or larger flow range than your system requires, it will suffer from reduced accuracy and will not provide the best measurement.
In some applications the flow range is too wide for one device to handle. This is less frequently an issue with Alicat devices, thanks to our turndown ratio of 200:1. This is 4 times larger than industry standard devices at 50:1.
As an example, imagine you require a 50 SLPM (Standard Liter per Minute) mass flow meter to measure your highest flow rates. But your application also requires you to measure a flow rate of 0.3 SLPM (300 SCCM) on the low end. This would mean that the device would need to measure a flow rate that is 150 times smaller than its full scale range. Below (or above) the specified flow range, your device may still detect flow, but since it is outside of specification, there is no scientific certainty to the numbers.
For the 50:1 turndown of typical devices, you’d have to buy two flow meters in this application. A 200:1 Alicat can handle both flow rates and remain within specification. The standard accuracy specification for an Alicat gas flow device is ± (0.8% of reading + 0.2% of full scale). A high accuracy calibration is also available. Both accuracy ranges offer 200:1 turn down capability, saving set-up time and money.
If you need an even higher turn-down ratio, such as 1000:1, two or three Alicat devices can be daisy-chained to fully cover the range of flows you are measuring.
Knowing your fluid matters
Although it is not the case with Alicat flow meters and controllers, many mass flow measurement devices are calibrated for only a single gas and a single temperature and pressure (STP) condition. In the case of such single-calibration devices, inaccuracy increases if the calibrated gas or operating conditions must be changed. Where some devices offer gas selection, the result is not always a recalibrated state: they typically only provide a single point offset (a K factor), which doesn’t account for the full range of viscosity, density and compressibility changes to volumetric flow of gases at varying temperatures and pressures. Take care in reading specifications to note the introduction of additional accuracy uncertainty when changing fluid, or when temperature and pressure conditions depart from the normal or standard.
Alicat devices are much more flexible than this, and offer the ability to change which gas is being flowed without reducing accuracy. This is accomplished in Alicat devices by the storage of internal models for the physical properties (viscosity, compressibility, and density) of each gas as functions of both temperature and pressure.
You can change the calibrated gas easily by toggling through the front panel display and selecting the gas or custom mixture you are flowing (without the use of K factors.)
Liquid condensation or other ingressing will destroy most gas mass flow instruments. While it’s not fatal to an Alicat, it’s not an ideal situation. Sometimes it is unavoidable. If condensation or liquid ingress is a risk in your flow system, Alicat’s MS/MCS series is your best choice, equipped with a 316 stainless steel sensor that can handle moisture as well as aggressive gases such as ammonia, hydrogen sulfide, refrigerant gases, and more. If you find that gas is condensing in your application process, our support team can advise you about measures you can take to minimize condensation in the meter or controller.
Floodgates: Sizing the valve
The proportional control valve on Alicat mass flow and pressure controllers is selected to give you maximum control of your device. The valve actively controls the set point given to the device.
Since having the wrong size can complicate your efforts, it is important to determine what size best fits your application. If a valve is too small, your process will not stabilize quickly, and the flow will become choked regardless of your supply pressure. With a valve too large, the slightest adjustment in the valve’s position will affect the stability and control of your system. By providing our support team with your inlet and outlet pressure conditions along with the flow rate expected, you can help us to select for you the most appropriate valve for your mass flow controller or pressure controller, one that will provide the best behavior in your system. A wide variety of valve sizes are available, to accommodate many different flow range and pressure conditions.
PID: Proportional Integral Derivative Tuning
Whether you are giving your new Alicat mass flow controller a set point for the first time or putting a trusted Alicat pressure controller into a new system (PID doesn’t apply to flow meters), your device sometimes may need a few adjustments to achieve the highest degree of control stability. In our labs we are able to make adjustments for the application conditions you provide to us so that the unit will provide the best control out of the box. Should your application change, Alicat devices’ PID profiles are also user-adjustable, so you don’t necessarily have to send it back to the factory to optimize the control valve behavior.
Our unique dual-valve controllers, the MCD and PCD series, use the PID terms, but they use them differently than in a traditional algorithm. We use a special PD2I algorithm created by Alicat that incorporates a predictive function into the algorithm. This is why standard methods of PID tuning will not work for these devices. The PD2I algorithm is more complex, and single-valve controllers typically don’t benefit from it. We recommend working an Alicat application engineer if you having trouble tuning a PCD or MCD.
