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Member postings for Robert Atkinson 2

Here is a list of all the postings Robert Atkinson 2 has made in our forums. Click on a thread name to jump to the thread.

Thread: Triangular Screws
08/10/2019 19:14:40

Hi Neil,

Diecast box screws, including Taptites are normally M3.5. This odd size appears to be for compatability with 4BA which was used on the original Eddystone boxes (and electical "back" boxes)

This rather than the tri-lobular form is likely why ir seemed small for a M4.

Robert G8RPI.

Thread: Question from a customer
07/10/2019 12:54:22

Great Post SOD

Re "Ought to be emphasised that the size of the boiler is all important. It limits their capacity to store energy. "

So true, this is where the 250 Bar/Litre "rule comes from. If the product of pressure and volume is less than 250 many of the regulations don't apply. 250l tank at 1 bar is same energy as a 1l at 250 bar.

Robert G8RPI

Thread: Tingling from Myford Super 7
07/10/2019 12:46:58

The problem is poor insulation and earthing of the Dewhurst switch. First thing is to check (or more likely provide) the earthing on the switch chassis and cover.
Theses old switches probably would not meet current safety standards. A modern cam switch (proper make and suppler, not junk off ebay or alibaba) in a proper enclosure would be a better choice.

Yes it is something you should be worried about.

Robert G8RPI.

Thread: Question from a customer
07/10/2019 07:29:12

When you are talking about ultra high vacuum (UHV) such as is used in semiconductor manufacturing and electron microscopes, pressure is no longer measured as a force. You are talking about how many molecules of gas are present. Measurement is usually electronic as in how much current will flow. The pressures are measured in Pascal and UHV is below 1x10^-7 Pascal or 0.0000001 Pa. One atmosphere is about 100,000 Pa (101325 standard sea level 1013.25 millibar) so that is 1x10^-12 Bar or 1.47^-11 PSI 0.00000000000147 PSI.

The pumps used look like multistage axial turbines running at very high speed but are actually sweeping individual molecules out of the UHV volume rather than pumping. They have to be "backed" by a high vacuum system of less than 100 Pascals (0.015 PSIA).

Robert G8RPI.

Thread: Alternative metal sources?
06/10/2019 20:19:52

CMM is a coordinate measuring machine. A surface table with a touch probe and 3 axis measuring system

The one in the skip even had the Rennishaw touch probe still fitted. )-:

Thread: Question from a customer
06/10/2019 16:34:46

As to vacuum gauges not being made for more than 1 bar, there are plenty, evey vacuum braked train has at least two and they used to run at 21". Brake cylinders also work on differential pressure between the vacuum developed in the train pipe to hold the brakes off and the atmospheric on the other side trying to put the brakes on!



Not on this planet. The maximum differential you can get with a vacuum is dictated by the local air pressure. This is variable but at sea level is about 1Bar, 14.7 PSI, or 30" of mercury. (Hg) The brake gauges are 21" of Hg not 21 PSI.
30" hg is 407" of water so that is the absolute maximum you can suck fresh water UP with a pump about 30 foot in practice.


Robert G8RPI

Edited By Robert Atkinson 2 on 06/10/2019 16:50:18

06/10/2019 14:20:13
Posted by JasonB on 06/10/2019 13:45:26:

Can I ask why the differential will only be 14-15psi. If the heat were removed when there was say 100psi in the boiler would there not be a drop of 85psi on the volume of steam?

Also why add a snifting valve to the setup as the existing feed water clack is doing the same thing.

Edited By JasonB on 06/10/2019 13:46:59

There would be a relative drop of 100 PSI but even with a full vacuum inside the boiler there is only atmospheric pressure, 1 Bar, outside. The pressures quoted for steam work are "Gauge" or relative to local air pressure. If you connected an absolute pressure gauge (which is referenced to zero or a vacuum) to the same point it would read 14-15 PSI higher depending on local air pressure.

Robert G8RPI.

06/10/2019 13:36:32

As Dave says, maximum differential is less than 15 PSI (higer at sealevel on a nice day)

Tanks can implode see Anyone wo has see an old, non-protected TV tube smashed will testify to the forces involved with larker volumes.

Robert G8RPI.

Thread: Oiling an old clock
06/10/2019 12:55:17

If doing the immersion trick, white sprit is a lot safer than petrol.

Robert G8RPI.

Thread: UK stepper motor supplier
06/10/2019 12:17:27

I've used Motion Control Products professionally with no issues

Head office is in Bournemouth

Robert G8RPI.

Thread: Alternative metal sources?
06/10/2019 12:01:58

It's two things,

Health and Safety - no digging in skip in case you cut yourself. That I can sort of understand.

