Barometric pressure

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Are there any clever sorts on here that can explain the difference between absolute and relative barometric pressure.

Please only use words with less than four syllables.

Thank you

Yours sincerely

Absolutely confused.
 
ABP is the weight of a column of air above the instrument/you at sea-level. RBP is the same but at the altitude you actually are. /forums/images/graemlins/tongue.gif
 
Hi Stingo
I don't know if this the sort of problem you have but here goes. Say you have an air bottle which is fully discharged, that is in normal speak 'empty'. The gauge on it will usually register zero. However there is really still air in the bottle at the surrounding (ambient) air pressure - that is usually refered to as one BAR. So RELATIVE to the surounding air the pressure is zero but its ABSOLUTE pressure is 1 BAR.
 
As someone who has built a life around pressure measurement, designing and manufacturing pressure transducers for all sorts of applications, including barometers, I can honestly say this is the first time I have ever heard the term "relative atmospheric pressure".

As far as I can see, it has no logical significance. The bishopsperformance definition looks like it comes from someone who may be knowledgeable on engines but knows little about pressure.

Pressure measurement comes in three forms: gauge, differential and absolute.

Absolute pressure is defined as the pressure above a perfect vacuum.

Differential pressure is the pressure difference between two pressures, for example, across a flow measurement orifice.

Gauge pressure is a special case of differential pressure and is defined as the pressure relative to the ambient pressure (as in a conventional pressure gauge) and is simply a useful and easy means of looking at what's going on in every day applications, e.g. car tyres. The ambient (normally barometric) pressure can be anything and it doesn't matter.

A pressure transducer will either have its reference sealed, usually under vacuum, or vented to atmosphere.

Barometric pressure is always absolute.
 
[ QUOTE ]
As someone who has built a life around pressure measurement, designing and manufacturing pressure transducers for all sorts of applications, including barometers, I can honestly say this is the first time I have ever heard the term "relative atmospheric pressure".

As far as I can see, it has no logical significance. Barometric pressure is always absolute.

[/ QUOTE ]

I thought I knew the difference, but now am confused!

I have come across this term on various occasions apparently used to differentiate between atmospheric pressure at sea level, and a 'corrected' or 'relative' reading of a barometer which is positioned at a height above sea level but which has been adjusted so that it gives a pressure reading 'relative' to sea level

My confusion arises from the fact that barometric pressure is constantly changing - which in my limited and totally amateur understanding of the term - means it can not be an 'absolute'.

Well, not in the UK anyway, for more than about 3 minutes! /forums/images/graemlins/smirk.gif
 
The use of the word 'absolute' is not referring to it being a final figure; it is in reference to an 'absolute' namely the perfect vacuum.

The reading may be constantly changing but is still an 'absolute' reading.

I monitor gas pressures with gauges and refer to these readings as psig or barg, in other words psi(gauge) or bar(gauge). It is term and means that if someone wants to convert to an 'absolute' pressure reading then they can add the one atmosphere themselves.
 
I can find two uses of the term "relative barometric pressure".

One is as described by Fireball namely that is is the offset to correct a barometer for altitude (height above or below sea level). However I think he is wrong in saying that the sea level pressure is the absolute barometric pressure I think absolute pressure means the actual pressure that an uncorrected barometer (or a Fortin barometer) will read at its location. For example when the sea level pressure is 1000mb, where I live, 300 ft or so above sea level, the absolute pressure is only about 987mb.

The second use seems to be to describe the presure trend bar graphs on some electronic weather stations.

It would help to know the context of the expression to which the original question refers.
 
Please only use words with less than four syllables.

A short sentence , which is why it was apparently missed by all that have posted so far , because none of the replies complies .

Would somebody that knows the answer , but also speaks KISS please respond
 
Hi John
I agree with your definitions. I too spent some time on tranducer work and have never heard of this 'relative barometric pressure' but I can see where it could be coming from. Obviously barometers work at any altitude but when someone refers to Barometric Pressure I assume he means the absolute pressure at sea level - unless he says otherwise. Airfields etc. often say Local Barometric Pressure so pilots can set their ana. altimeters
 
[ QUOTE ]
Please only use words with less than four syllables.

