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Old 12-13-2012, 06:54 PM   #11
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and if we can fill the tubes with water, at least the distance between the two ports, we could measure the wort to water difference more precisely
Yeah, this was why I was asking AJ about why not just filling the tubes with the wort. If you couldn't use wort, I don't know that you'd want to use water, since the wort would diffuse/dissolve into the water. An alternative option would be to use something like food grade mineral oil which will stay separated from the wort.

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But I think the escaping bubbles still affect the pressure difference by reducing the density of the wort. What seems to matter is the weight of the wort column between both ports and that will be less when there are bubbles present.
It's not clear to me why bubbles in the wort would affect this measurement. I'd have to think about it a while to be sure, but it seems like if any bubbles form they'll displace the rest of the wort, increasing the absolute pressure at each port, but keeping the pressure difference between the two ports the same.
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Old 12-13-2012, 07:04 PM   #12
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Bubbles lower the overall density of the beer. Just like a hydrometer sinks deeper into beer when bubbles are still rising in the hydrometer jar. They lower the absolute pressure at the lower port more than at the higher port since there are more bubbles above the lower port than there are above the higher port.

According to my math, the pressure difference between 0.25 m beer column and a 0.25 m water column is 0.001 inches H20 at a density of 1.001. It's ~0.5 in H20 at 1.050. So the 2001 from Dwyer may work: http://www.dwyer-inst.com/Product/Pr...000/ModelChart

I think it is important to fill the other tube with water compared to wort. That way we can messure the difference between water and beer/wort which is much smaller than the difference between wort/beer and air.

Ad for the diaphragm, that separates water and wort/beer, a think rubber membrane should work since it doesn't not have to move much anyway. There should not be any noticeable compression of the air above the water.

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Old 12-13-2012, 08:02 PM   #13
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The diagram shows how I conceive doing it with the conical. Your last diagram shows how it could be done in a carboy and how I suspect these guys may be doing it.

The math is on the diagram. The pressure difference depends on the differences in liquid level in the two lines, the density of the liquid and the density of whatever gas is in the lines. Dry air has density of about 0.00112 g/L which is about 0.1% of the density of water and moist air is less dense than that. CO2 is, of course, appreciably denser than air.

CO2 shouldn't move into the tubes nor should wort because there is no flow through the sensor. Yes, there would be a little diffusion and if temperature were crashed in the fermentor (your drawing) the gas in the tubes would contract and draw some in. It is l2 - l1 (see drawing) that is the most important parameter and it should be possible to calculate the change in (l2 - l1) from PV = nRT (i.e. air/CO2 is near enough to ideal). Note that in the cylindroconical the gas in the tubes would not be subject to the effects of cooling (unless the brewery got cold). In your drawing this effect could be minimized by making both tubes the same length (coiling one) so they held the same gas volume, both shrink by the same amount, draw in the same amount of liquid and leave l2 - l1 the same.

I think I see what you are saying about water but I'm not sure how you would get and keep it in there without these isolators which I can't find. I was thinking that we would just do calibration with water as we need to find (implicitly) the local value for g anyway.

I've looked briefly at the Ashcroft differential sensors which are reasonably priced at about $180 and at the Honeywells which are not (don't know but their similar altimeters are about $1K). Both seem to have an accuracy of about 0.1% of FS which is, of course, about 0.001 SG. The Honeywells are really nice because you just hook them up to the RS-232 port on your computer an you are in business.

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Old 12-13-2012, 08:11 PM   #14
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Why can't you fill the tubes with wort? Because if you do the pressure across the meter will be 0. Look at the last equation on my sketch. The second term would be equal to the first and cancel it out: Pz = Pc.

As to the need for water: look at the first term - it is g times the density of the wort. The density of the wort is the specific gravity times the density of water. If you fill the system with water and adjust to read 1.000 then your adjustment has compensated for local variation in g and you are set. Plus, as clearly a computer is going to be involved here, you have the ability to compute the density of water at any temperature and thus to correct observed SG for temperature. The second term, if the tubes are filled with air, might even move your computation closer to the apparent specific gravity on which the ASBC tables are based but I'd need to check that thought out.

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Old 12-13-2012, 08:38 PM   #15
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Why can't you fill the tubes with wort? Because if you do the pressure across the meter will be 0. Look at the last equation on my sketch. The second term would be equal to the first and cancel it out: Pz = Pc.
Haha, yes, very obvious when you actually write out the equations. One of these days I'll learn and do that before I post.
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Old 12-14-2012, 03:11 PM   #16
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I was doing some searching on Ebay regarding devices to measure differential pressure. I'm finding things like this: http://www.ebay.com/itm/Digital-Air-...71121305903%26

The price is acceptable but I'm not sure about the precision. The goal would be a precision as good or better than a standard hydrometer.

As A.J. mentioned, the pro of filling at least the long tube with water is that you'll get instant temp correction, compensation for g and your output will be a small pressure difference with no large offset. The con is that you need a flexible membrane to keep the water from mixing with the beer.

W/o filling the tube with water the device could be as simple as I have shown above. But you need to use a differential manometer that still gives you sufficient precision to detect the slight change in pressure while there is significant offset. That pressure delta should be 0.1% of the pressure offset. The pressure offset can be changed though the length difference of the tubes. For 25 cm the pressure offset (pressure for using water) would be 0.025 bar and every gravity point would increase it by 0.000025. The listed resolution for the cheap Ebay mannometer is 0.001 bar. Looks like that this is not all that suitable after all.

