What I want to do

Create a version of the modified aquarium pump in the formaldehyde test kit that is flexible and can be adjusted across a widest flow range, and also protected from damage by particulates getting into the pump.

Matching commercial pump volumes

Following @Nshapiro's directions I've been modifying one side of a Tetra Whisper 100 aquarium pump:

a DIY test kit:

A single-sided pump has difficulty pulling more than 300ml through a formaldehyde test tube. Modified on both sides, it is capable of pulling 2+L/m of air through a tube or ten times as much. Joining both sides seems to more than double the pump's capabilities, which can be adjusted from 100ml/m to 2L/m which should accomodate a range of testing tubes, and is the same spec as the $1800 Draeger X-Act 5000. The pump came with a "T" to plug both sides together:

Filters for reliability

The air pump comes with an intake filter by default, by modifying it to be a vacuum pump (pulling air) rather than a pressure pump (pushing air), we bypass this intake filter. While in limited testing the pumps have held their calibration, fouling will definitely ruin the pump long-term.

Most bio lab-grade vacuum pumps have 1um filters inside that are $100+, which is more than 3x the cost of the pump. This guarantees a clean airflow out the end of the pump, but if we're only concerned with fouling the interior of the pump, I think a courser industrial-grade filter will do the trick.

I found a number of $3-10 filters that filter from 25-65um in McMaster-Carr and Grainger supply catalogs. The only brand I could identify was a KMC HFO-006, which is about $4 in quantity.

All the filters I found were for 1/4" hosing, rather than the 1/8" hosing that comes with the aquarium pump and fits the formaldehyde test tubes. I therefore had to order some 1/4" air hosing and two 1/8" - 1/4" reducer barbs.

here's what the air setup looks like:

Here are the additional parts to add to the pump's parts list:

• Mcmaster filter or KMC HFO-0006
• requires 1/4" tubing and 1/8"-1/4" reducer barb fittings