Having an accurate scale can save you a lot of headaches and arguments with parents and will ensure that no racer has an unfair advantage due to weight. There are a lot of choices out there as to different kinds of scales, calibration weights, and scale accuracy and precision. This page attempts to help you to make the right choice in scales and to help you determine if your scale is accurate and precise and thus fair.
Types of Scales
- Counter Balance Scales - These kind of scales use a counter weight to measure an object's weight by correlating the distance that the counter weight is moved to the object's weight. Generally these are simple to operate but may not be the most accurate.
- Balance Beam Scales - These scales go back to ancient times and can be very accurate, depending on the amount of friction at the pivot point. These, however, may not be the easiest to use since the object's weight is balanced against graduated weights until both are equal and the scale balances.
- Digital Scales - These use electronic means to measure and calculate an object's weight and are generally pretty accurate and easy to use. Prices have come down significantly on these scales, so they are within reach of most organizations. There are many types out there on the market, so you do need to be careful with which one that you choose.
Digital Scale Buying Tips
In trying to purchase a new digital scale or in evaluating your current one, these are some of the things you should consider.
- Weight Capacity - Since scales come in different weight capacities, you should get the lowest possible capacity that will measure 5 ounces (141.75 g). A scale with a higher weight capacity will also have a higher measurement error, since a scale's accuracy is a percentage of the weight capacity. This can be as high as 1% - 2% of the weight capacity.
- Display Resolution - I recommend getting a scale that will display out to 0.01 ounces. It has been found that a difference of a few hundredths of an ounce can have a measurable effect on finish times. A scale reading out to 0.1 oz just isn't accurate enough and one that reads out to 0.001 oz will be too sensitive to air currents and table vibration so it won't provide a steady reading.
- Scale Calibration - A good scale will have a calibration feature that will allow you to adjust the displayed measurement to match a known "standard" weight. If the scale can't be calibrated and is off when measuring a standard weight, you may have problems with parents that have used a more accurate scale. Some scales come with the calibration weight, some don't. If not, you can purchase one.
- Accuracy - Accuracy is a measure of how close the scale can measure a known weight to its "true" weight (taking the average of repeated measurements). To be accurate enough for check-in purposes, a scale should measure a known weight to within 0.1% or better of its value. A 5 ounce weight should weigh in within +/-0.005 ounces or better on a scale with a resolution of 0.01 ounces.
- Precision - Precision is actually more important than accuracy and is a measure of how closely repeated measurements are from each other. Since variation is inevitable, the scale will not always hit the bull's eye every time, but the variation of the measurements should be very small. I recommend that you get a scale that has a precision of at least +/-0.0035 ounces (0.1 grams). Unfortunately, many scale suppliers use the term "accuracy" when they should be using the term "precision".
- Overload Protection - A good scale has overload protection to keep the scale from being damaged when the maximum weight capacity of the scale is exceeded. Have you ever see a kid go up to a scale and press down as hard as they could to see what happens? -- I have. Regardless, a digital scale should never be left unattended.
You should be able to find a good digital scale for under $50 that meets these criteria.
Official Weight
Having an official 5 ounce weight will help to guarantee that your scale is measuring accurately, and will reduce the arguments from parents that used a scale from the Post Office, their kitchen, work, or elsewhere.
- Commercial Sources - When I tried to find an official 5 oz. weight, I found that Awana International hasn't carried them for several years now, hobby stores don't have them, and of the calibration weight sets that I found, all were metric. The are some websites that carry a certified 5.00 oz weight, like GrandPrix Software Central, Maximum Velocity and ScalesUSA
- Weight Certification - Some weights that you will find on the web (like eBay) do not make any guarantees as to the accuracy of the weights that they sell, so buyer beware. To be used with a 0.01 ounce resolution scale, the weight should be certified to be +/- 0.001 ounces or better by a calibration lab. The weight should come with some documentation as to the certification of the weight.
Two Scales are Better than One
If your organization can afford it, I recommend having two scales on race day, one for the Check-in Area, and the other for your Pit Area. The scale in the Pit Area is for use by racers making their final weight adjustments. This will help keep your registration going smoother, without racers trying to reweigh cars and clogging your Check-in Area.
- Check-in Scale
- This should be your most accurate and precise scale.
- Make this scale available as much as possible before the race, so the racers won't have problems upon weigh-in during registration.
- Pit Area Scale
- Make a sign, or station a person, to stress that the purpose of this scale is to help the racers get close to the 5 ounce limit, but only the weight on the check-in scale will be considered as the car's entry weight.
- Since the scale is only meant to get the racers close to 5 ounces, you may use a less accurate scale.
- If you can't afford 2 digital scales an inexpensive counter balance or balance beam scale may work well enough for this purpose.
- Make an "Unofficial Weight" to go with a balance beam scale. I made one using pennies and masking tape. Though not as accurate as our official weight, it is good enough for this use.