Review: Bell & Ross BR V2-94 Black Steel Chronograph
Deployant Events: Roundup of the month's horological events for the month of August 2017

Technical corollary: Analysing the Bell & Ross BR V2-94 Chronograph crystal with the SAM

Geeking out with the Scanning Acoustic Microscope.
by Sharon Tan on August 30, 2017

Ultrasonic scanning is not new. A familiar application would be ultrasound scan of an unborn baby, during which we would probably hear excited yelps when parents catch a glimpse of their child for the first time. The Scanning Acoustic Microscope uses this technique, and we use it to look at the crystal of the Bell & Ross BR V2-94.

An even earlier use, maybe the first ever use, are the bats. Bats make their own ultrasonic sounds to “ping” their path, then time taken for them to hear their own echo would tell them how far away a cave wall is.The physical concept is all the same- sending out a sound wave and detecting the time taken for an echo to be heard. The longer the time taken (time of flight) to hear an echo (reflected noise), the further away the wall is.

Acoustic scans of the Bell & Ross BR V2-94 Chronograph crystal

We reviewed the Bell & Ross BR V2-94 in great detail here. This is a Technical support article to explain the SAM in more detail.

Graphical representation of images from the Scanning Acoustic Microscope (SAM)

Graphical representation come in many forms, the most frequently used would be B and C scans. Here’s an overly simplified analogy. Say I have a double cheeseburger and I’m doing a scan with the probe on top of the burger. A B-Scan would have the probe move horizontally above the burger. At each point above the burger, it transmits an ultrasound which hits multiple surfaces such as the bun, the lettuce, and then the cheese and then the patty. With each surface that is further away from the probe, the echo takes a longer time to reach the probe. So the probe does this all the way along the top of the burger. Then the image produced would be as if we cut the burger in half with the components all stacked up.

For a C-scan, the probe will move in a snake pattern above the burger, until it fully covers the burger. This image would be a little complicated to imagine. In any one image, we first have to decide what’s the sound time of flight we want to focus on. So for a short time of flight, we will see an image with just a top view of the bread. And a longer time of flight, we will see the top view of the patty. The longer the time of flight being imaged, the further away is the image slice from the probe. But of course, this is just an analogy to visualize B and C scans. A detailed and scientific explanation can be found hereThe main difference in this analogy to an actual scan is that the ingredients (materials) will actually absorb the sound, and the further we go, the less echo goes back to the probe. Also, with different materials, the speed of sound keeps changing and there are internal echos which affects the echo that ultimately reaches the probe.

So now that we are all experts at ultrasonic scanning, we are all ready to interpret some images that came out of a Scanning Acoustic Microscope (SAM).

Analysing the Bell & Ross V2-94 crystal

We were curious about the crystal on the V2-94. We couldn’t be sure if the underside of the crystal follows the ultra curved outside or is it actually flat. The rounded edges of the glass creates a magnifying effect, and at steep viewing angles it almost appears like the centre of the glass might even be thicker than the edges.

Graphically, if it were like the image on the left (A), or if it were on the right(B):


So what’s the cross-section of the crystal?


So we went to the lab, and put the V2-94 to be examined under the SAM. These were the images we got.

The B-scan

The B-scan looked like this:


B Scan that is very noisy and distorted


The B-scan didn’t let us make good sense of the detailed shape (bad frequency coupling, noise from the watch, internal reflection of sounds due to high impedance mismatch etc.) although that was intuitively the easier scan to give us our answer.

The C-scan

So let’s do it the challenging way and study a series of C-Scans at various depths. The picture below shows all the layers superimposed into one image.



What are we seeing here? Basically we are seeing the layers of the crystal, stacked on top of one another in one superimposed image. The composite below is perhaps a good visual:


The superimposed Cscan images on the right, and the corresponding dial on the left.


This scan reveals a bit more information to us. How do we analyse it?

First, we admit that the probe used was not ideal for sapphire crystal (we kind of hijacked an equipment used for studying much softer materials) and thus the images are not crystal clear. No pun intended.

Our verdict is that the glass is also ultra-curved on the inside. That is B.

The scan tells us that the crystal is basically approximately the same thickness and is curved sharply at the sides. Reason being that there are residual rings on the later part of the scans. A flat inner surface would have shown us steadily growing disc sizes instead of a ring.

The C-scan slices are not smooth and even. This is due to the method in which the crystal is manufactured. It is grown into a large block, and then material is taken away till it is close to the shape required. The crystal is then hand polished till it achieves the shine and smoothness required. As the last step is hand done, the scan reveals tiny imperfections in the curvature, consistent with this.

We’ve stacked them together in a helpful looping gif below:


C-scans moving from the top of the crystal surface towards the dial. Each image about 200ms apart from the previous.


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