Answer / sam

The question can be a bit confusing in that the word "range"
sometimes seems to mean different things to different people.
So let's just first of all clarify what we are referring to
when we use the words "range", "span" and "zero"

Range is the size or capability of the transmitter. This
means that if the specs on the transmitter says "range =
-300 to +300UOM (units of measure) it means you can
calibrate the transmitter for a total span of 600UOM and not
more. It also means you can calibrate the transmitter for
any span within this 600UOM range.
For instance you can calibrate this transmitter for -300 to
+250UOM or +140 to +300UOM and so on. Any values as long as
it falls within the range or capability of the transmitter.
You cannot in this case calibrate it for -400 to +100UOM
even though the top value is still withing the range the
bottom value is below the negative capability of the
transmitter. The area that you have calibrated is called the
calibrated span. In other words the value from zero to span
is also called the span.
a Bit confusing in that we also refer to the 100% value as
the span value and then we call the area from zero to full
scale the calibrated span as well. On Smart transmitter it
is less confusing since we here refer to the LRV and URV
instead of the zero and span values. Anyway the main thing
is to understand the difference between the range and span
values on a transmitter.
So, using the above transmitter again, you can say for
instance that you have a differential pressure transmitter
with a range of 600UOM but it has only been calibrated for a
total span of 350UOM. To give a more detail description of
how you have done this calibrated you can also say that zero
is -100UOM and span is +250UOM so the total calibrated span
is 350UOM.

Ok so back to the above question. Looking at it again I must
assume this person is trying to find out how to select the
correct range for a DP transmitter for a level application.
Obviously very important to be able to do but keep in mind
this procedure below is ONLY for a dry leg or open tank
level application and NOT for wet leg or capillary DPT's
selections.

The fist thing you need to do is measure the height of the
tank in mm. The next thing is to find the density of the
liquid you will be measuring and the third thing is to
decide where the transmitter will be installed. The first
two is easy enough to find but with the third one have one
or two extra things you want to consider. It is always
better to install any DPT below the bottom tap off point in
a close or open tank and you also need to think about the
Instrument Tech that needs to do the installation or do
maintenance on it later on. Try and install the transmitter
in a place that is easy accessible on a 2" pipe or something
like that. Also look at where the tubing or capillaries will
run, cable rack and details like this.
So finally if you have all the information you can now make
a small calculation to see what the maximum calibrated span
might be on a installation like this.
Measure the distance from the transmitter to the bottom of
your vessel and add this value to the total height of your
vessel. Let's say in total this distance is 3500mm. You
then miltiply this 3500mm with the density of the liquid you
want to measure and the answer will be the maximum
calibrated span that can ever be done on a instalation like
this. So in this case say the liquid is diesel with a sg of
0,85. The calculation will be 3500 x 0,85 = 2975mmH2O
Now you can convert this final mmH2O into any value you
prefer to work in. Sometimes the manufacturer might indicate
the range of the transmitter in Kpa or PSI so all you need
to do is convert you calculated mmH2O into that type of UOM
and you can then see how big your transmitter needs to be.

This maximum calculated value should fall in between 60 to
80% of the maximum positive capability of the transmitter
you select. If your transmitter is to big it will be very
insensitive and if it is to small you will not be able to
calibrate the correct span on it.
NB!!
There is one more important thing to remember when working
on open tanks or when you do a dry leg installations. The
calibration can only be in the positive so the range of the
transmitter you select must be marked as for exp in this
case: - 3200mmH2O to +3200mmH2O and not give a total range
of 3200 like -1600 to +1600mmH2O.
The calibration in open tanks or dry legs are always done
from atmospheric zero up into the positive area of the
transmitter and the negative area of the transmitter is not
used at all.
Watch out for this.
Good luck

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