​"Practical concept" of static pressure in DP transmitters

​In a DP transmitter one of the very important ingredient to check is Static pressure.

"IT IS IMPORTANT TO KNOW THAT STATIC PRESSURE AND DIFFERENTIAL PRESSURE IN DP TRANSMITTER ARE TWO DIFFERENT TERMS"

​So what is Differential Pressure Range 

When you are measuring a DP of 50 bar at your Transmitter (Tx) it simply means the difference between HP leg – LP leg is 50 Bar.

Case 1 :-
This could mean HP is sensing 200 Bar-g – LP is sensing 150 Bar-g = 50 Bar

Case 2 :-
Or it could mean 500 Bar-g at HP – 450 Bar at LP=50 Bar

So what is Static pressure?

In simple words what “common” pressure both legs of your Tx sense is termed as static pressure.
 
In case 1 above at HP leg 200 Bar-g and LP leg 150 Bar-g so 150 is Hitting common on both legs !!! Static pressure is 150 Bar-g.

In case 2 above HP leg is sensing 500 Bar-g & the LP leg is sensing 450 Bar-g, so the 450 Bar-g is hitting common on both legs !!

Static pressure is 450 Bar-g.
So you see the difference the DP is same 50 Bar in both cases but the static pressure is different in both cases.

What Difference Does the Static Pressure make in my engineering decision??

​Below 2 stories would make it clear

Story 1 – Where Max Static Pressure Comes Into Play

Datasheet states a DP of 0 to 20 inH2O

It means the difference between two legs of Tx must be 0-20in H2O

So you select range 1 of Rosemount 3051CD Transmitter (Refer snap below of Rosemount catalog)

 Image Courtesy : Rosemount
 
SO YOU ARE HAPPY !!! BUT WAIT A MINUTE DID YOU CHECK THE STATIC PRESSURE LIMIT IN CATALOG?

Below is a snap of static pressure limits of 3051CD (take from catalo g)

Let’s assume your datasheet says static pressure of 140 Bar so you will damage you sensor !!!!!

Because your 3051CD Tx can sustain a static of 137.90 Bar only!!

Story 2 – Where Minimum Static Pressure Comes in Play

Again if you see the lower static pressure limit is mentioned as 0.5 Psia

AND during my project we were to measure 0-8 mm HG so range 1 is ok

BUT the static pressure was 0.2 Psia

And the minimum static pressure as per catalog is 0.5 Psia so this violates the minimum capacity!!!!

if you still need to measure at conditions below 0.5 Psia then read below

Reason

The minimum of 0.5 Psia is mentioned because the Transmitter contains silicone fill fluid. And as you know at atmospheric pressure water boils (changes state to vapor) at 100 °C but water boils at 0°C at Full Vacuum .
 
Similarly the Fluid at Transmitter would get converted to vapor (Boil) at such low pressure 0.2 Psia

Thus we had to change hookups such that a positive head pressure was maintained and the transmitter never saw a pressure of below 0.5 Psia . (*1)

*1 – This was vendor clarification of stating 0.5Psia in Catalog.

Hope this article was useful

PS: this is as per best of my current understanding!!