A complete DIY project for creating a high-efficiency archival print washer (aPW) with Venturi aeration, calibrated flow control, and proven archival washing performance.
Introduction
Archival washing of photographic papers is often misunderstood. A good washer does not simply “push water through”—it must efficiently exchange, dilute, and remove thiosulfate (fixer) and its by-products. Commercial washers exist, but their cost and size can be limiting—especially for large prints such as 40×50 cm (16×20”).
The first modern archival print washer was designed by Henry Wilhelm in 1968, showing that effective washing depends not on huge water flow, but on controlled circulation, turbulence at the right locations, and repeated dilution. Later commercial designs from Kostiner, Summitek, and Kienzle refined these principles using narrow chambers, alternating top/bottom weirs, laminar transfer sheets, and balanced hydraulics to maximize fixer removal while minimizing water use.
Inspired by this lineage, the project described here presents a DIY three-chamber acrylic GS print washer, engineered for professional darkroom work up to 40×50 cm / 16×20″. The design incorporates a serpentine flow path, a submerged standpipe outlet, and a Venturi aeration system to enhance circulation in the first chamber. Every stage—from hydraulic modeling and flow calibration to acrylic construction and dye-tracing tests—is documented, resulting in a washer capable of achieving true archival washing at only 3–5 L/min. The first blueprint of the system can be seen in Figure 1. Tests of the commercial PW were perfomed by Vestal (1984) (Figure 2).
Part A — Design & Hydraulics
Residual Hypo Requirements
Archival Washing Standards recommended by Haist, Summitek, Ilford can be summarized as follows:
- Traditional fiber FB papers: < 0.5 µg/cm² = 0.005 g/m² residual hypo
- Practical darkroom target: ≤ 0.5 µg/cm² (matching Haist’s “Excellent Washing” curve)
- Typical flow recommendations:
- Ilford RC: 2–4 minutes with intermittent flow
- FB papers: 20–40 minutes depending on fixer and HCA
You can find practical guidance on archival washing in Lina Bessonova’s article on archival washing, where she also explains how to check your process with the classic Kodak HT-2 residual hypo test.
Kodak HT-2 working solution (approx. 1 litre):
- Distilled water: about 750 ml
- 7.5 g silver nitrate (AgNO₃), dissolve completely
- Add 125 ml glacial acetic acid (or 28% acetic acid)
- Top up with distilled water to 1,000 ml total volume
Then you can place a drop of HT-2 on a washed test strip of the same paper to see whether your washer really reaches archival levels.
Key Design Principles
Mysteries of the Vortex (Part 1, Part 2, French version) was a comprehensive article on print washing written by Martin Reed of Silverprint in 1996 and published in the American magazine Photo Techniques. The three key principles it emphasizes are:
- Fixer does not “sink”; circulation patterns dominate removal.
- Narrow chambers accelerate diffusion exchange.
- Weir exchange is more effective than a bottom drain.
This washer (Fig. 1) is built around three long, narrow vertical chambers arranged in series, creating a controlled serpentine path for the wash water. Water enters at the bottom of the rightmost chamber through a Venturi injector that entrains air into the flow. The mixture of water and fine bubbles then rises through the chamber, sweeping the print surface and disrupting boundary layers. At the top, the flow passes over a submerged weir into the central chamber, where it descends before being forced through a narrow slot close to the tank bottom. From there, it rises again in the outlet chamber, where a submerged standpipe fixes the operating level and establishes a predictable relationship between water depth and flow rate.
This alternating series of upflow and downflow zones is essential. It prevents short-circuiting, ensures complete water exchange around every part of the print, and maintains constant replenishment across the entire sheet. The submerged weirs silence the system and create stable water levels, while the bottom slot guarantees that no stagnant pocket remains in the central chamber. The standpipe—rather than a top spillway—provides precise control of the hydraulic head (Δh) and therefore of the flow rate (Q). By calibrating the standpipe height against actual bucket measurements, the washer can be operated at a known flow simply by reading the water level against an external scale.
The overall geometry follows the principles demonstrated in the most effective archival washers: narrow chambers, controlled velocities, intentional changes of direction, and a balance between turbulence (where prints are washed) and laminar overflow (at the weirs). This combination allows the system to achieve extremely efficient dilution at only 3–5 L/min, matching or exceeding the performance of commercial models while using surprisingly little water.
Additionally, my design adds an air diffuser (Fig. 3) in the first chamber, where the fixer concentration is highest. This diffuser produces controlled turbulence (bubbles) for boundary-layer breakup but does not disturb the upper weir. Reed demonstrated that small bubbles do not create the large, harmful vortices seen in poorly designed washers; instead they disrupt laminar pockets, enhance mixing, and keep the bottom zones active.
