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Continuous Measurement of Fluid Density, Part 2: Parts and Plumbing

Continuous Measurement of Fluid Density, Part 2: Parts and Plumbing

Before the hiatus, I made an attempt at creating a Continuous Fluid Density Sensor. Here are the primary components: Two dip tubes in the fermenter MPXV7002DP differential pressure sensor ADS1115 16-bit analog to digital converter Generic, chrome plated brass, 2.5 mm hose barb to M3 adapters and 2.5 mm pneumatic hose Generic 12V aquarium diaphragm pump (model no longer listed on eBay) Plumbing The first step was to install new dip tubes into the fermenter. These are just two tubes in the tank that terminate with their openings directed downward. They are connected to cam-lock adapters on the outside of the tank for attachment to hoses. I also wanted the portions of pipe inside the fermenter to have as few recesses as possible to limit contamination by undesirable microbes. I soldered the NPT stainless fittings using acidic flux (I used Stay-Clean). The 1/4″ stainless tubing connects through a compression fitting. These leaves less room for leaks and microbes. Now that we have dip tubes between which to measure the pressure difference, the first step in design is to calculate the expected difference in the pressure at the tip of each tube. The difference in their heights is approximately 15 cm. The conversion between Pascals and centimeters of water is , so the differential pressure between the two tubes due to a column of water is: The original gravity for a generic pale ale is approximately 1.05, so the differential pressure at the beginning of fermentation would be: The final gravity for a generic pale is about 1.011, so the differential pressure at the end of fermentation would be And finally, the change in the differential pressure is . The dip tubes need to be connected to a differential pressure sensor. The pressure sensors in the price range for this application connect via 2.5 mm pneumatic hose, but there are no adapters between pipe fittings (what I use in my brewery) and this diameter hose. So, I made adapters by taking a short length of 1/2″ NPT copper pipe and soldering a brass hose barb into it. The plating on the brass interfered with the soldering, so grinding down the threads on the barb before soldering was necessary. Diaphragm Pumps Lastly, the dip tubes will have a tendency to fill with fluid. Measuring the differential pressure will require a means to push the fluid out and fill the tube completely with gas. That’s accomplished with two diaphragm pumps. These simply take gas from the top of the fermentor and push it through the tubing leading to the dip tubes. MPXV7002DP This device is a differential pressure sensor. It operates at 5V and outputs an analog voltage between 0.5V and 4.5V that is proportional to the pressure detected by the device. Specifications dictate that this device detects a -2.0 kPa to 2.0 kPa range at 2.5% average and 6.25% maximum error. The first question is whether this is a significant error. At a range of 4000 Pa, the average error is 100

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