Measuring liquid density with ultrasonics

[Mosaic]

Hi All:
I am considering measuring Lead acid electrolyte density (sealed cell) with ultrasonics
https://www.newark.com/prowave/400s...=true&MER=ACC_N_L5_SensorsAndTransducers_None
I'd like to get any advice I could regarding this.

I envision placing the transducers in direct contact (petroleum jelly interface) on opposite sides of the battery walls externally around the level of the electrolyte above the battery plates. Then using opamps to detect and a microcontroller to do the calcs.

I suppose I could gen. the Ultrasonics with the uC as well.

 

[crutschow]

What type of change in the signal do you expect with a change in electrolyte density?

 

[alec_t]

Would the type of transducer in the link be suitable for dunking in petroleum jelly? Looks like it's meant for use in a gas (air) only.

 

[Mosaic]

Batt electrolyte varies between 1100 and 1300 or so relative to water being 1000.
The theory is with accurate speed detection, a state of charge can be estimated remotely and continuously.
https://www.engineeringtoolbox.com/sound-speed-liquids-d_715.html

 

[alec_t]

Speed of sound varies as the square root of elastic modulus / density. Does the elastic modulus also vary or is it independent of the state of charge of the battery (couldn't find out with a quick google)?

 

[Mosaic]

Alec...allow me to clarify...I just need to know when the SG of the acid stops rising. I also have a temperature ref. to correct for temp variation.

 

[alec_t]

Ok. But if the modulus were to change by the same amount as the SG (I've no reason to think it does) then the speed of sound wouldn't vary with charge state.

 

[jpanhalt]

I think this is an interesting concept. I could not find the speed of sound in sulfuric acid solutions, but as an approximation, I did find results for salt solutions (i.e., another ionic solution):

The right column is velocity of sound in m/S.

3.5% salinity is approximately 0.6 mol/L, which causes an increase of 2.3%. Battery acid is quite a bit more concentrated with solute (sulfuric acid) when charged. This approach will probably need to include a temperature correction. I will keep looking for some data on sulfuric acid solutions; although, looking for a maximum velocity with temperature correction makes sense. Let's hope velocity doesn't decrease past a certain point.

John

 

[jpanhalt]

OMGosh, we are reinventing the wheel. Here is an IEEE on this very subject (**broken link removed** ):

Unfortunately, I do not belong and can't access the full article. Nevertheless, someone who does can summarize the findings and post data from the tables without violating copyright.

John

 

[alec_t]

Nice find, JP. That would suggest that the elasticity modulus is essentially constant.