Figure 1.1 Circular scale - linear — Page 9, Lesson 11

Let’s start with the idea of a circular scale — that’s the classic round instrument face where a pointer sweeps around a dial. The first two figures I want you to see are Figure 1.1 and Figure 1.2. Figure 1.1 is labelled “Circular scale – linear.” Look at the numbers around it: 0, 5, 10, 15, 20, 25, 30, and the unit is “Kts x 10.” That “Kts x 10” means the pointer reading is multiplied by 10 — so if the pointer sits on 15, the actual speed is 150 knots. The scale is linear because the spacing between each number is equal — the same angular distance from 5 to 10 as from 10 to 15. That makes interpolation easy: you can read halfway between marks with confidence. Now Figure 1.2 is the contrast — “Circular scale – non-linear.” Look at the numbers: 0, 1, 2, 3, 4, then 5, then back down 4, 3, 2, 1, and the label is “RATE OF CLIMB.” Notice the spacing is not equal. The divisions are compressed in some regions and stretched in others. That’s deliberate — on a rate-of-climb instrument, the scale is designed so that the range you use most often, or the range where accuracy matters most, gets more physical space for the pointer to move. So a non-linear scale trades uniform spacing for better readability where it counts. Now, the core problem this chapter addresses: what happens when the range of values an instrument must show is very large, and you still need to read changes with a fair degree of accuracy? A single 360° sweep of the pointer may not be enough. The book calls these High Range Long Scale Displays. There are several solutions, and each has a trade-off. The first solution is to let the pointer make more than one revolution. That’s Figure 1.3, the single-pointer airspeed indicator. The pointer goes around and around, and the scale is marked 0, 10, 20, 30, up to 90, and beyond. The figure shows an airspeed of 300 knots. The problem: because the pointer can make multiple revolutions, you can’t tell just by looking at the pointer where it is in its cycle — is it on its first revolution, second, or third? That ambiguity is the confusion the book warns about. You see the pointer at 30, but is that 30, 130, 230, or 330? The single pointer alone doesn’t tell you. So the second solution, Figure 1.4, splits the task between two pointers. You have a long pointer moving over a fixed main scale, and a small pointer moving over a smaller inset dial. The book is specific: the small pointer and dial, inset at the top of the instrument, indicates the 100s — hundreds of knots. The long pointer and the main dial indicate the 10s — tens of knots. So in Figure 1.4, the small pointer reads 0 hundreds, and the long pointer reads 3 tens, giving 30 — but the figure caption says this indicator shows an airspeed of 33 knots. So the long pointer sits just past the 3, and you read the fine position to get the 3 knots. The key point: the small dial removes the ambiguity of which revolution you’re on, because it tells you the hundreds digit explicitly. The third solution, Figure 1.5, is the revolution counter — and this one is less confusing. Here you have two concentric pointers moving over two separate scales. Concentric means they share the same centre, like the hands of a clock. The small needle, with the inner scale, reads 10s of units. The large needle, with the outer scale, reads units. So you read the large needle for the ones digit and the small needle for the tens digit. The figure shows this indicator reading 25½ % rpm for a turbine engine. So the large needle points to 5½ on the outer scale, and the small needle points to 2 on the inner scale — giving 25½ percent of rated rpm. Because the two pointers are physically separate and each has its own scale, there’s no ambiguity about which revolution you’re on. The fourth and final solution, Figure 1.6, is the three-pointer display — and this is the one used on many altimeters. The book says it displays information in a similar fashion to a clock, with pointers showing hours, minutes, and seconds. But here the three pointers each cover a different range of altitude. The long pointer covers 1000 feet in one revolution, so each division of its scale represents 100 feet. The middle pointer covers 10 000 feet per revolution, so each division marks 1000 feet. And the smallest pointer covers 100 000 feet, with each division representing 10 000 feet. So you read the three pointers together: the small one gives you the tens of thousands, the middle one the thousands, and the long one the hundreds. Figure 1.6 indicates a height of 25 950 feet — the small pointer on 2 (20 000), the middle pointer on 5 (5 000), and the long pointer on 9½ (950). So the thread through all of this: when a single 360° sweep isn’t enough, you have four ways to extend the range — multiple revolutions of one pointer (confusing), a fixed scale with a moving scale for larger units, two concentric pointers on separate scales, or a clock-style three-pointer arrangement. Each is a different answer to the same problem of reading a large range accurately.