Draft system diagram
Density determination and error
Version 1 (pdf, gif)
I want to summarize what I see as the three general ways to fill/empty the E907 beam ckov's so a decision can be made as to which method to pursue. I welcome comments.
- Simply fill by monitoring temperature & pressure in the ckov and calculating density. This could be done manually or controlled by a computer. The problem with this method is thermal equilibium. As we quickly fill from a high pressure bottle to the low pressure ckov the gas will undergo expansion cooling (about 45F cooling from 2000 to 5 psia adiabatically). As the temperature rises, the pressure goes up, so its hard to know when the desired point has been reached and the ckov should be valved off. This cooling can be countered with a long inlet tube (~ 75 feet) or a heat exchanger to warm the inlet gas. This system also has error from two instruments compared to error from one intstrument for the 3rd method. Old ckov fill systems I located just had remote valve actuation and the operator watched a gauge to get the desired condition.
- Fill the ckov incrementally using a "standard volume." Say the ckov is 100 ft^3. Then there is a small pressure vessel "standard volume" of say 1 ft^3. This small vessel is filled to 100 psia by a regulator. When more mass is required in the ckov, this small volume is allowed to flow into the ckov by opening a valve. For arguments sake, a 100 psia 1 ft^3 vessel would increment the 100 ft^3 vessel by about 1 psi each time it was opened to the large vessel. This is how it would increment up a pressure curve, using a known mass of gas each time. The same vessel(s) could be used for helium and nitrogen. But the C4F10 goes to liquid at any pressure above about 35 psia so it would need a much larger "standard volume" to fill at the rate the N2 and He standard volume would.
I see some other issues with this method. If you fill the small volume quickly, it will undergo the expansion cooling. So say the regulator puts it at 100 psia and the expansion puts it at 50 F. The regulator closes when outlet pressure is equal to its setting. This small volume now warms up to 70 F and is at a higher pressure than intended. The temperature reached will vary each time you fill it based on bottle pressure/fill rate etc. so the amount of mass may vary slightly each time.
I think the ambient temperature will likely change significantly in MC7. The regulator maintains the pressure regardless of temperature. A 5 deg F temperature change globally in the gas is a 1% change in density if the regulator maintains the same pressure. So the unless this "standard volume" reaches the desired pressure at the same temperature all the time, I fear over time the mass stored will vary between charges.
And when the standard volume empties into the ckov, there will still be small thermal changes due to expansion such that some gas will want to go back into the standard volume. So there is still an equilibrium issue as to when you close the valve to isolate the ckov.
To come down a pressure curve pumping would have to be used as I don't see how the std. vol. could achieve this.
- Method 3 is to fill the ckov using a mass flow meter. Since what really matters is the mass of gas in the ckov, why not measure that directly? The mass flowmeter is really independent of temperature and pressure because it works off of specific heat which is a weak function of temperature and pressure. The mass flowmeter can easily be accurate to 1%. A controller could be set up that such that if 20 standard cubic feet of gas needs to be added, the mass flowmeter flows until 20 ft^3 has passed thru and then shuts off until the next increment is needed. No worry about expansion cooling or equilibrium. Its impractical to use a mass flowmeter to come down a pressure curve. Small pressure drop mass flowmeters are typically less accurate. It could not be inline with a vacuum pump because the flow rate would vary too widely so there would have to be a vacuum reservoir. To come down a pressure curve, it might be accurate enough to use temperature and pressure as its pumped because any expansion cooling would be very small.
It has to be investigated how many mass flowmeters are needed. Six may be needed, one for each gas for the two ckovs. Or it may be possible to scale for each gas such that only two are needed if the accuracy is satisfactory. A mass flowmeter would cost around $1500.
A brief document describing how I intend to calculate the density in C4F10 for the beam ckovs. (Tope 4/8/03)
I made a plot of the instrumentation error for the bckovs (attached .pdf). The blue line is the transducer I made the req for. The red line is an additional transducer that could be purchased for the lowest absolute pressures to improve density calculation accuracy.
The points I want to make are:
- RTD error is insignifcant compared to pressure transducer error.
- The pressure transducer error for the 0-14.7 psia instrument I want to buy could lead to a density error of 1% at 1.742 psia.
- Adding a second 0-2.321 pressure transducer would delay the possible 1% density calculation error until 0.545 psia.
So the question is then, is the error of the blue curve acceptable? If its not, then the only solution I see is to buy a 2nd transducer. Food for thought.