Electronic vs. Mechanical Scales

So you have this dilemma, you need to weigh something, be it a rocket engine, a spare screw in your next robot project, or the amount of glue you will be applying to your final project (this applies to just about anything) and you need to know the EXACT mass. Here we will be comparing the two options that (most) engineers have in small-scale projects. We have the electronic scale and the mechanical scale, so lets compare these two scales against each other and see what we get.

ELECTRONIC SCALE:



I have in my possession an OHAUS Explorer Pro electronic scale (seen above). This scale is quite large with rough dimensions of 32cm x 21cm x 10cm (length/width/height), requires a 12VAC / 1A output power adapter, and has a maximum measurable mass of 2100g. The scale has a large LCD display with several buttons surrounding it.

Buttons and Functions:



The positioning bubble (yellow) is like that found in the level of a construction worker, there is a small black circle and then fluid and a bubble underneath, to level the scale you must adjust two height adjustment screws at the back of the scale until the bubble is fully inside the circle, then you know the scale is level.

Warning: the scale MUST be leveled before any readings are taken to prevent invalid data.

Turning it on:

When you press the On/Off button (only once) the scale will turn on and initiate the startup sequence, this includes checking internal calibration automatically. When the internal calibration is off, the scale will take a few moments to recalibrate (as shown below).



The scale will calibrate itself and you will hear a series of motor noises, grinds, and clunks. When the recalibration sequence is complete a screen like that shown on the right will appear showing the corrections.


How it Works:

To measure the mass of a specimen (under 2100g) simply place it on the metal pan (after using the O/T button to zero the scale) and read off the output value on the LCD screen.


MECHANICAL SCALE:



I have in my possession an OHAUS Triple Beam Balance mechanical scale (as shown above). This scale is fairly large with rough dimensions 43cm x 10cm x 16cm (length/width/height). The scale is composed of 100% metal (high quality) and has a maximum measurable mass of 2610g.


How it works:

To measure the mass of a specimen, place it on the metal pan. Zero all the weights by moving them completely to the left (zero). Now slide the weights (there are 3) starting from the largest to the smallest in the direction of increasing mass. (note: in the picture the weights are at the maximum setting, corresponding to 2610g).

1. Slide the largest weight from the zero position to the right until the metal arm tilts downwards PAST the white zero marking. Now move the weight ONE position to the left (decreasing mass).

2. Repeat the process for the medium sized weight.

3. Now slide the small weight along until the arm is aligned with the white zero marking. Add up the values from all three weights (their positions) to find the total mass of the specimen.

How They Compare:

Electronic Scale:

Pros:
- Higher Accuracy
- Automatic calibration
- Large LCD screen
- Many extra functions (with buttons)
- Positioning bubble (to level) - Ability to ZERO at any mass
- Does not require any skill/knowledge to use
- able to measure hundredths of a gram
- able to measure in grams or ounces

Cons:
- Price: roughly $4000 CAD
- Requires an electrical outlet
- Quite large and heavy
- Not very durable

Mechanical Scale:

Pros:
- Price: roughly $100 CAD
- Does not require an electrical outlet (can be used anywhere)
- Durable

Cons:
- Lower Accuracy
- Requires skill/knowledge to use
- no other functions
- Manual calibration required
- Only one zero point


To test the difference in accuracy between the two scales I measured a standard Ti-83 calculator using both scales.



Results:

Electronic Scale: 260.00g

Mechanical Scale: 260.6g

This corresponds to a 0.23% error in the mechanical scale reading. As you can see the difference is not all that big. Therefore when spending an extra $3900 CAD to upgrade from the mechanical to electrical scale the major gains are an extra significant figure (hundredths of a gram) extra functions (able to print directly from scale etc.) and the ease of measuring specimens.