Improved Experimental Pendulum

What about fitting an ESP8266 WiFi uP ( or similar - there are many types) and asmall battery into the pendulum bob position(inside the bob?). Use a thin tube rather than a solid rod for the pendulum and run thin wires up midway with a thermistor at the end. Set the ESP to wake up and do a temp sense every 10min or so and collect that on WiFi. A CR2032 Li cell would last a year or more.

If a tube is a no-no, use the rod and superglue 34gauge ins wires neatly along its length to the thermistor.

Duncan, My comments were based on your initial statement re the dummy being outside the chamber...like me..

Edited By Joseph Noci 1 on 18/04/2023 11:26:54

Would it be simpler to mount the containment tube in its own container which is temperature controlled to a tenth of a degree?

Surely if reducing the pressure to eliminate atmospheric effects the best approach is to try to stabilise temperature as well? Of course this could be challenging as there's not much air to distribute the heat.

Ah, when I said mounted outside, I obviously meant mounted inside. I'm blaming predictive text, or perhaps I was just testing you all. It was late at night as well.

That has delightful symmetry, SOD is using an OCXO to do the measuring, so making an OCPO (oven controlled pendulum oscillator) has a certain logic.

Or you could have a double suspension spring like John Haine's, but insulate one half, then you can have a sensor on the pendulum without a battery

Might be! This is the world of the time-nut, where not matter how good a clock is, the poor sod building it can always do better.

Back when pendulum clocks were the last word in precision, they were usually bolted to a heavy masonry wall in a locked cellar, deeper the better. Partly to protect them from vibration, but also because temperatures underground don't vary much - natural air-con.

Now I'm thinking of buying a lorry load of two metre diameter concrete sewage pipes and using them to sink a 10 metre deep shaft in my back garden. The clock will live at the bottom...

Dave

Mildly disappointed [i.e. I will get over it ] that no-one appears to have given any thought to IR thermometers.

When it comes down to it, it’s the actual temperature of the actual pendulum that matters … not some proxy measurement from some surrogate device.

MichaelG.

Which is why I suggested running a calibration test with a temporary sensor on the actual pendulum to compare with the dummy.

regards Martin

I've gone off double springs! See my solder resist thread.

Well I have now, thanks! This module is available from Sparkfun ready to go on an Arduino:

I won't rush to buy one ($32 USD) until it's confirmed that my current temperature arrangement causes detectable errors. (The beast is experimental and may not keep good time for other reasons!)

What could possibly go wrong?

• As far as I know, no-one has ever used one of these in a vacuum to measure the temperature of a moving metal object. The device specs (MLX90614) don't mention any pressure limitations, so the case might pop in a vacuum! (I doubt it). Electrolytic capacitors feel more risky, because big ones come in a thin Aluminium cylinder that's designed to burst if the electrolyte boils. I guess the burst pressure at least 2 bar, but don't know.
• There's already an Infrared transmitter inside the container bouncing IR off the bob. It might interfere with an IR thermometer. Fixable by changing the beam break system to use visible light, so this could become a bigger change than expected.
• I'm still worrying about normally air-cooled electronics overheating in a vacuum. It's another unknown.

Meanwhile, I 3d-printed the sensor chariot last night. Always a nervous process because STL files are dimensionless and the slicer has to be told what the scale is. Get the scale wrong and the object comes out the wrong size. It's the sort of mistake I'm prone to make, but it fits correctly, phew!

Next job, remove the sprues, turn the slotted LED holders and fit the electromagnet. Once that's done I can spot glue the chariot in place and confirm the clock still runs. Unlikely, because the bob is much heavier and the electromagnet is near the cast-iron base, which is magnetic. The chariot position and software parameters, mainly impulse power, have to be tuned for acceptable amplitude and reliable running.

In air, the previous pendulum kept best time being lightly impulsed on every beat, basically keeping the amplitude constant. This may not be best for the new pendulum, which is mechanically superior and will be swinging in a vacuum. Therefore I have to retry the huff and puff governed approach, in which the pendulum is allowed to swing free for many beats, and only impulsed when necessary.