When you provide us with your expected inlet and outlet pressures on ordering, our technicians tune the device optimally for those conditions to provide you with a customized unit right out of the box.
Two Kinds of Speed of Response
As our specification sheet states, our mass flow meters have a response time of 10 milliseconds. Response time is the time it takes for a change in flow rate to be detected by the measurement device. Ten milliseconds is much faster than the 100ms-500ms response time common to other flow meters currently on the market. Changes to flow rate in an Alicat are reported to the display screen at the 10ms full bus speed.
If you are using RS-232 or RS-485 communication protocol, you can stream data from the device. The data takes longer to transmit than the response time, but still refreshes at a healthy rate of around 50 milliseconds per packet. This refresh rate might vary a bit depending on the exact software and hardware configuration you are using. You can also adjust the stream rate yourself by modifying the internal registers on the device, or by removing fields from the reported data frame, as illustrated below.
Control response time is the time it takes for a mass flow controller or pressure controller’s to achieve a new set point or recover control after changes in upstream flow or pressure. On an Alicat controller, we achieve your set point in under 50ms with proper tuning, compared to similar products with control response times up to 3 seconds.
Control response time is dependent on a few important factors, including the sensor’s response time, the valve response speed, and PID tuning. Response time is, simply the speed with which the sensor detects a change. Valve response speed is the speed at which the valve responds, which is dependent on the electro-mechanical properties of the valve you are working with. PID tuning is a factor which can be optimized by Alicat prior to purchase and further customized by you as explained above.
This video demonstrates the control response speed of an Alicat mass flow controller:
Environmental Effect on Your Process
Many manufacturers of mass flow devices measure flow rate and density based on an assumption of standard conditions. However, environmental factors such as pressure and temperature can have a significant effect on your flow process’s results, if not considered and adjusted for. Ambient pressure can affect your process: for example, when performing a leak test venting to atmosphere, should someone open a door, or the air conditioning shuts off in the test lab—a mass flow meter, flow controller or pressure controller will not be detecting or compensating for the change, unless the device is specifically fed the information in a closed loop. Likewise, a device under test can be affected by temperature changes in the room, and the flow meter or pressure controller will be unaware of the externality.
Sometimes, adjustments can be made to the flow process to control for environmental concerns. When measuring very low flow rates, such environmental changes will be registered on the flow device. You may be able to isolate the process to reduce these variations. If you become aware of temperature related errors, a temperature controlled box can be placed around the device under test in order to negate temperature changes in the room, or temperature changing “phantom leaks.”
By default, the STP (standard temperature pressure) on an Alicat device is set to 25°C & 14.696 PSIA unless a custom configuration is requested. The instrument also accepts normal temperature and pressure (NTP) settings. Regardless, it is easy to change your STP or NTP through the front panel display menus, in case your standard conditions are different than expected.
We acquire the mass flow rate by using this equation, and temperature is in Kelvin.
It is always wise to operate as close to standard temperature and pressure (STP) as possible, in order to be certain of a more precise flow measurement. By choosing an Alicat device, however, environmental variables are more easily controlled for, and less of a concern in getting accurate results.
Inlet vs. Outlet: Pressure Differentials
The difference in pressure at the inlet and outlet drives flow through your device. It is one of the most important parameters in a mass flow device. Our mass flow devices measure pressure differential to determine flow rate.
It is important that the pressure differential not be too great so as to prevent damaging the sensor: If you must operate your flow with a pressure differential between the device inlet and outlet in excess of 75 PSID, we recommend that you install a calibrated orifice on the outlet port of the mass flow device. Another mitigation technique is to program your system to come up to pressure (and back down in pressure) in steps of 15-20 PSIG increments.
At the other end of the scale, if your pressure differential is small, you may be able to use a low pressure drop flow meter or controller like our Whisper instruments.
Our mass flow controllers have a proportional control valve that will drop as much pressure as needed to maintain the given set point, so pressure differentials are less critical in the controllers—the valve will see the pressure drop instead of the sensor, in most cases. This too helps to mitigate pressure differentials that may harm a sensor.
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