Answer / sam

You first need to understand what pressure is. Without a
thorough understanding of pressure you will never understand
some of the basic concepts of instrumentation.

Air have weight. This can be proven by a simple primary
school experiment of taking a balancing beam and attached
two balloons to it. With the two balloons deflated the
balancing beam hangs exactly horizontal and the weight is
obviously equal on both sides of the beam.
If you now inflate one balloon the balancing beam will drop
down on the inflated end proving that air have a definite
and specific weight, since the volume of air on the one side
is now more than on the deflated side and therefore the
weight is more.
Pressure is measured on the surface of the earth as the
weight of the air from space pushing down on the surface of
the earth. The reason air and any other body have a weight
is due to the earth's gravity.
Since the gravity of the earth pull less the further you
move away from the earth's surface, the atmospheric pressure
changes as well and become less since the weight of air from
that point to space is less. Therefore on top of the
Himalayas the atmospheric pressure is much less than at sea
level. It takes a lot of years to get use to the low density
of the air up there and us normal people that are use to
higher density air find it difficult to breath up there. The
reason is the lower atmospheric pressure or the lower
density of the air due to the shorter distance from space to
that height.
We all know that water is always at the same level due to
the earth's gravity being the same all over the world, so
therefore if we look at the level of the sea it will be at
the same level in Canada as it is in Africa and the same at
the coast in China. It is therefore accurate to use the sea
as our reference point when we measure the atmospheric
pressure. So we therefore always refer to world standard
atmospheric pressure as it is at sea level and is therefore
a standard throughout the world. In simple terms the
distance from sea level to space at any place on the surface
of the earth is exactly the same. Therefore the atmospheric
pressure at sea level any place on earth is the same. This
atmospheric air weight is the reason why we have a
measurable amount of air pressure and is therefore referred
to as the atmospheric air pressure pushing down onto the
surface of the earth.
This atmospheric pressure is a specific amount of pressure
and is the same all over the world at sea level. The amount
is 101,3 Kpa (a) or 14,7 PSI.(a) or 760mmHg(a). You should
memorize these values. These values is also refer to as the
ABSOLUTE pressure or 1 atmosphere.
Note the (a) on the end?
If you write atmospheric pressures down you have to add the
(a) to indicate to other people that you are referring to
atmospheric pressure and not gauge pressure.

Gauge pressure is very simply what we call zero pressure at
1 atmospheric pressure at sea level. Just makes life a bit
easier to work in gauge pressure than trying to work in
atmospheric pressure all the time.

So to summarize, zero gauge pressure is equal to 1
atmosphere or 14,7 PSI atmospheric or absolute pressure.

Be aware you will find that some people will also write it
like this: 14,7PSIA or 101,3KpaA.

NB!!
Zero pressure absolute, is a complete vacuum and a complete
vacuum is impossible to achieve on earth. The closest you
will ever get is -0,99999Bar but never -1,013bar.

Where most of the confusion comes in is to find out from
what platform someone is refering to when they talk about
absolute pressures.
What you are suppose to do is to always stay on the platform
of gauge pressure, since this is the world standard we work
on in instrumentation, and then refer to absolute pressure
from there. This means you will refer to absolute pressure
in the negative like - 30Kpa or -200mmHg. In this case it is
in reference to gauge pressure so you do not add the (a).
Should you put yourself on the absolute pressure platform
you would refer to the same pressure as +70Kpa(a) and
+560mmHg(a).
Most of the time people refer to absolute pressure in
mercury but you may use any UOM you feel comfortable with.
In mercury, absolute pressure is -760mmHg standing on the
gauge pressure platform or 0mmHg standing on the absolute
pressure platform.
NB!!!
Be careful with this since there is a big difference to
calibrate a pressure switch for instance to 200mmHg(a) and
-200mmHg(g).
200mmHg(a) is equal to -560mmHg(g) and -200mmHg(g) is equal
to 560mmHg(a), so by calibrating the switch for -200mmhg(g)
and the specs refer to absolute pressure, your pressure
switch will not trip your application on 200mmHg(a) but only
on 560mmHg(a). The same when you work on a absolute pressure
transmitter. Try to stay on the gauge platform where
possible since it makes life a bit easier and less confusion
will occur.

Absolute pressure transmitters are normally used on vacuum
applications or applications where you work in gauge
pressure most of the time, but with a possibility of
negative pressures that could occur.

Referring now only to smart transmitters, we calibrate a
normal smart pressure transmitter for example to something
like this, LRV = 0Bar and URV = 2Bar.
The calibration of a smart absolute pressure transmitter
will look something like this, LRV = -1Bar and URV = 0Bar,
or LRV = -760mmHg to URV = 0mmHg, again saying on the gauge
platform.
It is not recommended but you can, if you need to, calibrate
your tx to absolute pressures as well but then you need to
chance your units of measure on the transmitter to absolute
pressure. That is if your transmitter have the facility to
do so, otherwise the transmitter will work with the measured
pressures as if it is gauge pressures and your transmitter
will not work.
Try and stay on the gauge platform to avoid confusing
yourself and others.

Watch out for the following as well.
With the previous era of pressure transmitters where we had
to use hand pumps and zero and span pots to calibrate the
pressure transmitters we also had to install absolute
pressure transmitters. The golden rule when working with
these transmitters was that you NEVER do a zero calibration
on them since they have already been calibrated by the
manufacturer for as close to absolute zero as possible. So
when you install the transmitter it will immediately
indicate atmospheric pressure and all you need to do is to
pump the transmitter up and adjust the span pot to where the
max pressure should be. Remember span adjustments do not
affect your zero but a zero adjustment will affect your span
calibration.
Now based on this you will find that someone will blow a
casket if you tell them you have done a zero trim on a smart
absolute pressure transmitter. This is a joke since all
smart pressure transmitter can measure up to -1Bar so all
smart pressure transmitters are really all absolute pressure
transmitters as well.
So as a final word from me, don't make things complicated.
Before you start your absolute pressure calibration on a
smart transmitter, open it up to atmosphere and do a zero
trim on it and then just add your -1Bar to +1Bar or whatever
the calibration should be in with the HART and give it back
to production. Simple as that. It will work perfectly, just
make sure the calibration values are the same as on the DCS
faceplate.
Good luck

Answer / s vedpathak

Dear Sam, although my basic knowledge about absolute &
gauge is clear, I am really glad to see your very
elaborative answer, from which my knowledge improved a lot.

Answer / allenbradley

I agree with sam except for "Remember span adjustments do not
affect your zero but a zero adjustment will affect your span
calibration".

When you adjust zero, span will not be affected, its just like you are moving your zero reference point but span is still the same, no change. When you adjust span it will affect both your zero and span.

Remember your zero shift and span error in basic calibration.

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