Years of work ahead...

Dave

Edited By SillyOldDuffer on 19/04/2023 11:06:10

It's not clear to me whether the above board includes a focussing lens. I think you'd have to arrange for the pendulum rod to fill the field of view all the time, otherwise it would be sensing the temperature of the background (containment tube when it's fitted, rest of the word when not). It could probably be sorted, but the proxy rod is more straightforward

.

Most unlikely to be able to add a focussing lens … [they are expensive at the relevant wavelengths]

A quite detailed data-sheet is available via the page that Dave linked,

… but for convenience : **LINK**

https://cdn.sparkfun.com/assets/8/b/3/2/9/MLX90614_rev001.pdf

I had assumed that filling the field of view with the pendulum bob would be quite simple … although I don’t recall what arc of swing Dave proposes to use.

MichaelG.

Edited By Michael Gilligan on 20/04/2023 06:39:18

Should you be blacking the pendulum bob to get it as near as possible to a "black body"? If it is reflective it will reflect IR from other sources into the temperature sensor as well as what you want.

Although using a 4" soil pipe as the vacuum tube creates more space, it's still cramped inside:

The pendulum is intended to swing up to 5 degrees, so the bob can never be more than 40mm from a sensor. The obvious place to put the IR thermometer module is between the pillars opposite the electromagnet. Could be done, though it means another wire has to penetrate the vacuum seal.

Space for a dummy pendulum looks tight as well, maybe the easy answer is an Invar rod...

Dave

This is why I need to improve my soldering!  Having made a new single-spring design which promptly fell apart and needed re-soldering, without much prompting the old one did too!  The idea is to get the small gap between the chops completely filled with solder so the length of the spring is completely defined by the edges of the chops.  The problem is if you get too much solder between the cheeks it tends to bulge out on to the spring at the edge.  In this case the springs have hardly "wetted".  So I need to be able to put enough solder paste on to the job when assembling into the soldering jig and also apply resist on the spring to stop it spreading.  I have a plan...

Edited By John Haine on 20/04/2023 09:21:59

I was too cowardly to solder mine so the spring is clamped with M2.5 nuts and bolts. The rod top weighs 9g, the rod 8g, and the bob 95gm, total 112g. As the pendulum will swing about 40,000,000 times a year, I'm hoping clamp pressure is enough to stop the spring from moving!

Dave

The bob has a very different area/volume ratio to the rod, so I'd expect it's temperature to lag the rod. Thermal conductivity across metal/metal joints in vacuum is not that good, but gluing the bob to the rod would help. Not sure about heat sink compound, it might evaporate. A drop of high vacuum oil in the joint would do. I'm still backing the proxy rod.

Solder paste is not a good option as it contains too much 'other stuff'. Since all mating surfaces should be jolly flat tin them and while hot wipe off the solder (dampish cotton cloth). Then hold bits in alignment with a jig / clamp and heat the whole lot up making sure the clamp pushes the joint together firmly.

I searched ( tediously) through all the pendulum threads and posts to find what was said about using carbon fibre as the pendulum rod - All I found was related to a carbon fibre in rod form, but not much help beyond that.

Since a single carbon fibre extracted from some high modulus Carbon Tow has a thermal expansion coefficient perhaps 6x less than aluminium, perhaps up to 3x less than steel, and that single fibre will easily support 10kg, is it not practical to use a fibre to support the bob - each end of the fibre fixed say 100mm apart at the top of the pendulum support structure and the bob fitted at the apex of the fibre down below. That would eliminate lateral swing, with a pure arc - along the lines of the dual support spring. I question the 2nd order effects of steel/brass springs - they will resist and assist the swing in their own way. . Perhaps this application of carbon fibre has been covered and rejected for good ( or bad?) reasons, but I could not find any reference.

If you really wanted to compensate the Carbon Fibre thermal performance with ambient temp, simply measuring the temp inside the tube would be sufficient - the fibre is so thin, it would hardly differ in temp from the sensor and surrounding low air pressure ( I assume not a high vacuum..). 

Edited By Joseph Noci 1 on 20/04/2023 12:53:41