Lamppost cable. What are they for?

[Harry Bloomfield]

Indeed - and in the days before washing machines, my grandmother had a vessel (which I'm almost certain was made of steel) in which she boiled up her 'washing', which was known as a "copper"

Because they were, indeed, once made from copper.

 

[JohnW2]

Because they were, indeed, once made from copper.

Exactly, and that's my point! ... just as telegraph poles were once for telegraphy and plumbers one worked largely with lead - but (I personally think very reasonably) we haven't felt the need to invent new words because of the changes which have occurred over time.

 

[Harry Bloomfield]

If I had used the word 'suction' when at school, my Physics teacher would have freaked out, since he was always reminding us that such a thing (which implies that one can have a 'negative pressure') does not exist

You still suck on a straw, never vacuum the water

 

[JohnW2]

You still suck on a straw, never vacuum the water

Try telling my Physics teacher that

His (valid) point was that 'sucking' (or 'suction') related to reducing pressure to a ('positive') pressure below atmospheric pressure, so that that the 'net' atmospheric pressure then 'pushed' air in the direction one wanted. In the absence of atmospheric pressure, no amount of 'sucking' would achieve anything!

 

[ebee]

A bit like the bar when relating air pressure to atmospheric.
It used to concern me that plumbing and heating bods talked about 1 bar to 3 bar etc .
To me 1 bar was atmospheric (pretty much) so 2 bar should be 1 bar above atmospheric but it seems they talk about the bar using the atmospheric 1 bar or thereabouts as the datum rather than using zero pressure as the datum.

I do suppose we could view a few things similar - temperature - we do not universally measure in absolute zero or kelvins but we take an easy to refence like 0 degrees C quite often and 100 degrees C as another and most instances we refer to the relationship of what we are thinking as, usually, somewhere between those two refence points and we can go negative from zero if we want to and it does ok for everyday usage.

C is for Celsius but used to be for Centigrade , how does one "sound more metric" than the other? I give in!

 

[bernardgreen]

What ever happened to "differential pressure" ?

 

[ebee]

Good question Bernard

 

[Harry Bloomfield]

To me 1 bar was atmospheric (pretty much) so 2 bar should be 1 bar above atmospheric but it seems they talk about the bar using the atmospheric 1 bar or thereabouts as the datum rather than using zero pressure as the datum.

I have never thought about that, never realised before, so many things, are referenced to zero, at atmospheric pressure, or roughly 1 bar. My water pressure gauges all drop to zero, my tyre pressure gauges all show zero.

 

[SUNRAY]

What ever happened to "differential pressure" ?

In daily use.

MPXV5010DP | NXP Differential Pressure Sensor, PCB Mount | RS

uk.rs-online.com

to quote one of hundreds of sensors except seems we now use Pascals.

 

[bernardgreen]

and the good old and reliable glass U tube manometer

Nerrad Tools U-Gauge 30mb Manometer NTUG30 - Baker and Soars - Leicester

This Nerrad Tools U-Gauge 30MB Manometer NTUG30 is designed to test gas pressure. It is also made in durable plastic

www.bakerandsoars.com

 

[IJWS15]

Pressure can be "absolute" (zero is absolute vacuum) or "gauge" (zero is one atmosphere, approx 1 bar/15 psi). Can't be bothered to quote it in lb/sq ft (should really be pounds force per square foot), pascals or butterflies per acre.

Using gauge pressure you can have a negative pressure of up to one bar.

It is important to know what your pressure gauge measures and IIRC manometers are all "gauge"

 

[SUNRAY]

It is important to know what your pressure gauge measures and IIRC manometers are all "gauge"

In essence yes but the calibration marks can be anything relating to system pressure

 

[JohnW2]

A bit like the bar when relating air pressure to atmospheric. .... It used to concern me that plumbing and heating bods talked about 1 bar to 3 bar etc .
To me 1 bar was atmospheric (pretty much) so 2 bar should be 1 bar above atmospheric but it seems they talk about the bar using the atmospheric 1 bar or thereabouts as the datum rather than using zero pressure as the datum.

Yes, I've also often wondered about that. However, I suppose it's understandable that we talk of pressures 'relative to atmospheric pressure', since in all normal circumstances we are unavoidably 'within the atmosphere', hence experiencing atmospheric pressure (as a 'background reference point').

This also relates to my Physics master's problem with 'sucking'/'suction'. Whilst it is not possible to have a negative absolute pressure, one certainly can have a negative relative pressure (pressure difference) - hence if what you and I would call 'sucking' resulted in the pressure in our mouth/straw/whatever reducing to, say, 0.6 bar, that would be a 'negative relative pressure'of 0.4 bar (relative to atmospheric.

As you say, it's sort-of smilar with temperature, although we do then use °C to indicate when we are talking about a relative temp (relative to the freezing point of water) but use K to refer to absolute temps - less confusing than using 'bar' for both absolute and relative pressures.

However, pressure is not the only thing we measure relative to the environment in which we normally live, hence making relative things seem absolute. We happily talk about speeds/velocities relative to the surface of the earth, and probably think of them as 'absolute' speeds/velocities, despite the fact that the surface of the earth (and hence anything moving relative to that) is moving at incredibly high speeds/velocities relative to other things in the universe.

Kind Regards, John

 

[SUNRAY]

As you say, it's sort-of smilar with temperature, although we do then use °C to indicate when we are talking about a relative temp (relative to the freezing point of water) but use K to refer to absolute temps - less confusing than using 'bar' for both absolute and relative pressures.
Kind Regards, John

and people will happily accept a change from 10°C to 20°C is doubling the temperature but so is a change of 50°F to 100°F despite the fact 10°C=50°F and 20°C=68°F and 38°C=100°F

 

[JohnW2]

However, pressure is not the only thing we measure relative to the environment in which we normally live, hence making relative things seem absolute. We happily talk about speeds/velocities relative to the surface of the earth, and probably think of them as 'absolute' speeds/velocities, despite the fact that the surface of the earth (and hence anything moving relative to that) is moving at incredibly high speeds/velocities relative to other things in the universe.

... which, of course, essentially means that there is no such thing as 'absolute' speed or velocity ...

... and that's when I start getting confused about things like kinetic energy and momentum. They are both, mathematically, functions of velocity, yet velocity can be anything, depending on what it is measured relative to.