Is that self-pressurized N2O? My visualization of what Ben is suggesting is
sensing the level in a tank of cryogenic liquid that is being expelled by
relatively warmer pressurization gas.
-p
On Mon, Sep 14, 2015 at 5:50 PM, Henrik Schultz <henrik@xxxxxx> wrote:
Based on experiments with N2O a while back, we found that the temperature
inside a tank with a cooled liquid - in this case not cryogenic in the real
sense, just very cold - did not change a lot between the vapor phase and
the gas phase. Its going to be cold inside the tank no matter what, and the
walls will also work to equalize temperature between your optocouplers. In
a tank made of e.g. aluminum everything is pretty much same low temperature.
For the temperature shift to be noticeable in a short time frame, i.e.
without so much delay that it skews the calculated predictions, the sensors
must obtain heat from somewhere fast. If you think of old air speed
sensors, they deliberately added heat to the sensor by running current
through the sensing wire.
Further, some liquids will be in indeterminable state, notoriously for N2O
which has a soft boundary between liquid and gas phases. Add to this
sloshing, etc. and your optocouplers may not be so accurate after all.
The capacitive sensor has the advantage of being a "straw" which cancels
out some of these effects. We never got to test it, but we had a suspicion
that it would in fact work for N2O at the critical gas-liquid
transformation stage, since the capacitance is a function of the dielectric
constant, and that this would likely correlate to the N2O density in some
function. Capacitive sensors are not hard to make.
My personal afternoon thought would be to measure the acoustic resonance
of the tank. Resonance frequency would be a function of volume, density and
pressure/temperature. Temperature and pressure can be measured accurately,
and density derived from that, leaving free volume to be determined. With
some empirical testing this could conceivably also be made to work in zero
gravity.
Lastly, some people use ultrasonic distance sensing from the top of the
tank down to the surface. There are Arduino kits for this used in robotics,
but they are probably not cryo rated ;-)
/H
------ Original Message ------
From: "Pierce Nichols" <piercenichols@xxxxxxxxx>
To: arocket@xxxxxxxxxxxxx
Sent: 9/14/2015 5:16:20 PM
Subject: [AR] Re: Semiconductor cryo level sensing
I like the idea of connecting them all together to get a single reading --
much simpler wiring. However, it seems to me that it would be better to
wire them in parallel, so one or a couple of dead diodes don't kill it
dead. That also reduces the peak voltage from ~n*Vf to ~Vf.
-p
On Mon, Sep 14, 2015 at 5:08 PM, Ben Brockert <wikkit@xxxxxxxxx> wrote:
On Monday, September 14, 2015, Pierce Nichols <piercenichols@xxxxxxxxx>
wrote:
Nifty idea! My understanding is that all plain LEDs shift their
wavelength with temperature because the size of the band-gap is temperature
dependent. It seems to me like the same physics would affect the receive
side of the optocoupler as well... so it's not clear to me that you'd
actually get the response to expect.
That would certainly kill it if true. I'll have to get some ln2 and try
it.
However, the current through a diode is also temperature dependent,
which points the way to a more elegant implementation. Perhaps you could
just have a line of plain diodes and sense the change in current through
each one as the liquid level passes it. Thoughts?
If it's a large enough change then you could have a bunch of them in
serial and have basically the same analog sensor as centaur but with a
current change rather than capacitance. Easier to turn into something a uc
reads.
-p
On Mon, Sep 14, 2015 at 2:14 PM, Ben Brockert <wikkit@xxxxxxxxx> wrote:
One of the few disadvantages liquids have over solids is propellant
depletion. A solid burns nearly all its propellant, though some of it tends
to be at lower than nominal pressures during the tail off. For a liquid
stage to get maximum impulse it is imperative that it deplete both
propellants simultaneously. The last kilogram of propellant is the most
important it. A real world example of this is that Stiga flew to ~90km but
had 30lb of fuel remaining after it burned out the LOX; with equal
depletion it would have reached roughly 120km.
Centaur accomplishes this with capacitive sensors in the propellant
tanks. A rod is inside and electrically insulated from a tube that is open
at bottom and top. This forms a capacitor. As the level of the propellant
goes down, more of the capacitor contains gas than liquid and its
capacitance changes. This is used as an input to the mixture ratio
controller, which drives a harmonic drive on a throttle valve to slightly
tweak the mixture. With this control a stage carrying thousands of
kilograms of propellant is able to deplete down to tens of kg, helping give
the stage it's incredible (and consistent) performance.
One downside of this system is that it is analog and it requires some
extract structures and mass to hold up the tube and wire, and keep them
electrically isolated.
Cryo and electronics geeks know that some types of light emitting
diodes (LEDs) change their emission wavelength when they are cooled to
cryogenic temperatures.
An optocoupler is a solid state semiconductor device essentially
composed of an IR LED mechanically coupled to an IR transistor. The two
components aren't electrically coupled, so it's a way to send information
between two isolated DC systems without them needing to share a ground. I
used optocouplers to run some of the solenoids on Xombie and Xoie; it was a
convenient way to amplify a TTL signal to coil current while isolating the
noise associated with quickly turning on and off inductive loads.
Background complete. You could make a chain of optocouplers inside a
cryogenic tank, with them turned on all the time. It would consume just a
few mA per sensor. If the wavelength alteration trick works on IR LEDs, the
optocouplers would only pass a signal when they're above the level of the
cryogen.
By watching the timing as the sensors toggle, you can then measure the
depletion rate of the propellant in the tank and adjust mixture ratio
accordingly. With SMD optocouplers, the whole thing could be quite light.
There is a similar concept with small heaters coupled to temperature
sensors, where sensors that get warmer are above the liquid level. But I
think my concept would put less heat into the tank and has digital outputs,
rather than analog.
So that's my idea of the afternoon.
Ben
