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Pasco TD-8551A Magnetic Counter Upgrade

The “Mechanical Equivalent of Heat Apparatus” device from Pasco (Part Number:  TD-8551A) is used in many of our lower division labs.  The device works very well and gets good results, but the counter sub-system is prone to failure.  It consists of a small rubber tab on the handle and a mechanical counter.  Each time the rubber pushes on the metal finger of the counter, the mechanical counting system advances by one.  The rubber and the metal finger both break over time.  Sometimes the rubber doesn’t hit the finger hard enough to advance the counter.  Dr. Campbell suggested using a magnetic system instead.  This project details the replacement of the mechanical counter with a magnetic one.  The goals are to reuse as much material as possible.

The Magnetic Counter

The magnetic counter shown in the picture below was purchased from Amazon  It is a generic part that is available from many vendors in China and Hong Kong.  The package contents consists of the sensor and cable, the counter, a magnet, two screws, and two mounting bars.  The mounting bars are not used.

The counter unit is 63.5mm wide, 37.4mm deep, and 34.7mm tall.  The top lip on the counter unit is 3 mm tall.  The sensor is 12mm in diameter.  Any counter and sensor that is similar in size should work.

The 3D-Printed Parts

Two parts were designed to hold the magnet (click for STL) and the magnetic counter (click for STL).  The parts were printed using black generic PLA on a Monoprice printer.  The settings I used were a fill density of 10%, bed temperature of 60C, and extruder temperature of 195C.

The Upgrade Procedure

1. Remove the allen bolt from the handle.  Put aside the washer and the rubber piece.
2. Put the magnet into the 3d-printed magnet holder and attach it to the handle using the original bolt.  The magnet should go on the side of the handle toward the counter as shown in the final picture below.
3. Remove the old counter from the base and put it aside.
4. Attach the 3d-printed counter holder to the base using the four screws from the old counter.  It should be orientated so the 12 mm hole is toward the handle.
5. Trim the sensor cable to 8 cm and bare about 0.5 cm of each wire.
6. Thread the sensor into the 3d-printed housing and use the two black plastic nuts to secure it.  It should protrude from the housing just enough that the yellow sensor surface is flush with the plastic bolt as shown in the picture below at left.
7. Insert the counter into the housing so that the wire terminals are not on top of the sensor.  With the top of the counter flush with housing, mark the position of the side holes on the counter with a pencil.
8. Using a #40 bit, drill two pilot holes 3 mm into the counter where you marked them with a pencil.  The finished hole is shown in the picture below at right.
9. Connect the two sensor wires to the terminals on the counter labeled sensor.
10. Slide the counter into the counter holder and secure it with the two screws that originally held the metal piece to the counter.

The Finished Product

The magnetic counter at 48 mm in height is taller than the mechanical one at 33 mm in height. The magnetic counter is 5 mm shorter in length and 10 mm longer in depth. There is no interference with the operation of the machine due to the differences in length. The comparison of the size of the two counters is shown in the pictures below. The magnetic counter is on the left and the mechanical counter is on the right of each picture.

Page maintained by Tom Ekkens | Last update on April 11, 2019