To that end, I made a series of flow restrictors, which are just some smaller diameter tubes I could insert in the tube between the chiller and the fermenter vessel.
I made the restrictors out of some wooden dowel rods I had lying around. Yes, I know that wood is one of the worst materials I could have used for this purpose since it's porous, and therefore could leak air and water and be a wonderful home to unwanted bacteria. I started with wood because I already had it on hand and I'm a woodworker so I'm better equipped to machine something out of wood. So if the experiment works I'll definitely be re-making the restrictor tube out of metal or food-safe plastic.
To start making the restrictors I found a dowel rod that would fit snugly in the 1/4" ID hose I was using, which I think ended being a 3/8" dowel. I cut a bunch of 2 inch lengths of dowel and then picked 3 diameters I wanted to try first: 1/8", 3/32", and 1/16". I clamped each rod in a wooden clamp, checked that it was perpendicular, and used my drill press to drill straight through the center of the dowel. After that I checked each rod into the drill press and used a file to taper the ends so it would insert into the tubing more easily. They ended up coming out pretty well, but not perfectly centered which should be fine for this experiment.
Measuring Initial Air Evacuation
Before I got to testing my new flow restrictors I wanted to investigate the initial start of the pumping process. I suspected that it would take some amount of time to evacuate most of the air from the system before the liquid would start flowing. To test this I used my usual volume of 16oz. of water to pump, but before the container was empty I poured in another 16 oz. of liquid. By timing both a 16 oz. and 32 oz. sample, and subtracting the difference between them I should be able to get an idea of the amount of time required to evacuate the air.
For this test I ran the pump full speed at 5 volts. Here are my results:
|16 oz. water||0m 41s|
|32 oz. water||1m 06s|
|air evac time||0m 16s|
Breaking the results down a bit, this means that the time to transfer 16 oz. of water is really about 25 seconds and it takes 16 seconds to remove most of the air from the inside of the pump apparatus. For the following tests I won't be adjusting for this initialization period, but it's something good to keep in mind.
Full-speed Flow Measurement
For the next round of tests, I kept the pump on full speed at 5 volts for the full duration of time it took to transfer 16 oz. of water. As you can see in the picture below, I inserted the flow restrictor in the hose that connects the output of the graham condenser to the input of the fermenting vessel.
Here are my results:
|restrictor size||16 oz. transfer time|
It seems that the restrictors are working though their effect is definitely non-linear.
Modulating the Pump Motor
Next I wanted to try to introduce another variable into the equation, which is to vary the on-time of the pump. In other circumstances I would use pulse-width modulation to allow a microcontrollers (like an Arduino) to control the motor, but for now I don't want to mess with setting up the motor drive circuitry. So I'm going to do "human powered" pulse width modulation and just turn the pump motor on and off in terms of seconds.
Here are the patterns I came up with:
|pattern num||init period||repeating pattern|
|1||0s||10s on, 10s off|
|2||20s||10s off, 10s on|
|3||0s||20s on, 20s off|
|4||10s||10s off, 5s on|
|5||10s||10s off, 3s on|
|6||10s||10s off, 2s on|
Each pattern has an (optional) init period where the pump is on, with the goal of initially evacuating the air from the system. After the init period, each pattern has a repeating pattern with the goal of replicating the duty cycle and period based control used in pulse width modulation.
I ran these patterns using the 3/32" restrictor since this restrictor seemed to give the best balance of hole size vs. flow restriction. I want to keep the hole size as large as possible because I'm concerned that if the hole is too small it will get clogged when I run actual wort through it.
Here are my results with the 3/32" restrictor:
|pattern num||16 oz. transfer time|
So it seems that my simplistic pump motor modulation is working, at least enough to proceed. I could have tried a pattern with an even shorter on time than pattern 6, but a 2s on time was about the shortest amount of time that I could consistently maintain over over the course of several minutes.
It's worth noting that by the end of my test runs the 3/32" restrictor was allowing a small amount of outside air to be pulled inside the tubing. I think this may change the results slightly, but I don't think it would have changed things enough to alter my conclusion that the strategy seems viable.
Conclusions and Future Work
The results from using a restrictor while modulating the vacuum pump seems to show enough promise to proceed. If you multiply out the time to transfer 16 oz. of water with around a 20% duty cycle, it would probably take around 24 minutes to transfer a gallon of wort through the chiller which seems like it should be sufficient to chill down to room temperature.
My next steps are to get my test rig ready to pump some near-boiling water though it, make a 3/32" restrictor out of something more durable than wood, and add some automation to control the vacuum pump motor.