When you rent an airplane, you are usually charged flight time against one of two devices - the tachometer or the Hobbs. The two measure time quite differently, and knowing how they work can help to alleviate some confusion.
The Hobbs is simply a small electric clock that reads in hours and tenths of hours - a tenth, of course, being six minutes. In the vast majority of airplanes, it is triggered by oil pressure so that it runs only when the engine does. A small sensor on the engine checks for oil pressure, and when it rises above a certain level, usually around 10 psi, the contacts close and the Hobbs begins emptying your bank account. The meter counts the same no matter how fast the engine is running or whether you are flying or waiting in line for takeoff. It has become popular as a means to measure an airplane's total hours (for both revenue and maintenance purposes) because it is difficult to fool. It is not usually connected through the master switch, either, so you can't shut down the electrical system and save money.
Measuring time by the tach, on the other hand, is full of inconsistencies and inaccuracies. A tach-based hourmeter is designed to count up accurately only at a certain engine speed, usually at high cruise or, say, 2,500 rpm in a typical training airplane. That means that while you swelter on the ground as number five for takeoff, the tach hourmeter is not ticking off anywhere near real time.
So which is better? That depends upon your point of view. If you want to spend your time doing touch and goes, measurement by the tach is best because it will ring up less time than you really flew. That's because you will spend little time at a power setting that makes the tach hour equal a clock hour. On a long cross-country, the tach hour is very close to reality, especially if you are cruising at the indicated rpm that corresponds to the tach hourmeter's set point.
Naturally, you won't likely find an FBO or flying club that charges the same for tach and Hobbs methods. Generally speaking for a training aircraft, the difference between Hobbs and tach time is 10 to 15 percent, with the tach showing fewer hours flown. On cross-countries, that difference will probably be about 5 percent or less. You'll want to make sure that the variance in rental prices accurately reflects these differences.
Not all Hobbs meters are wired alike, however. In some complex airplanes, the Hobbs might be hooked to the gear or squat switch, meaning that it clicks along only when the gear is up. Still others might be hooked to an air switch, measuring time only when the airplane has exceeded takeoff speed.
Often, when renting an airplane with both a Hobbs meter and recording tachometer, you will be asked to write down both indications. This is a safeguard for both you and the renting agency so that a malfunction of either instrument will be easily noticed, and you don't have to go on your good word that the Hobbs stuck open - and counting - that cold morning. (Which, incidentally, has been known to happen on very cold days after unsuccessful engine starts when the congealed oil can hold the Hobbs switch open. Be careful out there.)
One final item: Always check the rental logs against the tach and/or Hobbs time before you take off. Few pilots have never had a bookkeeping error or an undocumented maintenance flight show up on the monthly statement. There is, after all, no reason to pay for the good time you didn't have.
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