A short sentence , which is why it was apparently missed by all that have posted so far , because none of the replies complies.

[/ QUOTE ] Difficult, since "barometric" contains four syllables and was in the original question.


[ QUOTE ]
Would somebody that knows the answer , but also speaks KISS please respond

[/ QUOTE ] I thought my final sentence, from someone who only thinks he knows, was just that:
"Barometric pressure is always absolute."
 
A mechanical altimeter is simply an anaeroid barometer with appropriate calibrations. It must always be "zeroed" at the start of a flight to allow for the barometric pressure of the day. It still isn't very accurate, because the lapse rate of pressure with altitude is non-linear, and other things. However, that is not so important so long as the zero remains reasonably stable and you know when the ground is approaching and you can look out of the window!

Normally a relative measurement has a stable reference point, e.g. temperature. Degrees Celcius is a relative temperature measurement - relative to the freezing point of pure water at specified temperature and pressure. Degrees Kelvin are absolute, relative to absolute zero temperature. The reference point doesn't move.

With barometric pressure, it varies all over the place with altitude, air movements, temperature.... A barometer always measures absolute pressure at the point where it is. When you take that measurement, you have little idea what the pressure is somewhere else, so what is it relative to? It is possible to extrapolate making certain assumptions, but that does not make a relative measurement, simply an approximation.

Would you think of calling a temperature measurement "relative" simply because you take it half way up a mountain and assume that the adiabatic lapse rate is valid?

Barometric pressure is always absolute.

Sorry Trevera.
 
OK Stingo - this is it - the absolute definition.

I fitted a new barometer a couple of years ago and it worked fine. Perhaps the pressure wasn't quite the same as anyone else's but it moved up and down between Rain and Stormy RELATIVE to everyone else's and I was happy when filling in the log to be writing - Glass is really low and we are in for a storm etc.
Whilst on a pontoon last year we were parked next to a really nice boat and the woman in it was an ABSOLUTE cracker. We were discussing blood pressure - mine seemed to have risen at the time and then we got onto barometric pressure as I noticed hers was higher than mine and that she was beyond the point of change and heading nicely towards fair whilst I appeared to be wet.
I fiddled with my screw and got my reading up to the same as hers. Shortly after this the sky went black as her husband returned and he casts a big shadow - fortunately he and Dear Heart had been chatting in the bread shop so he thought I was probably ok as DH comes across as a decent sort etc. ABSOLUTE and RELATIVE...there you have it
 
... a bit less confused now, thanks! I guess the definition problems come from sales blurb written to sound convincingly esoteric by salesmen, not scientists!

And thanks for clarifying the term 'absolute' Fatlady.

<Would you think of calling a temperature measurement "relative" simply because you take it half way up a mountain and assume that the adiabatic lapse rate is valid?>
No, but atmospheric pressures quoted in weather forecasts are always related to the pressure obtained at sea level. So 250 m up a mountain the barometer wil naturally give a lower reading, which is only useful to the weatherforecaster if it is converted to the relative sealevel pressure.

Maybe this is where the confusion in terminology between different functions arises, particularly when 'related ' to the watered down 'idiots guide to the weather' that Tv companies see to think we need nowadays!
 
Re: Barometric vs Atmospheric pressure

Looking back, I can see that the question did NOT enquire about R and A <span style="color:red"> atmospheric </span> pressure - in which context these terms can only be used, and they are used by people in that milieu, as distinct from fat lady experts.

My simple answer to a simple question was directed purely to atmospheric pressure, the reply would otherwise be meaningless. /forums/images/graemlins/tongue.gif
 
Dear Madam,

It has come to my attention that you have professed to an expertise in pressure terms and measurement which is quite extraordinary. The Wikepedia definitions require your kind examination, in order that you might write and set them straight, esp on the subject of relative or gauge pressure.

