what is the difference between mmhg & mmwc?. i want to
calibrate a transmitter in the range of -100 to 300 mmwc.
Which is the reference for calibrate the Tx,if mmwc not
available?. Is it mmhg or mmho2?.
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Answer Posted / sam
Hi there,
mmwc (millimeters Water Column or Water Gauge)
mmH2O milimeters water)
Let me explain the pressure value of each since I can see
you have asked the same question earlier as well.
We all know that the gravitational acceleration of the earth
is 9.80665 m/s2 as a accepted standard around the world.
Our pressure system was also derived from this Universal Law
of Gravitation first developed by Sir Isaac Newton.
I think we all heard the story about Newton sitting under an
apple tree, an apple fell on his head, and he suddenly
thought of the Universal Law of Gravitation.
Anyway long story short, memorize the following formula and
you will not ever get confused again about pressures and levels.
1 meter height of any volume of clean water (1mWC) is equal
to 9,81Kpa. That is the true and accurate pressure value of
1meter height of water, regardless if it is in a small
tubing or the swimming pool or the ocean or the measured
height in the vessel you want to do a level calibration on.
a 1 meter height of clean water is always 9,81 Kpa.
Ok I know still a bit confusing.
Due to the above information we can therefore say 1000mm of
Water Column is = to 1000mm Water = 9,81Kpa or
1000mmWC = 1000mmH2O = 9,81Kpa
Now we all know the formula to calculate the calibrated zero
and span of our transmitters,
sg x g x h = Kpa
So for a vessel with a zero to span distance of 1000mm and a
product sg value of 1(clean water) we can do the calculation.
1 x 9,81 x 1 = 9,81Kpa
or if you want to work only in mmH2O,
sg x h = mmH2O
1 x 1000 = 1000mmH2O
So now you can calibrate your transmitter for the above
example for LRV = 0Kpa and URV = 9,81Kpa or if you set the
UOM on the transmitter to mmH20, you can calibrate it for
LRV = 0mmH2O and URV = 1000mmH20.
If the height or the sg value in your application differs
just substitute your values in the formulas and you will be
able to do any level calibration on any PIPED DP transmitter.
Remember these formulas are only for piped DP transmitters
and not for capillary or wet leg applications.
Ok let's move on to:
mmhg (millimeters mercury)
This UOM is normally used the measure vacuum and 1
atmosphere = 760mmHg.
You can also say 0mmHg = -101,3Kpa(a) since 1 atmosphere =
101,3Kpa. The (a) indicates that we are talking about
absolute pressure and not gauge pressure.
If you really want to confuse yourself you can try and
relate what I have said above in the mmH2O example, to mmHg.
You can say that 1000mmH2O = 73,56mmHg.
Forget about mmHg and don't use it, not even when you work
on vacuum applications. Stick to Kpa or PSI or the UOM that
makes sense to you and what you are use to. All the UOM are
very easy to convert to so always first convert the
measurement to units you are familiar with then start your
calculations.
Personally I only work in Kpa and mmH2O and I don't get
confused at all anymore.
So finally if you want to do you calibration as per your
question and you are SURE that the calibration is right, set
you UOM on your transmitter to mmH2O and set the LRV to
-100mmH2O and the URV to 300mmH2O and put it back on line.
If you do not use a SMART transmitter convert your
calibration values to Kpa or PSI and pump your transmitter
up to these values and set the zero and span accordingly.
-100mmwc to 300mmwc is the same as 0 to 400mmwc so if your
transmitter cannot measure in the negative you can set the
calibration for 0 to 400mmwc as well, just make sure the
zero is good at atmospheric presure.
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