Hydraulic parameters
These are the key variables that define the hydraulic behavior of the washer:
| Parameter | Value |
|---|---|
| Print capacity | 40×50 cm (16×20″) |
| Washer length (L) | ≈ 600 mm |
| Washer height (H) | ≈ 450 mm |
| Water level (h) | ≈ 420 mm |
| Chamber width C1 (e1) | ≈ 40 mm |
| Chamber width C2 (e2) | ≈ 35 mm |
| Chamber width C3a / C3b (e3) | ≈ 40 mm |
| Outlet chamber length C3a (k) | ≈ 165 mm |
| Top weir C1 → C2 | 5–10 mm submerged |
| Top weir slot length (d1) | 3 × 120 mm openings |
| Top weir slot height (f1) | 10 mm |
| Bottom slot C2 → C3b | ≈ 10 mm high, full width |
| Bottom slot length (d2) | ≈ 460 mm |
| Bottom slot height (f2) | 10 mm |
| Vertical weir C3b → C3a | 20 mm |
| Standpipe crest | ≈ 418 mm (for 3–6 L/min) |
| Inlet fitting | ½″ PVC |
| Venturi injector | Inline, vertical or horizontal |
| Diffuser tube | ½″ PVC, 24 1–2.5 mm downward holes |
| Diffuser bleed hole | 1 mm in end cap |
| Target flow | 3–5 L/min |
Hydraulic Design
The washer uses submerged top weirs to distribute flow between chambers and a submerged standpipe in the last chamber (C3a) to fix both the water level and the total discharge. The top weir between C1 and C2 and the vertical weir between C3b and C3a run with a small, controlled submergence of about 5–10 mm. Their job is to create smooth, laminar sheet flow and force the water to follow the serpentine path (up → over → down → under → up). They are intentionally not the primary flow-control elements; instead, the standpipe sets the overall flow. The washer uses three weirs to control hydraulic exchange between the chambers. The first weir (C1 → C2) is a top weir with three 120×10 mm openings that, although cut slightly asymmetrically, turned out to improve small-scale turbulence and mixing. The second weir (C2 → C3b) is a bottom slot of roughly 460×10 mm, which provides rapid transfer and strong scouring of the lower part of the chamber. The third weir (C3b → C3a) is a top vertical weir about 20–40 mm wide, designed to remain submerged at all practical flow rates. Together, this alternating sequence of top and bottom overflows creates a pronounced “S-path” through the washer, promoting thorough mixing and preventing short-circuiting from inlet to outlet. Hydraulically, the standpipe behaves like a small submerged orifice. In theory, the discharge is given by \[ Q = C_d A \sqrt{2 g \,\Delta h}, \] where \(Q\) is the flow rate, \(A\) the cross-sectional area of the standpipe outlet, \(g\) gravity and \(\Delta h\) the water head above the standpipe rim. To ensure that the overflows behave as quiet sheets rather than noisy jets, the weirs were checked using the Froude number, \[ \mathrm{Fr} = \frac{v}{\sqrt{g\,h}} \] where v is the mean velocity of the water sheet, h its thickness and g gravity. For the chosen weir widths and the operating flow range of 3–5 L/min, the resulting sheet depths are only a few millimetres and the calculated Froude numbers remain well below 1 (Fr ≈ 0.2–0.4). This confirms that the flow over the weirs is sub-critical, with no hydraulic jumps, so the overflows stay smooth and nearly laminar while turbulence is concentrated in the aerated first chamber where it is actually useful for washing. In practice, the hydraulic design was refined iteratively. The weirs and slots were first sized using standard theoretical coefficients, and then the standpipe length was adjusted based on experimental calibration: measuring the volume collected in 60 s for different water levels. In its final configuration, within the practical operating range (about 2–7 L/min), the measured data are well described by a simple linear law: \[ Q \approx 0.73\,\Delta h, \] with \(Q\) in litres per minute and \(\Delta h\) in millimetres above the standpipe rim. This means that every additional millimetre of head increases the flow by about 0.7 L/min. Using this relationship, a small scale on the outside of the tank converts water level directly into flow; typical working values are: \[ \Delta h \approx 3\ \text{mm} \Rightarrow Q \approx 2\ \text{L/min} \] \[ \Delta h \approx 4\text{–}7\ \text{mm} \Rightarrow Q \approx 3\text{–}5\ \text{L/min} \] The weirs ensure even distribution and chamber turnover, while the standpipe provides a stable, predictable flow that can be set by simply adjusting \(\Delta h\).Aeration System
The system uses a Venturi injector positioned horizontally to mix air and water before the flow enters Chamber 1. This controlled aeration enhances boundary-layer agitation, increases the diffusion rate, prevents smooth laminar “sheeting” along the print surface, and helps flatten potential stagnation zones. The diffuser tube fed by the Venturi has two rows of 2.5 mm downward-facing holes (12 per row), plus a 1 mm bleed hole in the end cap to equalize pressure and spread the airflow along its entire length. After some experimentation with hole size and orientation, this arrangement produced an even and stable bubble distribution across the first chamber.
Part B — Construction and Assembly
Building the washer is much more about care and sequence than about exotic tools. Once the acrylic has been cut to size (by Kunststoffplattenonline), the whole project becomes a controlled gluing exercise where alignment, cleanliness, and patience are the key ingredients.
