A lot of biotech involves moving liquids around. Most of the time we use peristaltic pumps because they’re sealed and precise. Sometimes they’re massively over-spec though. That’s not the Scruffy way. To move water around our bioreactor’s water jacket we needed to shift large volumes quickly, but sterility was not an issue. Rather than spend our cash on peristaltic pumps we used screenwash pumps from cars.

Screenwash pumps work on a different principle to peristaltic pumps. They’re basically two interlocking gears that throw water through faster than it flows back.


One of the downsides of this is that when the pumps are off they permit backflow. If you have different levels of liquids, as we did, you get siphoning. We corrected this with some simple inline valves.

Screenwash pumps are driven by simple DC motors. You can control flow rates to some extent by changing motor speeds, but get too low and the pumping becomes erratic and then fails, so they’re not so good for low flow rates. For our purposes we wanted to move the water as quickly as possible. These are the flow rates for four identical pumps (Techniks CL270 Universals) on identical power supplies. As you can see, they’re rather variable.


Flow rate (L/min)









Screenwash pumps can be a good substitute when you want high flow rates and low cost (£6 each). Be aware that they leak and run hot if used for too long (in our water jacket controller they run for a maximum of two minutes at a time).