Hydrostatic formula

Has anyone used (2) pressure sensors andthe hydrostatic formula to determined the fluid density?

Nope. But it sounds interesting.

You've got to have a pretty big depth differential between the sensors or very sensitive sensors to make it work.

You know, you need roughly 33ft (10 meter) of water for 14.5 psi (1 bar) of difference.

Why do you ask?

Pete.S. said:

Nope. But it sounds interesting.

You've got to have a pretty big depth differential between the sensors or very sensitive sensors to make it work.

You know, you need roughly 33ft (10 meter) of water for 14.5 psi (1 bar) of difference.

Why do you ask?

Click to expand...

I'm going to attempt to throw something together this week to try to see if I can get a correct density reading. I was hoping someone has done it before so I could pick their brain.

I was planning on having 2 ports for compressed air and teeing both airlines off to a pressure differential sensor that way I can manipulate a bigger pressure differents and then compensate that factor in the program some how??

The more I think about it, an ultrasonic transmitter and receiver to measure the travel speed would probably be more a fool proof way to determine the tanks density.

Snap25 said:

I was planning on having 2 ports for compressed air and teeing both airlines off to a pressure differential sensor that way I can manipulate a bigger pressure differents and then compensate that factor in the program some how??

Click to expand...

I'm sorry but that will not work. It's the difference in height and the mass of the fluid and gravity that causes the hydrostatic pressure. You can apply all the pressure you want. The differential will be the same.

Snap25 said:

The more I think about it, an ultrasonic transmitter and receiver to measure the travel speed would probably be more a fool proof way to determine the tanks density.

Click to expand...

Travel speed of what? This sound more like you're trying to measure viscosity.


Either way I suggest doing a search on the net for sensors that do what you need. You might not want to use them but that should give you an idea of what can be done and how it is done.

I have worked with several different in-line viscometers if thats what you are looking for, there are a several factors that will change it and you may need to know the values, such as temperature will affect it a lot and needs to be compensated for, the sampling is a little on the slow side so not sure how fast you need to react if out of spec, also some viscosity cups but the inline worked better for our apps

D =(PL - PH) / L
D = Density
PL = Lower Differential Pressure Transmitter
PH = Higher Differential Pressure Transmitter
L = Length beween PL & PH measured datum to datum

1 = (16-4)/12

---
| |
| |----O ATM or Top of Tank if Pressurized EX Reading 4
| | |
| | | Constant Distance EX 12 Feet
| | |
| |----O-> ATM or Top of Tank if pressurized EX Reading 16
---

Your tank level must always be higher than the upper sensor.

ps gama you are missing the gravity constant..

DeltaP=Density*Gravitiyconstant*distancebetween sensor

Pete.S. said:

Travel speed of what? This sound more like you're trying to measure viscosity.

Click to expand...

Measure the travel time time from the transmitter to the receiver. I actually think their our a few ultrasonic sensors out their to measure density.. I'll have to do a little more research.

psgama said:

D =(PL - PH) / L
D = Density
PL = Lower Differential Pressure Transmitter
PH = Higher Differential Pressure Transmitter
L = Length beween PL & PH measured datum to datum

1 = (16-4)/12

---
| |
| |----O ATM or Top of Tank if Pressurized EX Reading 4
| | |
| | | Constant Distance EX 12 Feet
| | |
| |----O-> ATM or Top of Tank if pressurized EX Reading 16
---

Your tank level must always be higher than the upper sensor.

Click to expand...

joaco1993 said:

ps gama you are missing the gravity constant..

DeltaP=Density*Gravitiyconstant*distancebetween sensor

Click to expand...

So, Density = (PH - PL) / (L * gravitation acceleration) ?

Snap25 said:

Has anyone used (2) pressure sensors andthe hydrostatic formula to determined the fluid density?

Click to expand...

Whenever density is done by hydrostatic pressure, it is done with with a single differential pressure transmitter.

The greater the distance between the two, the better the measurement, because the measurement range is higher; pressure is a percent fullscale accuracy measurement, not percent reading, so the greater the distance the closer to a full scale value, the better the accuracy.

Using 2 pressure transmitters will disappoint you because of the accuracy issue.

Even differential has its issues because it is typically done with at least one remote seal and a capillary and capillary tubing is subject to adding an error for temperature change (it's closed system and the pressure reading reflects the temperature of the closed system).

Last time I spec'd one out, it was with giant 4" (or was it 5"?) diameter diaphragm seals in order to get the sensitivity needed for the 1 meter elevation distance between the diaphragms. It was indoors and heated so the temperature swings were marginal. They calibrated with water first to get an exact value for the distance between installed diaphragms. They used flush rings between the diaphragm seal flange and the mounting flange in order to flush out sediment. It worked.

Nuclear (radiation) is used for density.

Yes a single filled capillary pancake flange type DP Transmitter is good for this application. I've used them often.

psgama said:

Yes a single filled capillary pancake flange type DP Transmitter is good for this application. I've used them often.

Click to expand...

Have you ever compared your data with results from a hydrometer? How accurate? Tolerance?

My tolerance needs to be pretty tight. I'm starting to think sensotechs ultrasonic density sensor lmay be a better option?

danw said:

Nuclear (radiation) is used for density.

Click to expand...

I actually was reading about this last week..that would be a hard instruments to convince a customer use.

Let's get picky.

deltaP / L = specific weight

Specific weight is weight (force) per unit volume = pounds force per cubic ft or Newtons per cubic meter, for example.

Density is mass per unit volume, pounds mass per cubic foot or kg per cubic meter for example.

For some calculations it doesn't matter, but or some it does. You should always keep track of your units properly, or you could be in deep doodoo some day.

The formula with the picture posted by Snap25 , for example, has feet and pounds and inches mixed in higgledy piggledy. You'll end up with pig slop for your result if you don't watch your units!

Done right the result of this calculation will be as accurate as the differential pressure measurement accuracy and the tolerane on the distance between the two pressure taps. I would personally take it over ultrasonic or other instrument prone to various error inducing influences. This is fundamental physics.