Xray on de move, wit de fault.

It reminds me of a riddle I used to ask my kids.
Make a 12 word sentence with these words. Defeat, deduct, defence, detail.
Answer: De feet of de duck go under de fence before de tail.

Honestly, I thought it was clever, even if the kids thought Dad had lost it.

This time, I had a mobile xray machine (xrays on de move) that had a drive fault (de fault). What a cow of a problem this was!
Turn the key, press the push bar and the machine would trundle away quite happily. After a while, it would hiccup, splutter (metaphorically) and roll to a stop, throwing up a Drive Fault message. The machine is very heavy as you would imagine. It carries a series of batteries, like car batteries, an xray generator, plus big drive motors to move the machine around.

Looking through the service manual gave some clues as to what the problem might be, what sections of the machine to look in and some tests to try.
None of this information was any good. Drive Fault error.

Fortunately, there is an identical machine in de hospital (oops) so I could swap parts between machines. The problem was in the push bar. It has miniature sensors that detect push pressure. That tells the drive logic to deliver battery power to the drive motors.

The push bar had been pushed one too many times and caused the sensors to fail.
A new push bar and presto, xrays on de move, widout de fault.

Next, a tale of flash and bang.

Over exposed - not enough ion

An ion chamber is a little capsule of gas, about the size and thickness of a business card. When the gas is exposed to X-rays, it ionizes. That means some of the gas atoms have electrons knocked off them, creating an ion. The ion chamber has a high voltage applied across it.
Because the ion now has a positive charge (fewer electrons), it rushes toward the negative side of the chamber, created by the high voltage. A small electrical current is produced. We can take that current and measure it. When the ion chamber receives enough xrays, ie enough ionization, the measuring circuit tells the X-ray generator, "that's enough X-ray thanks" (measuring circuits don't say words, but you get the drift) and switches off the X-rays.

This is actually a very good thing. If you're having an X-ray taken, you want that X-ray to turn off when the X-ray film is properly exposed. That means the radiologist (doctor) gets a good view of your broken collar bone, lungs, skull or whatever, and you don't get ionized too much. Yep, you ionize too when exposed to X-rays. But don't worry. The amount of ionization is much, much less than the cosmic rays you're exposed to while flying in a plane.
So to make a good exposure, the ion chamber is placed in the X-ray beam, and it's adjusted to turn off the X-ray generator when it receives enough X-rays to make a good exposure on the film. The X-ray beam starts at the X-ray tube, then passes through you, then the ion chamber, then the film.

So I was called to investigate why films in this particular room were always dark, ie too much X-ray. You're way ahead of me here. You've probably already guessed that the ion chambers weren't working. Why was that? Because someone had unplugged the circuit that measures the current from the ion chamber. It wasn't me I promise.

So plugging the circuit back together fixed the problem.

Next, Xrays on de move, wit de fault.

Light's on, credit's good.

Today, a ceiling suspended xray tube wouldn't lock into its central position.
To explain, in some general xray rooms, the xray tube (the part where the xrays originate) is suspended by rails from the ceiling. The tube can be moved in the X, Y and Z directions, that is, up/down, left/right, back/forwards.
To make it easy for the radiographer to make an xray film, the xray tube generally locks into a position that centres it over the film container, called a buckey. Centre of the film (and the patient) is then easy to locate, every time a film is exposed.

On this job, the central position lock didn't operate. My job is to find out why and fix it.
Using a ladder, I climbed up to a point where I could see if the index mechanism was actuating the lock. In many cases, it's simply a switch that operates as it passes a bump or groove in the suspension track. But not on this job.
I discovered that this particular room has a small infrared light beam, that reflects off a reflective dot stuck onto the suspension rail. As the light passes by the dot, a sensor next to the beam detects the reflected beam and triggers the lock for the centre of the buckey. A tiny little red light comes on at the sensor when it detects the beam. No little red light came on as the beam passed by the reflective dot. What gives?

What could I use as a mirror to test if the light beam was present. I needed a tiny little mirror about the width of a pencil. Where do you find one of those? I've seen one somewhere before. Aha! I remember that my American Express card has a very reflective strip on the back. And it was in my back pocket, saving a trip down the ladder.
Waving the credit card across the light beam . . . . clunk, the lock engaged. The little reflective dot had gone dull with age. Now how to fix it? Where could I find a bit of reflective something. No, my credit card has business most places except stuck to the suspension rail in an xray room.
A trip to the vending machine in the hallway; a packet of chewing gum with silver wrapper, a bit of sticky tape and viola, problem solved.

Now; if shining a light on my credit card could fix more problems. . . .

Next, Over exposed; not enough ion.