2" Heating oil tank Dip Stick cap

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I recently invested in a Sensit, Oil tank contents gauge but was a bit skeptical on its accuracy, especially throughout the range due to tank shape and the changing volumes per divisions of height. So I made a spreadsheet which enabled me to log the level height of the oil measured by the Sensit, and carry out a physical dip of the tank to enable a comparison of measured height accuracy from the ultrasonic sensor and the dip height. The Sensit reports daily to an App on the phone and displays a percentage value, accurate to the nearest 5% and calculates remaining volume of oil in the tank, and a predicted run-out date.Taking the regular tank dips was proving to be a messy business, initially using an old tape measure, it dripped oil all over the place, as did the bamboo stick I started using and then measuring with the tape measure, so thought I'd make a semi permanent dipstick which could remain inside the tank.My tank is manufactured by Harlequin and is a 2000 ltr bunded horizontal style. It has a nominal depth of 1100mm, but I considered the 'Lost volume' at the bottom of the tank due to the outlet being around 50mm from the top of the outlet pipe to the tank base. For calculation purposes, I considered this 50mm to be the zero point, as it's not advisable to drain down beyond this without fear of pulling sludge and sediment through to the boiler pump. The tank has a standard 2" BSP fill pipe at the top and the dipstick is designed to replace the standard plastic screw on cap.The dipstick itself is made from acrylic tube and rod, enabling a tape measure to be viewed when inside the tube. Following items were ordered from a mail order plastics company, (Trent Plastics, in my case in UK). Parts list as follows:Acrylic tube, 16mm OD, 12mm ID x 1000mm lengthAcrylic Tube, 12mm OD, 08mm ID x 500mm lengthAcrylic rod, 8mm diameter x 1000mm lengthAcrylic rod, 8mm diameter x 500mm lengthAcrylic solvent, (Extru-fix) 50ml bottle2 x M4 x 18mm cap head screwSelf adhesive measuring tape 300cm x 10mm width steel. (From Amazon UK). Link: https://www.amazon.co.uk/gp/product/B0BL3NQ3KL/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1Once the depth of the tank is established, you can cut the 16mm tube to length. the distance from the sealing face of the cap, to the end of the tube support of the cap is 50mm so remember to take this into account before cutting. I allowed for the unusable depth of 50mm as I didn't want the dipstick to make contact with the base of the tank, which meant the 1000mm length was actually ok. Could have done with an extra 25mm to be perfect but I never run the tank that low anyway.I cut approx 25mm of the 8mm x 500 rod and the 12mm x 500 tube to make a plug for the bottom of the dipstick. Rough up the surfaces to be bonded with some Scotchbrite or wire wool and clean with some meths or IPA before applying the solvent weld and glue inside the tube, ensuring there are no leak paths. i.e. 8mm rod, inside the 12mm tube, inside the 16mm tube. Allow to fully cure overnight.Cut another 40mm of the 12mm tube to act as the sleeve for the 8mm rod at the top of the dipstick. Slip this over the 1metre long 8mm rod and solvent weld at a position of 50mm from the end of the 8mm rod. The purpose of this 8mm rod is to support the tape and stop it kinking inside the 16mm tube, and the 50mm protrusion at the top is where it slips into the cap, to be secured by the two set-screws.At this point, I set about calibrating the dipstick so, Step 1, took a measurement of the current oil level using a standard tape measure, from the base of the tank. Subtract the un-useable depth value which will give you the oil level above the set zero height, and make a note of the value. Now carefully dip the dipstick tube into the tank, keeping it vertical and line up the top of the 16mm tube with the top of the 2" fill pipe. Do not submerge the dipstick beyond this level as we're now going to take a measurement from the wet level on the dipstick and need it to be representative of the actual level. So withdraw the dipstick tube carefully and note the level mark of the oil level. You can put a piece of masking tape on the outside of the tube to mark the oil level.Measuring time. The easiest way I found to do the next bit is as follows. Lets say that the measured depth of the oil when using the tape measure in step 1, was 375mm from tank base. Assuming 50mm un-useable depth, subtract 50mm from the 375, = 325mm of useable oil volume. Slide the steel tape ribbon, zero end first, into the tube until it touches the top of the plug in the bottom of the tube. Make a note of the reading on the tape ribbon, where the oil level was marked on the tube. Now add 50mm to allow for the additional protrusion of the dipstick cap support spigot from the top of the 2" fill pipe. In a perfect world, the value on the tape ribbon at this mark will be 325mm but we know this won't be the case. it will be less. So subtract the marked value from the original useable oil level value measured in step 1, and this is the amount of tape ribbon required to be snipped off the zero end. Once snipped, slide back into the tube and re-check the correct values are now aligned. Also note the value on the ribbon where it aligns with the top edge of the tube. Deduct 40mm from this value and snip off the excess tape ribbon.Originally, I intended to stick the tape ribbon to the 8mm rod but I found the steel didn't want to conform to the 8mm curve, so I just inserted the rod carefully behind the tape ribbon, to secure it in place against the inside of the 16mm tube. Rough up and clean the top 40mm of the inside of the tube and the outside of the 12mm sleeve fitted to the rod earlier, apply solvent weld and insert until flush with the top edge of the 16mm tube. Again, ensure no leak path evident in the solvent cement and allow to cure overnight.Push the dipstick into the 2" cap support spigot and secure with the pair of M4 set screws. Don't overtighten or the acrylic rod can shatter. Place dipstick into the oil tank and screw the cap onto the 2" fill pipe until it is home. Unscrew and observe indicated oil level on dipstick. It should tally with the measured useable depth of the tank from step 1. A small amount of calibration adjustment is available by slackening off the set screws and moving the dipstick in or out of the cap slightly, if required but if the steps above were followed, it should be pretty much millimetre accurate.Update. The dipstick has been in use for well over 2 months now with no issues, other than it could be a bit tricky to see the oil level on the shiny surface of the acrylic tube. So I masked off an axial section to the side of the tape ribbon scale, and roughed it up with a bit of Scotchbrite along its full length. The oil level can now be clearly seen against the matt surface whilst leaving the scale readable through the rest of the crystal clear tube.