Gas Blending and Working With Flow Ratios –Part B
What is the right way to deliver precise ratios of gas flows?
Many applications require a fixed ratio of two or more gas flows. Setting each of the flows independently is certainly one approach. But in cases where you need to change the total flow while keeping the ratio constant, the “set each flow” procedure becomes cumbersome. So for applications like combustion control, gas blending, or vacuum furnace control, setting the flows using a “master/slave ratio” approach is far more dependable and convenient. The Hastings PowerPod 400 performs this function with the capability for up to four gas flows. Once properly configured, the total flow is adjusted with one parameter to provide a settable flow of a constant blend ratio without recalculation—avoiding many sources of operator error. This note outlines specific procedures for setting up the PowerPod 400 for master/slave ratio control and provides examples for some typical applications.
Objectives
- Determining ratio settings for specific flow rate combinations.
- Configuring the PowerPod 400 for master/slave operation.
- Adjusting the master flow or the ratio values.
Method for Determining Flow Ratios
Proper set up for master/slave operation begins with defining the desired flow ratio. Then, further calculations depend on how the units of flow measurement have been set, as actual flow units or as percent of full scale (FS). Once this calculated set point is programmed into the PowerPod for each slave MFC, subsequent changes in the master flow will cause proportional changes in the slave flow set point to maintain the ratio.
Full scale ranges are the same
For example, let’s say you are using a 1000 sccm full scale (FS) oxygen MFC and a 1000 sccm FS hydrogen MFC and the desired flows are 500 sccm oxygen to 400 sccm hydrogen. The actual flow ratio is Ra = Q2/Q1 = 400sccm/500sccm = 0.8(See Gas Blending for Stoichometry-Part A). In this example, the MFCs have the same full scale, so the “percent of full scales” ratio, RFS, is the same as the actual flow ratio, 0.8.
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