Another thought I had is, why not simply use a water filled U tube to measure the differential pressure? But with this design beer is expected to enter the longer tube, which has to be taken into account since this changes delta H. But I'm concerned about the ability to read the scale. With a delta H of 200 mm each change by one gravity point is only 0.2 mm. That's a bit difficult to read.

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Old 12-14-2012, 05:02 PM   #17
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I think there is some misunderstanding about my thoughts on the use of water as the other fluid. I do not think the tubes should contain water. They should contain air (or hydrogen of helium). I updated the image in #13 to show the rest of the math. The bottom line shows how one would calculate the apparent specific gravity (what brewers use) from the differential pressure reading. Note that it depends directly on the differential pressure reading divided by the distance between wort levels in the tubes conveying pressure to the pressure sensor (carboy arrangement or conical). Thus if the pressure instrument is in error by 0.1% the SG reading will be in error by 0.1%. The nominal pressure difference depends, of course, on the delta L - the separation between the liquid levels. If that is about 27" the pressure difference will be about one psi (for water) and thus your instrument needs to be able to measure to an accuracy of 0.001 psi if you want accuracy of 0.1% in SG which, as SG is about 1 is 0.001 SG. For 13.5" separation you would have to measure to 0.0005 psi.

If you use a water filled manometer with 27" separation then, unsurprisingly enough, the height of the water column in the manometer will be about 27" and you have to be able to read that to 0.027" to achieve the 0.1% accuracy goal.

Now the other factor in apparent SG is 1 plus the ratios of the density of the gas in the tubes divided by the product of g, the density of water at the reference temperature and the separation. What I am suggesting is that this factor be set to the pressure difference when the setup is filled with water as water has apparent SG = 1 (to 3 decimal places). There is no need to measure or obtain a value for g and no need to know the density of what is in the tubes or indeed the density of water at a particular temperature.

In the conical case there is one potential problem with doing things this way if the tank is spunded and appreciable pressure applied. If, for example, 1 atmosphere CO2 is allowed to build in the head space the air in the tubes will be compressed to approximately twice its density. At atmospheric pressure the 1 + density ratio terms is approximately 1.001. With an additional atmosphere's pressure in the headspace that would increase to 1.002. That's really only 0.1% change so perhaps not a problem after all but the idea of using hydrogen or helium as the gas in the tubes was prompted by consideration of this.

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Old 12-14-2012, 05:40 PM   #18
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A.J, my thinking behind filling the longer tube with water up to the port for the shorter tube is that the resulting pressure differential will only depend on the density difference between the beer and the water. So you should end up with an equation that should have the following structure

sg = 1 + A * delta l * delta P

I don't have the time to do the math right now. But the factor A can be calculated. My point is that you can measure delta P with a more sensitive manometer since it only has to deal with the pressure that comes from the density difference between beer and water and not the larger pressure difference that you would have if you put beer against air or some other gas.

This also assumes that there is no actual movement of air such that the water column in the long tube is pushed up a little.

Kai

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Old 12-14-2012, 09:28 PM   #19
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I didn't mean to imply that I don't think using water will work - but rather that it isn't necessary IMO and therefore not, given the necessity to keep the fluids apart at the boundary, something I'd consider (though my mind is not closed on this by any means).

If you look at the pressure difference formula on the image in #13 it is

D_P = g*D_L*(dens_wort - dens_fluid)

where D_L is the level differnce and dens_fluid is the other fluid (air in my calculations). If the other fluid is water then

D_P = g*D_L*(Dens_wort - 1)

approximately and the developed pressure differential is much smaller. At first look you might say that's bad because the 'signal' is less but if you solve the D_P equation for dens_wort

dens_wort = D_P/(g*D_l*dens_water) + dens_fluid/dens_water

and take the partials with respect to pressure and the density of the other fluid you get

partial WRT pressure = 1/(g*D_l*dens_water)

partial WRT density =1/dens_water

it's clear that selection of a fluid other than air does not improve accuracy for the same absolute accuracy in pressure measurement.

Now if the absolute accuracy of the available pressure gauge improves as it's span gets smaller then using water could be a viable strategy. With a 27" separation and air the pressure differential will be about a pound (27" WC). To measure 0.001 SG you'd need accuracy of 0.027" WC. With water as the fluid you'd still need 0.027" accuracy but the pressure differential for SG 1.040 would only be 27*0.04/1.04 = 1.038 " WC. Now 0.027/1.038 is 2.6% which is a more relaxed accuracy requirement than 0.1% for sure. The problem is that whereas the accuracy seems to be a function of span at higher pressures it seems to become an absolute error at lower pressures. For example, the Honeywell PPT series has a 0.0375% FS typical error spec for 2psig and above but it doubles (on a percentage basis) to 0.07% percent for the 1 psig unit (which is the most sensitive in the line). There may be other instrumentation with better performance but given the cost of these Honeywell things I'm not interested.

Using a manometer you will need to be able to read 0.027 " (0.7 mm) whichever fluid you use but the height of the column will be less.

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Old 12-14-2012, 09:56 PM   #20
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Originally Posted by ajdelange View Post
Now if the absolute accuracy of the available pressure gauge improves as it's span gets smaller then using water could be a viable strategy.
That's the point I was trying to make. In addition to the fact that it is easier to read a 0.027'' change on a gage that goes from 0-0.25'' compared to a gage that goes from 0-30''.

But finding a diaphragm that could be used to hold the water in the pipe doesn't seem to be easy.

Kai
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