Controlled materail #most of the work material has to be traceable and there is a concern that somehow uncontrolled stock would get re-used either by us or someone making bogus parts. They won't even put off-cuts back in stock. If making some 0.51m parts from 1m length of stock 0.49m goes for scrap.

A couple of years ago there was a complete CMM in the skip.

They have recently agreed to put timber and office furniture in a bar for a week for staff to take but no electrical or mechanical equipment (H&S again). And they have donated some old commputer equipment to charitable causes. Our WEEE skip is huch high quality they don't charge to take it away.

05/10/2019 20:55:40

What I find really frustrating is walking by the scrap skip at work and seeing off-cuts of new stock still marked with the material specification and not being able to to take any. I've stopped looking.

Thread: Unusual GPO hammer?
29/09/2019 22:31:29

I inherited a small hammer from my father in law (an American) that looked similar but the back end was straight with a narrow slit in it. It looked like a claw but almot closed. I discovered that it is intended for holding pnel pins or tacks for the first blow. It's magnetised with one side North pole and the othet South so the pin is helad firmly and centered between the two halves. Similar to this

Robert G8RPI.

Thread: Aero Fuel
29/09/2019 20:04:31

Biggest problem with Methanol is the energy content. Methanol has around half the energy content of petrol so you would have to carry much more than twice the weight for the same range.

Robert G8RPI

Thread: Water in fuel
27/09/2019 20:16:43
Posted by RMA on 27/09/2019 09:35:41:


Nice one! I often wonder what happens to the samples taken from the main tank at my airfield when we have a delivery. I'll ask around, not sure what Avgas would do to the car.

The samples should be put in a waste container and disposed of properly. Eye witness evidence, stains on the ramp and dead grass on parking areas indicate it is often just dumped.

Avgas is leaded and will kill your catalyst / O2 sensors and is illegal for road use as the corret tax has not been paid. Jet A is not good for modern diret injection diesels du to poor lubricity and i likewise illegal for road use.

Robert G8RPI.


Edited By Robert Atkinson 2 on 27/09/2019 20:18:14

26/09/2019 12:43:58

Washing the ethanol out with water is pointless in this context because the washing will leave MORE water in the petrol than before. It seems pointless to me anyway.

Just looked at the ethanil kit £50 for a clear fuel can with a drain tap! And how do you dispose of the petrol contaminated water ethanol mix?

Not all old cars have the issue with modern fuels. When unleaded was starting to be introduced in the mid to late 80's it was a lot cheaper than regular leaded (usual tax incentive to lull us into a false sense of security) I had a 1975 Jensen Healey and used to use unleaded, if the cashier would let me! They could not believe that a 10 year old car was OK on unleaded and often took some persuasion to turn on the pump. The JH had a Lotus 907 2l, a modern design and the one I had was a LHD Californian spec example that was specifically meant to run on unleaded.

Edited By Robert Atkinson 2 on 26/09/2019 12:46:41

25/09/2019 20:12:22

Dave W beat me to it on ethanol affecting tank coatings. As I mentioned earlier it badly affects some light aircraft tanks. It has some odd effects. Materials that are fine with just petrol or just ethanol can be softened by a mixture of the two.

Robert G8RPI.

Thread: Silent compressor problem
25/09/2019 07:16:33

A company I worked for used lots of these and they should not run too hot to hold your hand on. Have you checked the oil level?
If that is OK the next thin to check is the capacitor (or thermal time delay relay if fitted, but most of these except the very small ones used a capacitor)

Robert G8RPI.

Thread: Water in fuel
24/09/2019 13:00:34


Sorry not a chemist but do know something about alcohol in aircraft fuels.
Ethanol in petrol does not attract water, but would rather dissolve in water than fuel so if you add say 2ml of water to a 50ml sample of fuel containing 2% ethanol, shake it and lit it settle there will be 4ml of "water" at the bottom of the container. This is of course 2ml of water and 2ml of ethanol. The ethanol does not attract extra water to the fuel. The test above is an approved method for determining ethanol content in motor gasoline "Mogas" (petrol) before using it in aircraft that are approved for it subject to ethanol limits. Amongst other things ethanol can cause some plastic or GRP fuel tanks to fail.

To get water out of your fuel, add isopropyl alcohol (IPA) This should ideally be 99% pure but 70% or higher is OK. It absorbs water and is miscible with petrol so keeps the water in suspension and allows it to be carried into the engine and discharged through the exhaust at a constant rate that does not affect combustion. Most commercial fuel "dryers" are just IPA.

Robert G8RPI.

Thread: Petrol Gen for 7 1/4 locomotive
15/09/2019 11:50:26

Where are you located Andy?

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