A Digital Hydrometer

[eBrew]

Hello, what are peoples thoughts on using a digital hydrometer? I have always wanted to use one myself for the purpose of automation, but was immediately put off by the price of commercially available units. I have seen the Beer Bug out there, which seems like an interesting concept, but a little clunky looking. It is considerably cheaper than commercially available digital hydrometers, but probably has the trade off of lower reliability/robustness, accuracy and perhaps annoying to sanitise.

I have been developing an ultrasound based density meter (hydrometer) in my spare time for a bit of fun (Electronic engineer by day). The sensor will be a solid state component. Importantly, it would be easy to sanitise and would be robust. The sensor itself would float inside the fermenter, so should have minimal impact on your standard equipment. It would of course be wireless too.

I am still in the early stages of development, and am at the stage of building a simplified prototype to verify the modelling I have done for the sensor. If the technology proves successful I may be interested in turning it into a product for home brewers, but the question is what is the max people would pay for a digital hydrometer aimed at the home brew market? This would ultimately be the go/no go for further development as a product. I will continue making one for myself regardless.

Cheers guys

 

[MHB]

Not the first time the question has come up.
My first thought would be to hang a sinker from a load cell, probably a piece of stainless pipe with end caps welded on, attach (weld) a piece of stainless TIG wire to one end to hang it from, makes it pretty easy to clean and sterilise.
The apparent weight of the sinker is inversely related to the density of the liquid it is in, so in water if it weighed 100g, in a 1.100 SG solution it would weigh (100/1.100)=90.909090...g
I haven't modeled it but I think the closer it is to being a float, rather than a sinker the bigger the relative change, that's why I thought of a hollow pipe with caps, I would set the density of the sinker above the brewing range (say 1.200), you are going to need a pretty accurate load cell, but if you can get 0.1g drug scales on eBay for less than $20, I cant see that being too much of a problem.
Mark

 

[Lyrebird_Cycles]

I have been developing an ultrasound based density meter (hydrometer) in my spare time for a bit of fun (Electronic engineer by day). The sensor will be a solid state component. Importantly, it would be easy to sanitise and would be robust. The sensor itself would float inside the fermenter, so should have minimal impact on your standard equipment. It would of course be wireless too.

I am still in the early stages of development, and am at the stage of building a simplified prototype to verify the modelling I have done for the sensor. If the technology proves successful I may be interested in turning it into a product for home brewers, but the question is what is the max people would pay for a digital hydrometer aimed at the home brew market? This would ultimately be the go/no go for further development as a product. I will continue making one for myself regardless.

Ultrasonic "densitometers" using propagation velocity have been tried before, they proved to be less accurate than the oscillating tube densitometers so they're not a commercial proposition. See US4934177 and this more recent discussion

Anton Paar make a specialist combined densitometer / propagation velocity meter, IIRC it's designed for fluids where tiny gas bubbles can disturb the measurement from the oscillating tube.

Another problem is that the propagation velocity of a trinary mixture (water / sugar / alcohol) is not predictable though any simple algorithm. There is software than can be used to predict it but that software is very expensive.

As to price / performance: if you can do 0.02 oP for less than a $1k I'm in.

 

[Lyrebird_Cycles]

I haven't modeled it but I think the closer it is to being a float, rather than a sinker the bigger the relative change, that's why I thought of a hollow pipe with caps,

The slope of the line of density change wil lindeed be inversely proportional to the density difference, and this in itself can be exploited to measure the density: very high precision can be obtained from what's called a two sinker densimeter which as the name implies measures the difference between two sinkers of different densities.

 

[MHB]

Makes sense, two equal volume different masses would compensate for temperature changes. might be overkill for what we need as brewers.
The reason I suggested thin SS wire as a hanger is its uniform density and that very little crud would stick to it (dried on krausen) and as above it would be easy to clean/sterilise.
Mind you I suspect the difference in a temperature controlled ferment (say from 8-25oC) wouldn't be very significant and could be compensated for either by a temp sensor and algorithm, or just from a reference table.
Still think its a low cost way to get pretty accurate data.
Mark

 

[eBrew]

Not the first time the question has come up.
My first thought would be to hang a sinker from a load cell, probably a piece of stainless pipe with end caps welded on, attach (weld) a piece of stainless TIG wire to one end to hang it from, makes it pretty easy to clean and sterilise.
The apparent weight of the sinker is inversely related to the density of the liquid it is in, so in water if it weighed 100g, in a 1.100 SG solution it would weigh (100/1.100)=90.909090...g
I haven't modeled it but I think the closer it is to being a float, rather than a sinker the bigger the relative change, that's why I thought of a hollow pipe with caps, I would set the density of the sinker above the brewing range (say 1.200), you are going to need a pretty accurate load cell, but if you can get 0.1g drug scales on eBay for less than $20, I cant see that being too much of a problem.
Mark

That is indeed a sound concept, but I am after a solution that is not comprised of multiple components, hence why I am going down the solid state ultrasonic route. I have a background in ultrasonics too, so it helps for taking some of the guess work out. I would assume you would need to be very careful not catch you sinker on something and damage the load cell when moving the sensor to and from the fermenter.

 

[eBrew]

Ultrasonic "densitometers" using propagation velocity have been tried before, they proved to be less accurate than the oscillating tube densitometers so they're not a commercial proposition. See US4934177 and this more recent discussion

Anton Paar make a specialist combined densitometer / propagation velocity meter, IIRC it's designed for fluids where tiny gas bubbles can disturb the measurement from the oscillating tube.

Another problem is that the propagation velocity of a trinary mixture (water / sugar / alcohol) is not predictable though any simple algorithm. There is software than can be used to predict it but that software is very expensive.

As to price / performance: if you can do 0.02 oP for less than a $1k I'm in.

Yes you are correct, and I am aware of this. If only it was that easy huh . By varying the ratio of sugar and ethanol, you can get the same time of flight (TOF), so it makes it extremely difficult to determine alcohol content. My method does involve time of flight measurements, but that is only one piece of the puzzle. There is other aspects of the reflected signals that are used, which then allows me to calculate density directly.

As for the accuracy, I would not be aiming for 0.02 oP. I know the limitations of my approach and that it will not out perform the oscillating u-tube method. I am aiming to resolve +- 0.001 gm/cm^3 density or +- 0.001 SG if you are considering specific gravity, and will be a happy man if I can achieve this. I think we can all agree that this accuracy will certainly suffice for brewing.

Thanks for taking the time to reply guys.