The tank is made from cast acrylic (Acrylglas GS), with a 10 mm base plate, 10 mm outer walls, and 5 mm internal dividers. Before anything is glued, all protective foil is removed only from the bonding edges, and those edges are cleaned with isopropyl alcohol to remove dust and grease. It is worth taking a few minutes to dry-fit every part: lay out the base, the two long side walls, the front and back, and the three internal dividers so that their relationships in Fig. 1 are clear in three dimensions.
The first step is to establish the internal geometry. Short acrylic strips are glued to the base plate as rails to hold the dividers in their precise positions. By bonding these rails first, it becomes almost impossible to misalign a divider later. Once the rails have cured, the two long side walls are glued to the base, forming a rigid U-shaped channel. Corner clamps or right-angle guides are used to hold everything square while a small amount of ACRIFIX 192 is applied along the inside seams by capillary action. After this has set, the back wall is added to close the box, again checking with a square that the corners are true.
With the outer shell stable, the dividers can be installed. Each divider is slid into its pair of rails and carefully pushed down until it rests fully on the base. If you want the option to remove them in the distant future, only the bottom edge can be fixed with a tiny amount of adhesive or a fine bead of silicone; otherwise, they can be bonded permanently along both bottom and side edges. At this stage, the weir openings and bottom slots should already be cut to their final dimensions: the top weir between C1 and C2, the bottom slot between C2 and C3b, and the vertical weir between C3b and C3a. It is much easier to machine these accurately before assembly than to try to cut them inside a finished box.
Once the acrylic body is complete and fully cured, the plumbing can be installed. The ½″ inlet bulkhead is mounted in the right-hand wall, connected on the outside to the water source and on the inside to the Venturi injector and diffuser tube assembly. The diffuser tube is cut to length so it sits near the bottom of Chamber 1 and drilled with its pattern of downward-facing holes plus the small bleed hole in the end cap. At the other end of the washer, another bulkhead fitting is installed for the outlet, feeding the standpipe and drain line. The standpipe itself is a straight section of PVC tube that simply pushes into an elbow or coupling inside Chamber 3a; by cutting this tube to different lengths during setup, the final operating water level can be trimmed very precisely.
Before doing any serious washing, the assembled unit should be leak-tested. The tank is placed where it will be used, all external hoses are connected, and the washer is filled with cold water to just below the top edge. Every seam and fitting is checked carefully for drips. If no leaks appear, the water is brought up over the weirs and standpipe so the flow path can be observed. At this stage the internal hydraulics can be fine-tuned: you can verify that the weirs run submerged, that no unexpected jets or vortices appear, and that the standpipe establishes a stable, quiet overflow.
Only after this wet test is the calibration performed with a bucket and stopwatch, linking water height to flow. By the time Part B is complete, the washer is not only watertight and mechanically sound, but also hydraulically configured exactly as required for the theoretical design in Part A.
Part C — Testing and Verification
Before trusting the washer with real prints, the hydraulic behaviour and flow had to be checked in practice. This was done in three simple steps: flow calibration, dye tracing and real-print trials.
Flow calibration
- The outlet hose was directed into a measuring jug.
- For each faucet setting, the volume collected in 60 s was recorded and converted to litres per minute (L/min).
- At the same time, the water level above the standpipe rim (Δh) was read on the wall scale.
- Plotting Q versus Δh showed an almost straight line, well described by the simple law Q ≈ 0.73 × Δh, with Q in L/min and Δh in mm.
- This made it possible to print a small scale on the outside of the tank: setting the water level now directly sets the flow.
Dye tests
- A small amount of dye was added to the water in the last chamber (C3a) at a normal working flow.
- The colour could be seen moving through the system, confirming the intended S-shaped path: up over the weirs, down through the bottom slot and back up to the outlet.
- An initial tendency for dye to linger at the bottom of C3a was corrected with a small movable deflector, which eliminated the dead zone and produced even clearing in all chambers.
Bubble behaviour and real prints
- The aerated inlet in Chamber 1 produced fine, downward-facing bubbles that rose gently through the prints but did not disturb the weirs, preserving the smooth, laminar overflow needed for a stable calibration.
- In use, RC papers wash very quickly, and fibre papers with a hypo clearing agent reach their recommended washing times comfortably at flows of about 3–5 L/min.
Together, the flow calibration, dye tracing and practical printing sessions show that the washer behaves as designed: the flow is predictable, the chambers exchange water efficiently and the prints receive a reliably archival wash without excessive water consumption.
First Print in the aPW2
For the first real test of the APW2, I chose a recent photograph of Cotopaxi — a subject with fine detail, deep shadows, and bright highlights. The washer ran at its calibrated flow, and the print came out clean, flat, and neutral, with no trace of fixer stain or colour shift. As a first run, the results were very satisfactory and confirmed that the design works not just on paper, but in day-to-day darkroom use.
