Improved Experimental Pendulum

I was tempted to say he took all the air out, then carefully let some vacuum in

Neil

When I worked on gas centrifuges, which run at far higher vacuum than a clock, we used Oxley pins to get power in. Oxley Developments, Ulverston. They might send you a few as a sample if you ask them nicely. Otherwise, a ptfe disc with interference fit pins would be a close replica.

Maybe you could avoid pins altogether? Use a "wireless power" setup (chips and coils readily available) to transfer power through the plastic pipe, put the processor inside the enclosure, Bluetooth or Wi-Fi to get the data out?

That's a good idea. A variant would be to run a single power feed through the base and use the base itself as ground. Easier to seal one connection than four, more when a display is added.

But would running normally air-cooled electronics in a vacuum cause them to overheat?

Dave

And then stretch your pendulum that your are trying to insulate from the environment..

As little as possible inside, then you can play around with it without breaching vacuum

.

+1 for that

MichaelG.

I can consider encapsulating because my rather complicated clock & logging electronics are already designed for remote operation, running over WiFi. Done so I can modify the software without going near the physical clock, but it could all be installed inside the pipe.

Mechanical adjustments are the main reason for opening the case, and they shouldn't be necessary once the beam break sensor is positioned. Not much reason to open up once the pendulum is swinging because rate, temperature and pressure are all compensated in software.

The main problem with the suggestion, I think, is keeping the electronics cool inside a vacuum. A vacuum is an excellent insulator and there's no point spinning a fan in one!

On balance, I feel it's easier to make an air-tight connector for 3 or four wires. Araldite should do it!

Dave

With a Fedchenko type drive you could mount everything outside the vacuum containment with a non magnetic diaphragm to isolate the pendulum. A second hall effect sensor would monitor amplitude.

You could even do this with a tee shaped pendulum, have the end of the tee running in a tunnel with the drive coil external.

Still inching along with the rebuild, old and new side by side:

Just testing the gantry goes together and making sure it fits inside the soil pipe. Next job is to finish the top-plate and make the suspension, rod and bob.

Decided my existing suspension design is too fiddly to make, so have to try for something simpler in CAD before cutting metal.

Dave

Getting closer: suspension and bob finished.

The levelling feet work wonderfully, and the bob swings freely with no obvious deviations.

I abandoned the carbon-fibre pendulum rod after pondering this FFT graph:

In theory a pendulum should have no harmonics, and this one does. Pretty sure it's because the carbon-fibre rod (which started as a spring), is twanging. The new rod is 2.7mm steel - much stiffer. It occurred to me that because the clock doesn't use actual period and temperature is compensated in software, the rod changing length significantly as it heats and cools may not matter. We shall see!

Next step will probably be the beam break and electromagnet assembly. When they're available I can test basic operation of the clock software without mounting the microcontroller, soil pipe, or vacuum pipework. Still haven't decided how to get the electric wiring into the vacuum tube without causing a leak.

Blood was spilled! The suspension spring is made from a razor blade and I forgot to blunt it...

Dave

Edited By SillyOldDuffer on 17/04/2023 21:42:22

What's that FFT of Dave?  Is this impulsed or run-down?

Edited By John Haine on 17/04/2023 21:52:53

Now you can accurately describe your timepiece as cutting edge technology Dave.

regards Martin

As with recent 'bench design' threads and previous lathe bed ones would the support frame benefit from diagonal bracing and perhaps being based on a great big lump of granite?

I would look at mounting your temperature sensor on a dummy pendulum, same diameter and length, mounted outside the containment. This might compensate for the time lag between air temp and rod temp. If the temp sensor is outside the containment this could be quite a long time as vacuum doesn't conduct heat very well.

Since Air temp is irrelevant ( no air in the column), surely only the temp of the pendulum rod inside the column is? It's temp is what makes it's length vary, and the lack of air means air density with temp has no meaning.

Joe confirms my thinking : it's the temperature of the rod that matters.

But this is yet more unexplored half-understood territory for me.

• At the moment, the design has the sensor mounted on a circuit board inside the vacuum about 20mm distant from the bob. I intend to 3D print the support and plastic is a good insulator. I'm assuming the temperature sensor doesn't need air to work. (There will be some air because it won't be a perfect vacuum, maybe far from it.)
• The vacuum container will be a plastic soil-pipe with a plastic cap. Also good insulators.
• The whole is mounted on a cast-iron base.

I think the main heat-flow would be:

• Cast-iron block temperature follows external ambient with a time lag
• Inside the vacuum containment, heat travels up the steel-braced Aluminium pillars until it reaches the top-plate.
• From top-plate, heat travels via the two Brass trunnions into the levelling axle
• From the levelling axle, it travels into the Brass suspension top. (By accident the top is counterbored so the metal to metal contact area is reduced, and I'd expect this to delay heat transfer.
• Heat then travels into the suspension spring (a 4mm wide slice of 0.1mm thick stainless steel)
• After traversing the spring, heat then enters the brass rod-top, and from there into the 2.7mm diameter Silver Steel rod, about 230mm long.
• The rod enters the Mild-steel bob (a 25mm cylinder) through a 4mm diameter hole, and screws into a 6mm diameter brass collar so that the bob is supported at its centre. (So the bob expands and contracts without altering the effective length of the rod.)
• Cooling would reverse the heat flow, with the bob providing a reservoir.

In short, I expect there to be considerable temperature hysteresis, and it's likely that the sensor and rod won't be at the same temperature. Also, the hysteresis of the rod will be slower than the hysteresis of the sensor.

I don't know how much this matters, yet! Possibly it would be better to put the sensor on the top plate rather than close to the base. Or maybe to measure temperature inside near base, inside at top-plate, and outside on base, and see how they correlate. If the hysteresis is predictable based on previous experience, I can compensate for it in software.

Anyone know of a way of measuring the temperature of a pendulum rod without disturbing it?

Dave

Duncan is thinking on the right lines in my opinion but the dummy rod needs to be in the chamber with the temperature sensor attached to it. If it is mounted in such a way as to mimic the thermal path of the main pendulum rod then it will track the temperature profile correctly. It obviously needs to be rigidly mounted to avoid any parasitic motion but should be achievable. A real test would be to mount a temporary sensor to the main pendulum and run a test over some days or weeks to see how well the dummy tracks.

regards Martin

“Anyone know of a way of measuring the temperature of a pendulum rod without disturbing it?”

.

In principle, an InfraRed thermometer should do it

… but it could be a very long journey getting it to work in practice !!

MichaelG.

Martin has described more eloquently what I was trying to get at, a way of measuring temperature of the pendulum rod without interfering with it's motion, put a non moving dummy in the same environment and measure that.

As well as the heat paths listed by SOD there is radiation from the pendulum to the containment tube, but in view of the small temperature differences this should be very small