2.5 Sourcing

Cat W/C Air Filtration System · 13 Oct 2026

The last research topic is where to buy it all in the UK, as one person buying small amounts. I sent this brief before the fan research had finished, to save time, so ChatGPT priced a fan that 2.3 has since ruled out. I've swapped in the right one below. It ran on GPT-6 at the Pro setting and worked for about 44 minutes.

The short version: the electronics and the particle filters are easy, mostly in stock and a few clicks away. The carbon is the hard part. Nobody I could find sells a few litres of a documented gas-phase carbon to a private buyer through a checkout, and the ammonia medium is harder still. Those need emails to UK suppliers. Measuring the room's ammonia needs a lab, and Gradko's diffusion tubes can see down to a couple of ppb. The parts with prices come to about £285, and the prototype to about £440 to £640 once the quoted items are in.

The shopping list

Prices are from 13 October 2026, including VAT unless it says otherwise.

Media

Filters, mesh and seals

Sensors and electronics

The SDP810 and SDP811 are the same package with different I²C addresses, so both can share the bus. The SPS30 needs its 5-pin JST-ZH cable. Adafruit doesn't say its load cell is a single-point type, which a one-cell platform needs, though its drawing looks like one, and the drawing gives a 1 to 10 kg range against the listing's 20 kg. That's one to check before I design the tray platforms. And The Pi Hut sells the same HX711 board for £4.80 when it's in stock, which would save about £11.

Fans and power

2.3 asks for 12 V at 3 A for two S12038-4K fans and the electronics. ChatGPT suggested Mean Well's GST25B12, but at 2.08 A it's too small. The GST36B12 is the same desktop supply at 3 A and 36 W. Its RS price came from a search listing, so I'll check it before buying. Farnell sells these too, but its terms are for business customers only.

The carbon problem

The chemistry is settled. Buying a few litres of it isn't.

Purafil's Purakol, the reference plain carbon, is 4 mm pellets at 0.45 to 0.55 kg per litre, so 2 to 5 litres is 0.9 to 2.75 kg. I found its data sheet but no UK small pack. Puracarb AM, the reference ammonia medium, turns out not to be carbon with an acid coating at all: it's 3.2 mm pellets of activated carbon, activated alumina and binders, at about 0.72 kg per litre, with an ammonia capacity of at least 5.8% of its weight. That's about 42 g of ammonia per litre. It's a capacity, not a lifetime: it doesn't say when ammonia starts getting through at the low levels in a WC.

Media should be compared by the litre, not the kilogram, because the bed is a fixed volume:

$$\text{£ per litre} = \text{£ per kg} \times \rho$$

where $\rho$ is the bulk density in kg per litre.

The plan is one enquiry, sent to Eurocarb in Bristol (which lists RGK, a respirator grade for ammonia), Carbon Activated Corporation's UK office, Puragen, Airclean, Camfil UK and Purafil. It asks for a paid, sealed and labelled repack of 1, 2 or 5 litres, and:

• The exact grade, its maker and batch, the current data sheet and UK SDS.
• Pellet size, bulk density, fines limits, and the pressure drop for a shallow bed.
• For the ammonia medium, what it's impregnated with and how much.
• Breakthrough curves, ideally at below 1 ppm to a few ppm of ammonia, with the test humidity, temperature, bed depth and air speed.
• Whether they'll sell to a private buyer at all, and the delivered price.

The enquiry also gives them my conditions: from 2.3, a 380 × 240 mm bed face, 40 mm per stage, air at 0.11 m/s normally and 0.18 m/s on boost, and a contact time of 0.37 s and 0.22 s per stage.

Two things that look like shortcuts aren't. A lab supplier's "acid-washed activated carbon" has been washed with acid to clean it, which isn't the same as being impregnated with it, so it says nothing about ammonia capacity. And aquarium ammonia removers work in water, not air.

Safety and disposal

The SDS for the exact grade comes before anything else, since acid-impregnated carbons differ. I'll fill the cartridges with the fan off, over a tray, away from the cats, and close them before they go into the WC. Impregnated media don't get washed or regenerated at home.

Spent media are less clear. Whether they go in the household bin or need the council's hazardous waste route depends on the formulation, so the council gets the SDS and a description and decides. GOV.UK sends householders in England and Wales to their council for this. Until then, spent media stay bagged and labelled, and never go down a drain, into compost or back in the litter.

