Member postings for James Hall 3

Andrew- I entered into academe late in life, something like a third career, but after your time:

DipComSci - 1992-3
RA at Computer Lab 1993-1999
PhD - 1999-2003
Research Associate 2003-6

SillyOldDuffer:

" I think some might interpret this as willy-waving."

Well, I've had more graceful responses to an apology

The Interweb is packed with nonsense, advice and 'howtos' posted by idiots who have no idea of which they speak so in my experience it's always a very good idea to check provenance before accepting anything too readily. I merely pointed out that I do actually know what I'm talking about - why should you apparently have some sort of problem with that?

Andrew: Cambridge.

Martin Kyte wrote:

" Any time varying current is analog. I suppose you would agree that a sine wave is an analog signal. Well a pulse train is a series of sine waves of different frequencies. Do the Fourier transform. For USB the data is encoded and decoded as digital on the analog carrier as I said which is what makes it a digital communications channel but the connection is analog. It is splitting hairs but that's the way it is."

That's more sensible and describes the mechanism - but actually supports the necessary differentiation between the digital and analogue. It really is not splitting hairs as an appreciation of the difference between signal, channel, carrier and their 'layering' is essential to any understanding of digital (or any other form of) communications.

I apologise if I may have overreacted to seeing incorrect statements regarding digital signals but teaching digcomms to half a generation of undergraduates (most of whom have achieved good degrees and gone on to successful careers in R&D and industry) has made me particularly sensitive to any misconceptions of this sort.

Martin Kyte: you may well agree with Andrew Johnston on this one but you would, with respect, both still bee wrong.

This really is DigComms 101 - kids' stuff. To say that a digital signal is really an analogue one because it is carried over an analogue channel is as ridiculous as saying that an analogue signal is really a piece of wire because the piece of wire carries the electrons. I've tried to think up an illustrative analogy with the difference between the meaning of word and a dumb spell-checker for you further amusement - but it proved to be too contrived.

A digital signal is most definitely NOT an analogue signal. Over copper it is transmitted by way of an underlying analogue signal, but that is a different matter and the two signals are quite discreet.

Loadsa steel to remove machining a blank arbour to make an adaptor for a non-standard chuck. Cutting happily away with an indexable cutter amidst great clouds of fumes from hot cutting oil in a not-too-well-ventilated workshop. Now I love the smell of hot cutting oil, second only to that wonderful combination of hot oil, coal smoke and steam from a loco, but common sense says it's probably bad to be breathing it.

Thoughts or advice please - a couple of centuries of machine tools must have come up with some evidence on this.

Cheered up by nice new Tshirt from No.1 son for my 72nd Birthday.

I think that someone has (apparently) been pulling your plonker.

IFF the bits are getting transmitted and received correctly the quality of the cable will make absolutely no difference.

Unless you are sending signals over too long a distance or in a high noise environment the chances are that a cheap cable should be fine (subject to correct manufacture).

Be wary of advice from hobby mags on these matters - particularly when talking about hi-fi - kit reviewers and bloggers have to maintain interest and hence sales; one of the ways they do this is by cooking up fashions for one expensive tweak or another. Remember when sounds weren't worth listening to unless your speakers were hooked up with cable capable of serving the National Grid - but with five million cores!

Tony Wright 1 - I certainly will in future.

As a matter if interest, I had to make a second cylinder/port-face assembly for my Slim Jim - the first, made in accordance with the drawings, had a design error - you can see what it is in the pic below. A valuable lesson to check plans/drawings carefully before proceeding - it should have been obvious to me, but I naively assumed published drawings must be right.

Problem simply solved by using a wedge-shaped port block so that ports were still covered at 'full swing' and at my embryo stage it's all good practice. Spotted the problem on first assembly, second assembly with modification ran immediately on compressed air without tweaks or fiddles so I was very happy.

Thanks JasonB and Jeff.

I'll probably stick with 3mm pipe for this setup as that's what I've got. Joining with flanges looks good and I'd certainly use them for authenticity on any future larger or replica models - but will stick with nuts and nipples for the present as less fiddly - union nuts are fast and simple to make. I'm currently soldering an appropriately sized washer to the end of a pipe sealed to the nipple with a sandwiched washer or o-ring but soldering this on square at this size is very fiddly. I'd rather use cones but have no idea where to obtain them for 3mm pipe (for 1/8" would do) - any suggestions?

Just to clear up any confusion over terminology, I use 'nipple' to refer to the fitting onto which the union nut screws and 'cone' to refer to the tapered fitting soldered on to the end of the pipe which is then held onto/into the nipple by the union nut. This comes from my one-time plumbing background, but if wrong in the ME world will happily be put right.

Have my simple boiler up and running and next step is to connect to my simple oscillator (both from Stan Bray's book 'Making Simple Model Steam Engines) but the book omits to give any guidance on size of steam pipework.
I've assumed 3mmOD as I recall this from childhood Mamods, but would hate to put a lot of work in on fabricating unions etc. for the wrong sized pipe.

The engine (a Slim Sam) has a bore and stroke of 10x11mm and is a very basic oscillator.

Guidance would be much appreciated, as usual.

Thanks for your reply Jeff Dayman.

Leaking at the valve. I used the safety valve design shown by Myfordboy in his Youtube video series on making a simple boiler. It is essentially a cone in a taper with adjustable spring force. Of course I didn't seal the valve with instant gasket - I used it to form a cone-shaped silicon washer which makes a good enough contact to build pressure but as it's only a washer still allows a blow when required. Tested and adjusts and operates reliably at the 2bar I want.

I'll check out the designs you suggest.

Well, today the moment of truth: 30mins at 4bar and 30mins at 6bar. No leaks or weeps so as it's designed to steam simple oscillators at 2bar I'm happy.

First failure also today - my safety valve refused to hold against any pressure. Cured the problem with Instant Gasket - but don't know how it will stand the test of time and it's not really exactly an elegant solution. Any suggestions for a relatively simple safety valve design?

Makes sense - thanks Brian.

Thanks for this Jeff. One thing puzzles me (or am I just being dense) - why fill with water? Compressed air and immersed in water to show any bubbles would surely serve the same purpose.

Seems to have turned out ok with all joints nicely filled with solder and good fillets where required. Fitments not made yet so no pressure test but in the meantime no drips or weeping when filled with water. A certain amount of expensive solder strayed onto areas where not needed (forgot the tippex) but only cosmetically damaging and not strayed into bush threads. I'm content so far with my first boiler-making and silver-soldering adventure.

Once again, thanks to all for the advice.

Thanks again for all of the helpful replies.

Just one minor point - should I rinse off pickling solution (whatever used) before soldering? Should soldering be done asap after pickling or can it wait a while?

Thanks.

Tony Wright - I was wondering about using battery acid - got a couple of old car batteries hanging around waiting for the tip to reopen.

Think that I can handle the stuff. Old enough to remember when you could go into the local chemist and buy concentrated sulphuric/nitric/hydrochloric on the way home from school for the chemistry set (and possible other things which we won't go into!).

No problem with the soldering caused by residual lead salts which battery acid is bound to contain, then?

Thanks to all for such speedy replies already received.

I must have been suffering from a misapprehension, I think: that citric acid is used to clean flux residues after soldering - but it seems it is good for cleaning up beforehand. Does the copper need to be mechanically cleaned before 'pickling' i.e., in addition to the obvious removal of any oil or grease?