Regards,

An admirer... /forums/images/graemlins/tongue.gif /forums/images/graemlins/grin.gif

PS did you not mean the ELR rather than the ALR??
 
Dear Sir,

Thank you for your kind words and worthy suggestion. However, since Wikepedia contributions are made by mere mortals such as I, who think they know it all but in practice may not, I do not regard it as a reliable source of information and I am content to leave it in whatever impoverished state it currently resides.

By the way, what is ELR?
 
My Dear Madam,

Thank you for your felicitous reply. As you say.

FYI...

ELR = Environmental Lapse Rate, ALR = Adiabatic Lapse Rate has children MALR and DALR children, aka 'Wet and Dry'

Environmental lapse rate
The environmental lapse rate or temperature lapse rate, which refers to the change of temperature with altitude for the stationary atmosphere. The environmental lapse rate at a given place varies from day to day and even during each day. As an average the International Civil Aviation Organisation (ICAO) defines an international standard atmosphere with a temperature lapse rate of 6.5 °C per km (3.57 °F/1000 ft or 1.99°C/1000 ft) from sea level to 11 km. However, the temperature of the atmosphere does not always fall steadily. For example there can be an inversion layer in which the temperature hardly falls or even rises with increasing height.

Dry adiabatic lapse rate
The dry adiabatic lapse rate (DALR) is the rate at which a rising parcel of unsaturated air, such as a thermal, will lose temperature. Unsaturated air has less than 100% relative humidity, i.e. its temperature is above its dew point. The term 'adiabatic' means that no heat will be gained or lost from outside the parcel. The DALR is a constant 9.78 °C/km (3 °C/1000 ft, or 5.37 °F/1000 ft). The adiabatic lapse rate can be explained by simple fluid mechanics. As a parcel of air travels downward through the atmosphere, it experiences a rise in ambient hydrostatic pressure. The pressure compresses the parcel, doing work on it and thus increasing the energy and temperature in the parcel. The converse applies for rising air: it experiences a drop in ambient pressure, so it increases in volume, loses energy, and its temperature drops. Air has very poor thermal conductivity, and the bodies of air involved are very large, so transfer of heat by conduction is negligibly small. Thus, the process may be approximated as adiabatic (perfectly insulated).

Moist adiabatic lapse rate
When the air is saturated with water vapour (reached its dew point), the moist adiabatic lapse rate (MALR) or saturated adiabatic lapse rate (SALR) applies. It varies with temperature and pressure, but is usually near 4.9 °C/km (2.7 °F/1000 ft or 1.51°C/1000 ft). The reason for the difference is that latent heat is released when water condenses. Even though there are no more than 10 grams of water in a kilogram of air at 15 degrees Celsius, water's high heat of vaporization creates a significant release of the energy when it condenses (and is an important source of energy in the development of thunderstorms). Until the moisture starts condensing, the parcel of air cools at the DALR so any air that is unsaturated can be assumed to be 'dry'.

Regards,

Your ever-faithful secret admirer... /forums/images/graemlins/tongue.gif
 
Kind Sir,

Thank you for that very interesting response. I have learned something today, more about terminology than anything else. However, I would think that using an average ELR has no more significance in practice than ALR, since both have fixed values which may be quite different from the actual lapse rate.

What happens when dew point is reached is far more complex than the basic polytropic equation (where the index is the ratio of the specific heats Cp and Cv) required for the adiabatic expansion of dry air, particularly so since the moisture content of saturated air is very non-linear with temperature. I would not even think of using it to work out whether I should take my anorak up the mountain.

Going back to your earlier post, I suspect that you really do appreciate the difference between weight and pressure. I am glad you didn't use mass, as seems to be the trend these days.

I am, sir, deeply indebted to you for making me less befuddled and introducing me to the cornucopia of terms which are now used to describe what used to be basic concepts.

I shall now sing.

Yours,

la donna grassa
 
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