The weight matters too. Five litres each of Purakol and Puracarb AM would weigh 5.85 to 6.35 kg before the filters and the case, which settles 2.3's point that the wall mount carries the media, not printed tabs.

Measuring the ammonia

2.4 left the room's ammonia level as the big unknown, anywhere from below 0.1 ppm to a few ppm. The options:

The conversion from µg/m³ to ppb at 25 °C is

$$\text{ppb} = \frac{\mu\text{g/m}^3 \times 24.45}{17.03}$$

where 24.45 litres is the volume of a mole of gas and 17.03 g is a mole of ammonia. So Gradko's 1.6 µg/m³ is about 2.3 ppb.

Gradko is the first enquiry: a two-week round, with two exposed tubes and a blank. It gives an average, not the peak after a cat's visit, but that's the question 2.4 left: is the room at tens of ppb, hundreds, or ppm? I'll also ask whether a room at a few ppm would overload a two-week tube.

The budget

ChatGPT's own list came to £295 for the priced parts, with the P12 Pro fan and two load cells per tray. My brief asked for "two load cells with HX711 boards per tray", which it took literally. 2.4 settled on one per tray, and 2.3 replaced the fan, which together take it to £285.

What's next

• Send the media enquiry to the six suppliers.
• Ask Camfil UK for an EcoPleat G panel near 390 × 250 mm, with its low-flow pressure data.
• Ask Gradko for a two-week ammonia round.
• Buy the room pod now (SPS30, SGP40, SHT45, ESP32 and buck converter, about £76), so the baseline can start while the quotes come in.
• Buy the fans and two IKEA filters for the test mule.

That's the research done. The next part is measuring the room and starting the baseline.

What I checked

I had Claude check eighteen claims against the shop pages and data sheets, on the day. I reran the budget myself, with the fan and the load cells corrected.

These held up: the Airclean and Purafil densities, Puracarb AM's make-up and capacity, General Carbon's details, Camfil's panel sizes, the ADD, Polymax, DigiKey, Pi Hut and Scan prices, and the Gradko, SKC, Gastec and Dräger figures.

These needed correcting, and the post above uses the corrected versions:

• ChatGPT said the SDP811 is a board-mount part and the SDP810 has a cable connector. They're the same package, and only the I²C address differs.
• It cited a Camfil sheet for 13% phosphoric acid in 4 mm pellets. The current sheet says none of that, so I've left it out.
• The Radiello price is from SLS, not Sigma-Aldrich, and SLS is discontinuing the pack.
• The Farnell price for the 2 A supply couldn't be checked, and the 3 A supply's RS price is from a search listing.
• The Hydro Bros carbon had sold out by the time it was checked.
• Inoxia doesn't say whether its price includes VAT.
• SKC states the badge's sampling rate is unaffected between 22 and 40 °C. It doesn't call that a validation range.
• Eurocarb's ammonia grade is a respirator grade.
• Adafruit doesn't say its load cell is a single-point type.

The allowances in the budget are ChatGPT's estimates, not prices, and weren't checked.

Sources

Media:

• Purafil, Purakol bulletin and Puracarb AM bulletin.
• Airclean, activated carbon.
• Eurocarb, impregnated grades.
• Carbon Activated Corporation, activated carbon.
• Puragen, UK.
• General Carbon, GC IPA and its SDS.
• The Hydro Bros, AC Infinity carbon refill.

Filters, mesh and seals:

• IKEA, FÖRNUFTIG particle filter.
• Camfil, EcoPleat G data sheet.
• ADD Filtration, F7 mini-pleat panels.
• Inoxia, 30 mesh sheet.
• Polymax, 10 × 3 mm foam strip.

Sensors and electronics:

• DigiKey UK, SPS30, SHT45 breakout, SDP810-125PA, SDP811-500Pa and HX711 board.
• The Pi Hut, SGP40 breakout, FireBeetle 2 ESP32-E, load cell, HX711 board and MPM3610 buck converter.
• Adafruit, load cell 4543.
• Sensirion, SDP8xx data sheet and SPS30 data sheet.

Fans and power:

• Arctic, S12038-4K, three-pack at Scan.
• Mean Well GST36B12-P1J, specifications and RS listing.
• Farnell UK, terms of purchase.

Ammonia measurement and disposal:

• Gradko, ammonia diffusion tubes and technical data sheet.
• SKC, UMEx 300.
• Gastec, 3DL instructions.
• Dräger, ammonia 0.25/a tube.
• SLS, Radiello RAD168.
• GOV.UK, hazardous waste disposal.