FAQ: How to Use a Tachymeter

 In FAQ

No doubt you’ve seen a tachymeter scale on a watch. Probably on a chronograph. You can also find it on some time-only watches, but without the chronograph complication, it’s completely useless. This article explains what a tachymeter is and how to use it.

What is it and what is it for

A tachymeter is a scale for measuring the speed/output/rate. It depends on what you’re using it for. Mainly, it’s for measuring the average speed over a fixed distance (a kilometer or a mile).

This only works for times of below one minute. If it takes more, the hand will point to, say, a nonsensical reading of 500 (kph, mph), unless the scale of your watch doesn’t continue to speeds below 60 units. If the speed isn’t constant, the result will be an average. This might be useful in racing, but that’s about it. And even there, it’s more of a trivia.

Anyway, all in all, it allows you to quickly get the rate of something within a measurable fixed range.

Just the speed? And in what units?

While the scale was often marked as miles per hour or kilometers per hour, it’s versatile. You won’t see the speed unit displayed on, say, a Rolex Daytona or an Omega Speedmaster.

How to use a tachymeter

A tachymeter on an Omega Speedmaster 145.022.

Rolex will give you a diplomatic “units per hour” description, which means “whatever.” Omega won’t give you that, so you get to figure out that it means “whatever units you wish to use.”

Omega’s adverts for the CK 2915 referred to it as a “tachymeter/productometer” and claimed that it had some uses in sports, industry, and science. Technically, that’s true. The base doesn’t need to be a distance. It could be a number of a different nature. Do you want the efficiency rate of a production line? Use the tachymeter, and there you go.

Any practical uses?

In fact, yes. OK, it’s a bit of a case study, although one of the “it could happen” kind.

Let’s say you’ve observed that while driving, the speed gauge shows you that you’re going at 100 km/h. But for whatever reason, the view in the windows seems to roll by faster than that. So, you don’t trust the speed gauge on the vehicle, and you want to check it.

Mark the starting and ending point of a mile or a kilometer, have the vehicle approach it at a constant speed, start the chronograph when it crosses the starting line, stop it when it passes the finish, and the chrono hand should display its speed on the tachymeter scale.

Is it accurate?

Don’t count on too precise a measurement. Of course, the result has some inaccuracy to it.

The higher the speed, the greater that inaccuracy will be. After all, if you do that at 200 kph, the point at which you’ve started and stopped the chrono might be far off from the actual moment that the vehicle crossed either line of the marked distance. This has to do with the reaction time.

So, you can’t use it to calibrate a speedometer, but you can use it to check if it needs to be calibrated, repaired, or both.

Stop the physics talk

OK, OK. I don’t like talking physics either. If you wondered if you’ll ever need the tachymeter, the answer is short and sweet. You likely won’t.

The example of a gauge that’s possibly broken is the only practical application that the tachy scale might have these days. Using it won’t save you time. But it just might save you from spending on a mechanic. That’s because you can use it to prove or disprove your suspicions.

All in all

In today’s society, the tachymeter doesn’t have much use. It’s an essential part of the design of some watches though, and let’s be honest. It just looks cool.

Have you ever had any practical use for the tachymeter scale on a watch? Let me know in the comments.

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  • Avatar
    R.F.
    Reply

    — “This only works for times of below one minute. If it takes more,
    the hand will point to, say, a nonsensical reading of 500 (kph, mph),
    unless the scale of your watch doesn’t continue to speeds below 60 units.” —
    – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –

    There’s a trap-door secret to the tachymetre bezel; after reaching the 60 second mark, the numbers on the tachy-ring become the seconds and the minutes on the dial become the Units per Hour.
    So, if the timing goes beyond 60 seconds, just add up all the seconds and go to that number on the bezel. Now read over to the minutes of an hour that’s adjacent to that number, and that’s the Units per Hour.

    For one minute and fifteen seconds (75 seconds), the rate is 48 per hour (48 minutes is across from the number 75 on the bezel), and 80 seconds is 45 Units/Hour. Some of the numbers that are usually chopped off are:
    4:00 (4 minutes flat) 240 sec. = 15 U/H
    4:35 (4 min., 35 sec.) 275 = 13.1
    5:00 (300) = 12
    6:00 (360) = 10
    7:30 (450) = 8
    8:20 (500) = 7.2
    10:00 (600) = 6
    12:00 (720) = 5
    15:00 (900) = 4
    20:00 (1200) = 3
    24:00 minutes (1440 sec.) = 2.5 Units/Hour

    3600 (sec. in a hour) ÷ elapsed seconds = the rate per hour.
    The numbers can go higher, but there’s no room on the dial.

    Chronometers are a scientific tool for measuring time. They have many uses, and can be entertaining. For example, measure how many seconds it takes a bug to travel one foot. Convert the feet per hour from the tachymetre ring to miles per hour (a mile is 5280 feet). Do you have a dog? how fast it can run. How fast can you throw a ball (this needs two people). Is your restricted-flow shower head delivering less that it should? The flow-rate depends on the water pressure in your area. In a city of 175,000, its can be as low 40 pounds per square inch during the day, but as high as 90 psi at night due to lower demand. How much money is that dripping faucet costing you? use a shot glass then count the number of shots to fill an 8 oz. cup (a gallon is 128 oz.).

    I hope this gives you some ideas; and you’re right, it does look cool.

    Your blogs about Omega Speedmasters are great: interesting variations.
    An “Ed White” would be nice, but, yikes $$$.

    R.F.

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