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Hardware__Pumps
For the mkturk hardware, we exclusively use pumps for fluid delivery even though traditionally valves have been used in behavioral training setups. The advantage of a pump system over a gravity-fed valve line is in the compactness of the fluid circuit and in pumps providing an approximately fixed displacement of fluid even when the water source is at or below the level of the output port. In a gravity fed system, the water supply often has to be held at a certain height to achieve a desired flow which was not ideal for mkturk where everything needed to be compact and robust to variance across configurations.
However, there are a number of parameters to consider in choosing pumps, and there is no perfect combination of flow rate, power consumption, and flow control as these can all trade off (see list of pumps below for examples). What follows is a very non-technical introduction based on experience with various pumps:
peristaltic, centrifugal, diaphragm...Centrifugal pumps use a rotating impeller that looks like a fan to create a vacuum in order to move fluid; think aquarium pumps. Peristaltic pumps are positive displacement pumps that use a rotating gear or rotor to move fluid and then pinch the compressible tube. Diaphragm pumps are also positive displacement pumps but use a chamber where the movement of a diaphragm displaces water through simple valve system.
Power is an important consideration for a low power, battery-operated system. Many pumps will rely on 12V DC, but we sought low power pumps (<1 Watt) that used 3-5V DC and drew <200 mA so that they would work with USB power (5V, up to 500mA). This is where piezoelectric mechanisms stand out as they are very power efficient. The Takasago 7mL/min pump draws ~0.1 W. By comparison, the most efficient, non-piezoelectric pumps that we have used draw 0.5-1.5 W.
This is an important consideration for a portable system where you would like to be able to start up and have the fluid line working even after changing out the fluid supply. Reduces number of components if pump is self priming. Self-priming pumps work by moving an air-water mixture which means that some water needs to be in the casing. Peristaltic and diaphragm pumps can be self-priming while only submersible centrifugal pumps are self-priming.
Some pumps are valveless and allow free flow in both directions. For example, the TCS submersible M100 is of this type. One way to prevent backflow is to put a check valve downstream so that flow is prevented in the reverse direction. Check valves have a cracking pressure that your pump would need to overcome to push fluid downstream. However, obtaining the right check valve and one that is robust introduces a layer of complexity. Rather, one could choose a peristaltic pump. The Adafruit peristaltic prevents flow in either direction by pinching the tube when no fluid is being actively displaced. This is not true of all peristaltics as the Takasago RP-Q1 did allow some flow, though attenuated, in both directions. Diaphragm pumps have valves that close, but if the back pressure or forward pressure are enough from the weight of the fluid column, these valves can be made to open. Therefore, it is best practice to always test your particular pump and fluid circuit configuration to see what direction fluid flows when the pump is not in operation. Our final solution for the Takasago piezoelectric which has flexible valves was to adjust its height until it was in the plane of the output (this pump was originally designed for "on the table" microfluidics). On the other hand, the TCS diaphragm pumps have strong enough valves to prevent backflow, so we can place the water source somewhat below the level of the output.
With a gravity fed system, flow can be adjusted by changing the height of the water column. Pumps, on the other hand, offer a fixed flow rate though some can vary pumping power with changes in input voltage within across a certain range. We found that pumps with flow rates in the 5-20 mL/minute range were good for delivering reward volumes in a timely fashion for smaller animals. Flow rates in the 100 mL/minute range were good for larger animals. Another issue with flow rate is that it determines how quickly you can flush the line so that a 0.3 mL/min flow rate may deliver reward precisely but then takes a long time to flush a long fluid circuit. Finally, more viscous fluids will decrease flow from the listed flow rate. Using a viscous milk solution halved flow rate in the Takasago piezoelectric relative to using water.
Drop size is an important factor because it determines the resolution of fluid delivery. This is a product of the pump mechanism, the diameter of the tubing, and the viscosity of the fluid being delivered. In general, pumps with larger flow rates will pair with larger diameter tubing increasing minimum drop size. Pumps which depend on discrete mechanisms such as the 3 tooth rotor in some peristaltic pumps or other stepping motor mechanisms will be limited to some resolution.
Piezoelectric pumps offer more smooth operation over time whereas pumps that rely on stepping mechanisms will tend to give give reward in a more discontinuous manner over long duration delivery. We found that the Takasago piezoelectric had a fairly continuous operating range and could deliver very small fluid amounts as its mechanism was more smooth.
Peristaltic Adafruit Peristaltic Liquid Pump (ID 1150)
Peristaltic Takasago RP-Q1
Centrifugal TCS Micropumps M100-Submersible-E
Diaphragm TCS Micropumps D250S-L + 3/32" ID nalgene tubing
Diaphragm - piezoelectric Takasago SDMP306D + 1.59mm ID Tygon 2001 tubing available from Takasago
Calibration curves for some of these pumps. The y-intercept gives a rough idea of minimum drop size.