Table 1.

Components of the mini-Secchi disk.

Name	Figure 2a Index	Description	Materials	Manufacture or Purchase Method	Dimensions
Weight	1	To ensure the Secchi disk sinks vertically.	We tested various materials and eventually chose marine grade brass as it is very dense and to minimise corrosion when operated in the marine environment. The weight is 100 g. Two designs are provided. One includes an attachment to allow extra weight to be added, when operated in currents, and for attaching additional sensors, for example, a temperature sensor (see Figure 6).	For single production the weight can be produced (home-made) using a metal lathe (see dimensions of weight in Supplementary data 1). For batch production, the weight can be computer numerical control (CNC) machined by any local CNC company from the STL file (see Supplementary data 2; we have used G&J Services Ltd in Kent in the past (http://www.gandjcnc.co.uk) for some of our batch runs). It is also possible to 3D print the weight using a suitable metal filament and appropriate 3D printer. If you try this you want to achieve a weight of 100 g or heavier (we would be interested to know how you get on).	Technical drawing and STL file for both designs are provided in Supplementary data 1 and 2.
Weight O-ring	2	To ensure weight locks into bobbin.	Rubber.	Purchasable from gasket and bearing supplier.	Provided in Supplementary data 1 and 2.
Weight washer	3	Separates disk from weight.	Brass or stainless steel.	Purchasable from a hardware store.	Standard M6 washer (12 mm outside diameter, 6.4 mm centre hole diameter and thickness 1.6 mm).
Secchi Disk	4	White Secchi disk.	Polypropylene. Very high resistance to UV. An oily plastic with good elastic properties. Resistant to becoming brittle and capable of retaining its colour over time.	Polypropylene sheet purchasable from hardware store or online plastic distributor. Can be manufactured at home or in a workshop using a compass to mark outer circle at 100 mm diameter, then cut using a bandsaw or by hand using coping saw. 6 mm hole drilled in the center. Can also be produced in batch using a laser cutter.	100 mm diameter, 2 mm thick and 6 mm central hole.
Weight bolt nut	5	To secure weight to Secchi disk.	M6 brass or stainless steel nut with ’nyloc’ or nylon locking insert incorporated.	Purchasable from hardware store.	Standard M6 ‘nyloc’ nut.
Weight attachment circlip	6	Joins tape to the weight and Secchi disk. This must be bent to allow the start of the tape measure to be at the correct distance from the disk so the measurement is accurate (tape starts at ∼20 mm from disk).	Stainless steel.	0.7 mm stainless steel rod cut to 30 mm with pliers, bent using a simple jig set into a 10 mm by 10 mm by 10 mm equilateral triangle and threaded onto the weight and stainless steel fastening on the end of the tape. We bought this from an online hardware store. Alternatively, you can buy a triangular stainless 0.7 mm corner clip from a hardware store and adjust it with long nose pliers and pincers to the correct length. Ensure circlip is strong enough to carry a sufficient load (in strong currents extra weight is required to sink disk vertically and there can be drag on the disk).	10 mm by 10 mm by 10 mm equilateral triangle that is 0.7 mm thick.
Tape measure	7	Tape used to measure the Secchi depth (printed in imperial and metric units). Stitched to bobbin at one end and attached to weight at other end by stainless steel fastening.	Tape made from fibre-glass reinforced polypropylene. Fibre-glass tape makes it resistant to stretching and polypropylene material makes it resistance to wear in aquatic environments. Nylon thread and needle required for stitching tape to bobbin. Stainless steel fastening.	Tape purchasable from on-line provider or can be extracted from a standard fibre-glass tape measure purchased from hardware store or online (we have used the 10 m senator fibreglass tape measure from Cromwell tools (http://www.cromwell.co.uk) in the past). Stainless steel sheet (for fastening) purchased from hardware store and riveted onto tape at a distance of 20 mm into the tape using a rivet tool (20 mm distance allowing for circlip, weight bolt and weight nut).	13 mm wide tape. Length of tape is dependent on size of casing and thickness of tape. The typical length used is 7 to 8 m. Stainless steel sheet 30 mm length, 13 mm width, and 0.2 mm thick. Two stainless steel 3.2 mm rivets.
