Chapter 8  Making a Linear Scale for a Meter.

8.1  Altering an Existing Scale.
8.2  Making a New Linear Scale Using MaxCAD.

Chapter 8

Making a Linear Scale for a Meter.

Analog panel meters come in a wide variety of shapes and sizes. They come from the factory or the surplus dealer with a scale that may not be what your application requires. Most meters can be taken apart and a new scale installed that matches your needs.

Most round meters come apart by removing 3 screws that go radially into the meter at the back edge. When you pull it apart the meter movement and scale stay with the back part.

Meters with a round body and a square face may come apart in a couple of ways. Often there are screws that are installed with their heads toward the back of the meter on the surface that will be flat against the panel. There are usually 4 although in large meters there may be 6. Be sure you have them all out before you start prying on the front cover. The entire works of the meter stays with the case and just the front glass and escussion come off.

If you have one of the above types and you can't find any screws, chances are the front just pops off. Insert a small screwdriver and pry gently, with the emphasis on GENTLY.

There is another type of meter that you should walk away from. It is characterized by the front glass being curved convex (outward). Also the zero adjust screw does not penetrate through the glass rendering it inaccessible. I have heard variously that these meters are evacuated or have been filled with dry nitrogen. The curved glass seems to point toward a vacuum. A flat glass would have to be very thick to withstand the pressure difference.

You might feel that the meter is of no use to you in its present form so you have nothing to lose. There are likely to be flying bits of broken glass associated with an attempt to open such a meter. If you are determined to try, don't say I didn't warn you.

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8.1 Altering an Existing Scale.

This method does not employ MaxCAD but it is included for completeness. Occasionally A meter has the correct scale but the legend might not be what you want. For example, you may have a surplus meter that is calibrated from 0 to 1 and the legend says "Crystal Current in mA". You want to shunt the meter to measure 0 to 1 amp and use it in a power supply. All you need to do is alter the legend.

  1. Carefully disassemble the meter and remove the metal plate which holds the scale. Reassemble the meter leaving the screws out so as to protect the works from dust and grit. Note: (A meter movement is similar to a watch. It doesn't take much to gum up the works and stop it from working.)

  2. Place the meter scale in your scanner backed by a dark piece of paper. This is to make the mounting holes show clearly on the scanned copy.

  3. Load the scanned image into your favorite photo editor. Instead of trying to erase the old legend, use the color picker to get the exact shade of the "white" face and paint over it. If you do need to change the scale numbers do the same.

  4. Call up the lettering feature and draw the new legend on the meter face. Do the same for the numbers if you are changing them.

  5. Print out the revised meter scale on photo paper. Be sure to use a paper that contains some plastic. Ordinary paper changes dimensions as the humidity changes. High quality photo paper is designed not to do this.

  6. Cut off most of the excess paper. Turn the metal plate that holds the meter scale over and glue the new scale to the back. That is so if you should ever want the old scale back it is there to be used again. Do not use a glue that contains water as a solvent. Some spirit glues are dark and will soak through and stain the paper. Don't use one of these glues. I have installed new scales on several meters using rubber cement.

  7. If you have a light box that is the best thing for the next step. Place a light source behind the metal plate and align the dark spots that mark the mounting holes with the holes in the plate. Wipe off as much excess glue as possible and weight the scale down with something heavy and flat, (books), and leave it to dry overnight.

  8. Using an Exacto knife, trim down the paper to match the edges of the metal plate and cut out the mounting holes.

  9. Disassemble the meter, install the new scale, and reassemble.

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8.2 Making a New Linear Scale Using MaxCAD.

  1. Carefully disassemble the meter and remove the metal plate which holds the scale. Reassemble the meter leaving the screws out so as to protect the works from dust and grit. Note: (A meter movement is similar to a watch. It doesn't take much to gum up the works and stop it from working.)

  2. Now here's the rub. There is no way to get a scanned image into MaxCAD. Make some measurements on the existing scale and use the measurements to make up a new one employing the scale editor that is built into MaxCAD. Place the scale on the drawing board and mark the center of curvature with two crossed lines. Also place two small circles to mark the mounting holes.

  3. Print out the scale on tracing paper. Lay the paper over the existing scale and figure out what you need to do to make it match the scale. Too much curvature? Make the radius larger. Curvature is right but the new scale overlaps the old one at each end? Make the angle smaller.

  4. Edit the scale and pay attention to the mounting hole markers as well. I have found this to be pretty much a trial and error process.

  5. Print out the changed design and compare it to the original scale again. Go through as many iterations as it takes to get it right.

  6. Print out the revised meter scale on photo paper. Be sure to use a paper that contains some plastic. Ordinary paper changes dimensions as the humidity changes. High quality photo paper is designed not to do this.

  7. Cut off most of the excess paper. Turn the metal plate that holds the meter scale over and glue the new scale to the back. That is so if you should ever want the old scale back it is there to be used again. Do not use a glue that contains water as a solvent. Some spirit glues are dark and will soak through and stain the paper. Don't use one of these glues. I have installed new scales on several meters using rubber cement.

  8. If you have a light box that is the best thing for this step. Place a light source behind the metal plate and align the small circles that mark the mounting holes with the holes in the plate. Wipe off as much excess glue as possible and weight the scale down with something heavy and flat, (books), and leave it to dry overnight.

  9. Using an Exacto knife, trim down the paper to match the edges of the metal plate and cut out the mounting holes.

  10. Disassemble the meter, install the new scale, and reassemble.

  11. Enjoy.

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