Monday, October 1, 2012

Your car is probably lying to you.

Most cars have an array of gauges in front of the driver. There's always a means of indicating road speed, and usually a gauge for fuel level. What else there is varies depending on the target market, but nowadays a tachometer to show engine speed and a temperature gauge to show if the engine's overheating are normal.

Most of these gauges are fully functional, but one, the temperature gauge, tends nowadays to be a skeuomorph. It's not technically a temperature gauge (which in automotive terms is actually a suitably calibrated ohmmeter, reporting the varying resistance of a bit of wire that is the temperature sensor), but a multi-state indicator. This is called compensation, and it's annoying to me.

The major reason for compensation is to avoid worrying inexperienced drivers. When first sitting at the controls of a car, one sees this apparently huge panel of dials, very few of which are all that obvious in function. To avoid overwhelming the novice, much of what used to be handled by dials (oil pressure, electrical system health, and so on) has been relegated to a row of go/nogo "idiot lights". The temperature gauge was a candidate for this, but it turned out not to be a good idea; there is still a need for more granularity than a simple light can provide. However, by filtering the signal fed to the gauge itself, the needle can be held rock-steady.

Why is the filtering needed for a steady needle? Because engine temperature is affected by multiple factors working to both increase and decrease it. Demand more power, and the extra fuel will result in more heat energy going into the engine; it heats up. The thermostat opens, and it cools down; the electrically driven fan comes on, and it cools down further, closing the thermostat. Heat begins to build again. And so it goes, with the temperature confined to a surprisingly narrow range so long as everything's working as it should. If the filtering weren't there, the needle would move around, and an experienced driver would be able to see when the thermostat's opened, and so on. This would be useful should something in the complex chain of equipment stop working, since it would be reflected in the gauge's reading, making figuring out what's broken easier.

However, a nervous driver will be distracted by this wavering needle, wondering, not unnaturally, what they're doing wrong; and making the needle stay still is easier than explaining that "it's meant to do that". So in goes the filtering circuit, and the needle now has five positions. When the engine's cold, it's just off the stop, since it has to indicate that the engine is running (despite the tachometer; it's expected that a gauge will be "live" when the engine is running, so even if a cold engine should have the needle sitting on the stop, it's moved up); when the engine's warm, but not to full operating temperature, it'll be between there and the middle. When the engine's in normal operating range, the temperature needle will be rock-steady at the middle of its travel, since there's a strong mental association between "the middle" and "where it should be". Then there are "moderate overheat" and "severe overheat" positions, although the severe overheat will usually be heralded by a cloud of evaporating coolant and a cessation of drive.

But you're not seeing what the engine temperature actually is. Instead, you're seeing that it's within what the designers of the circuit deemed acceptable. This doesn't just make troubleshooting harder; it introduces another thing which can fail. The small circuit board that handles this for Volvo 240s is notorious for failing, and it's normal for enthusiastic owners to simply remove the board and substitute a short wire which restores the temperature gauge to being an actual gauge, rather than an indicator. Most vehicles built after the later 1980s will tend to have some degree of compensation in their temperature gauge; so unless you're driving a classic, it's probably lying to you about the engine temperature.

I'm not a fan of skeuomorphs in general. I much prefer that an interface be designed from the ground up for the device it's to be used on; skeuomorphs hinder accessibility and force compromises which lead to less powerful interfaces. It could be argued that skeumorphic design leads to less usable interfaces as clutter and non-functional elements accumulate.

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