Mini-Secchi casing	8a,b	Structural casing of the mini-Secchi disk. Framework encasing the measuring tape and bobbin.	The casing is made from polylactic acid. This is a biodegradable thermoplastic devised from renewable resources or natural starch. Modern polylactic acid plastic has good structural properties and when deposed can be composted.	3D printed using the Ultimaker 2 and 2+. Any 3D printer would work for this manufacturing process as the files are available in STL format. We used the Ultimaker as it is a robust, low-cost printer, and the software is open source and managed well by the company. We have clocked up almost four years of solid run time and these printers are still operating like new. Ultimaker Cura software allows for the nesting of components to make the printing process as efficient as possible. Once printed all parts require a post-print clean up. Depending on the quality of the print you have achieved will determine the clean up required. For FDM printers, a set of modelling making chisels are useful for the task.	STL file provided in Supplementary data 3.
Fixings 1	9a,b,c	Bolts and nuts to fix handle to bobbin.	Stainless steel.	Purchasable from hardware shop.	M2.5 nuts and bolts. Bolts are 10 mm long and have a countersunk flat screw head.
Bobbin	10	Cylinder holding the tape and for storing the weight. Rotates to wind tape in and out of casing.	Polylactic acid biodegradable thermoplastic (see mini-Secchi casing for details).	3D printed using the Ultimaker 2 and 2+ (see mini-Secchi casing for details). Two parts printed separately. In order to have a very accurate fit for the brass weight we printed the bobbin with a high-quality fill and finish. The two parts were glued together with an industrial super glue using a 19 mm bar inserted into the hole in the middle of each component to insure parts were accurately aligned. 20 mm reamer bit used to finish inside hole for accurate fit to brass weight (either by mounting reamer bit in the jaws of a metal work lathe and sliding the bobbin over from end to end, or reaming by hand using a decent pair of gloves).	STL files provided in Supplementary data 3.
Handle	11a,b	Handle used to wind tape in and out of casing. Two separate components to allow for efficient rotation. Fixed together with a stainless steel nut and bolt (see Fixings 1).	Polylactic acid biodegradable thermoplastic (see mini-Secchi casing for details)	3D printed using the Ultimaker 2 and 2+ (see mini-Secchi casing for details).	STL files provided in Supplementary data 3.
Colour scale	12	Vinyl-laminated Forel Ule colour scale sticker. Red Green Blue (RGB) colours for each Forel Ule colour were taken from Wernand et al. [19], see their Table 5.	Synthetic textile fibre (long-chain polymer) consisting of vinyl alcohol units.	Printed using a vinyl printer.	Example vinyl print file provided in Supplementary data 4.
Lanyard	13	Standard camera wrist lanyard for carrying the device and for preventing the device from dropping when in use.	Nylon.	Purchasable from a camera shop or online.	180 mm length.
Finger strap	14	Used to hold the device safely when in operation (by slipping one or two fingers under the strap).	0.5 mm polypropylene. Very high resistance to UV. An oily plastic with good elastic properties. High resistance to becoming brittle and capable of retaining its colour over time.	Technical drawing can be overlain onto polypropylene sheet as a template (stuck down using masking tape or spray mount) and cut out with scissors or a craft knife. Can also be laser cut from the template.	Technical drawing provided in Supplementary data 5.
Fixings 2	15a,b	Bolts to fix casing together. These screw directly into the 3D printed casing.	Stainless steel.	Purchasable from hardware shop.	M2.5 nuts and bolts. Bolts are 10 mm long and have a countersunk flat screw head.
Fixings 3	16a,b	Bolts, washers and nuts to fix finger strap to casing. Nuts fit into body of the chassis.	Stainless steel.	Purchasable from hardware shop.	M2.5 nuts, washers and bolts. Bolts are 10 mm long and have a countersunk flat screw head.