Refinement of the weighing arm of tinman

March 2009

The present weighing arm has two pivots. One suspends the water container, the other is the centre pivot.

Both pivots use the principle of a sharp point resting in a shallow conical depression. This arrangement is easy to make but has the disadvantage that the spacing between the apices of the cones is not exactly the same as the spacing between the pins that rest in them.

This gives uncertain repositioning after the contact is broken, which happens every minute. This is a possible explanation for the poor precision of the weighing, compared with the precision when one repeatedly pushes down the beam without breaking the pivot contact.

The problem can be solved by retaining one of each pair of cones but converting the other into a groove with a 90 degree angle. This is sketched in the attached drawing.

The even bottom of the groove need only be 2 mm long, centered on the present position of the hole, though the ends of the groove may need to be bevelled to give space to the conical pin.

There is one practical consideration: the spacing between the beam and the support for the central pivot should be measured before dismantling the apparatus, because the position of the pins will beed to be adjusted afterwards. This is because the conical receptacle screw and the milled groove in the other screw must be furnished with lock nuts, so the groove cannot turn. This will raise the pivot point, which must be compensated by a corresponding raising of the pins on the beam.

The beam can be removed without disturbing the rest of the apparatus, though a careful slight twist is necessary to avoid damaging the water stirrer. Note that the counterweight is heavy! The tank end of the beam can be unscrewed for milling. It is best not to mess with the counterweight. If you do, record the exact placing of the weights, because it is important that the counterweight just overbalances the full tank of water. When you replace the beam, fill the tank with water, then measure the weight. Tim can use this value in the control program to stop the peltier condensing more water and overflowing the tank.

This is the centre pivot (enlarged detail bottom left). It may be that an entirely different way of making the groove is easier: I have sketched a milled base of a set screw. That may be difficult to do. The height of the groove bottom relative to the screw is not critical. There can be an extra 2-5 mm of height, compensated by screwing up the pins. It would be wise to measure the distance between beam and support base (the vertical arrowed line) before disassembly, to save time when re-assembling.

This is the tank pivot (enlarged detail bottom centre). The groove can be extended to the edge of the beam, if that makes milling easier. There is no adjustment possible for the pins, so the bottom of the groove must be equal to the depth of the hole at present. Some minor adjustment is possible by moving the pins of the centre pivot to tilt the beam.