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更新日期：2014-04-26
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Gas blending using VpmMixer program
The idea behind VpmMixer program is to separately adjust oxygen flow and helium flow reducing the problem to same level as mixing nitrox. That reduces the time needed to set correct oxygen and helium flows and increases the mixing accuracy and saves the expensive helium.
VpmMixer will do topping up calculations for all three cases and all the necessary calculations for topping up mix: necessary flows of each gas, helium price and filling time.
On the bottom of the screen there are all the necessary calculations for three stage process of setting the correct gas flows for both “oxygen first” and “helium first” procedures.
1. Keep the oxygen and helium banks closed and calibrate both oxygen analyzers
2. Open the oxygen bank and set the correct oxygen flow by setting the oxygen needle valve until the correct reading on both analyzers (not the same as any of end readings).
3. Open the helium bank and set the helium flow by setting the helium needle valve until the correct reading on both analyzers.
After that you can open the dive tanks and start filling them, taking just minor adjustments on both needle valves.
Most of continuous gas blenders use the same principle of operation. The air is mixed with the low pressure gas in a first mixing stage, analyzed for oxygen share, mixed with second gas and analyzed for oxygen again. First and second gas is pure oxygen and pure helium in any order, based on personal preference and desired mix. Desired oxygen and helium flow is adjusted using needle valves that are feed from gas regulators.
There is no need for expensive helium analyzer in this process; everything there is to know about end mix can be calculated from two readings of oxygen share.
Gas mix is than compressed using any diving compressor and filled into a scuba tank.
Based on which gas (oxygen or helium) is mixed with air in the first mixing stage and which gas is added in a second mixing stage, two flavors of procedure can be used:
• Oxygen first procedure
• Helium first procedure
The choose which procedure to use is made based on few criteria
• Avoiding oxygen rich readings (more than 40%) for safety reasons – oxygen first procedure produces higher readings on first analyzer. Mix richer than 40% must never be feed into compressor inlet.
• Avoiding oxygen low readings (less than 10%) for accuracy reasons – helium first procedure produces lower readings on the first analyzer
• Personal preference
The mix being filled can be calculated by the readings of the first and a second oxygen sensor. Commercial blending units use a microprocessor unit to calculate the mix in real time (plus they can provide some safety features like shutting off the oxygen supply in case of too rich oxygen concentration being feed into a compressor inlet). In fact the processor unit is the priciest part of commercial blender and that is why most home-build blending-units will use double nitrox analyzer instead.
Determining the reading of both oxygen analyzers that corresponds to desired mix is pretty easy math. In fact if “oxygen first” procedure is used the fist analyzer always reads the same value (that depends on MOD of the mix being filled), the reading of the second analyzer is the same as the oxygen share of the mix being filled.
The tricky part is setting both needle valves to set the desired oxygen readings for both analyzers. On oxygen first procedure opening oxygen valve will increase oxygen reading on both analyzers, opening helium valve will decrease reading on second analyzer and increase reading on the first. The situation is similar on helium first procedure.
Knowing that typical response time of oxygen sensor is 5 sec one can imagine that setting correct flows of oxygen and helium is long iterative process of adjusting small changes on both needle valves.