Tech note #002

My second week of engineering in Tanzania was, of course, more frustrating than the first. The 26th of June was my second visit to Mount Meru Hospital in Arusha, and afternoon labs have continued during the week in preparation for working full time at hospital in Moshi next month.

Mount Meru Hospital

After my first visit I was halfway through a repair of a hydraulic jack from an operating table. I begin the day with a trip into town to buy new hydraulic fluid, to replace the original lot which ended up all over the floor. Two litres and a tub of grease cost 30,000Tsh, or about £9.

Hydraulic fluid bought from a garage in Arsuha
Hydraulic fluid bought from a garage in Arsuha

The spanner in the works was a group of American doctors who had set up a cleft pallet surgery, so the operating table was being in use, sans jack. Unfortunate for the repair, which will have to be completed next week; good for anyone with a cleft pallet.

Leaving a job half-finished is frustrating, but a look in the hospital store rooms turns up a near limitless supply of projects. Almost all hospitals in LEDCs have rooms like this, full of broken or uninstalled equipment donated from western countries. The trick is working out what’s there because it’s useless and unwanted, and which bits are worth fixing.

One of the store rooms for broken equipment at Mount Meru
One of the store rooms for broken equipment at Mount Meru

The partial solution is to trust the doctors to know their needs, which is how I got to working on a wheelchair. The problem was general grubbiness and flat tires, one of which fine after a pump up while the other needed a replacement inner tube. Fortunately, a local bike shop carried the obscure size required. Unfortunately the valve on the tube we bought was just as obscure as the size and we didn’t have a pump that fitted it. Most unfortunately of all, we only realised this after returning to the hospital, so a sheepish second trip to the bike shop was required.

Wheelchair with punctured tires
Wheelchair with punctured tires

The final wild goose chase of the day was to find some screws, needed to fix a pair of laundry trolleys. At work last year I would have called GTSS and screws would have been delivered to the office that day. Not so in Tanzania. Several trips to several duka vya umeme [electrical shops] were required to find the correct combination of diameter, length, head shape and thread length. This simple task took several hours.

Sign for an electrical shop in Arusha
Sign for an electrical shop in Arusha

Spending lots and lots of time tracking down parts will be a recurring theme. That and very few of these repairs being very technical at all, raising the question of why they’re not being done. To be continued.

Labs

The other technical aspect is the afternoon labs I’m undertaking on the course run by Engineering World Health. Following some simple labs last week we’re now building more complex circuits like a variable power supply capable of converting Alternating Current (AC) from the wall into variable voltage Direct Current (DC) for say, charging batteries or supplying a machine.

The circuit is made up of three parts: a bridge, a variable resistor, and a bunch of capacitors combined with a voltage regulator chip. It looks like so:

Circuit diagram for a variable power supply
Circuit diagram for a variable power supply
The circuit once built
The circuit once built

The bridge is the part of the circuit that takes the AC and makes it DC: it’s the diodes on the left of the diagram arranged in the diamond shape. Diodes are analogous to valves in a fluid pipe, they let current though in one direction only. Careful inspection of the diagram shows that no matter which way the current is flowing on the input, the bridge makes it so that the current on the output always flows in the same direction. Less careful inspection leads to a lot of fiddly desoldering.

The bridge
The bridge

The variable resistor essentially lets you dump energy from the system, allowing the output voltage to be varied up and down. Increase the resistance and dissipate energy as heat, lowering the output voltage. If just the bridge and the variable resistor were used the output would look something like this:

Current once passed through the bridge
Current once passed through the bridge

Between them, the capacitors and the regulator chip smooth this out into a nice flat DC signal. The big capacitors are good for smoothing low frequency signals, the small ones for smoothing high frequency signals. The regulator chip is made of magic.

  1. Really enjoying these Con……….Particularly amused by the cleft pallets, are they like a low tech version of a cleft palate?

    It ain’t hot in Galway.