Any device that is used to measure anything has to go through a calibration process. It could be a radar, clocks, speedometer and even truck scales. The process of calibration is to verify that the thing that is measuring an object has been compared against something similar with a known value.
In the case of truck scales, calibrations are usually done with weights of a known mass. Weight is added to the scale to make sure that the readout reflects the weight of the known object. Over time, scale readings can start to drift and produce errors. When this happens the calibration process is done again and the scales are brought back in line.
For example a 1,000 lb weight will be placed on a truck scale and if the readout says it is 1,000 lbs the scale is calibrated and if the scale says it weighs 950 lbs some adjustments are made to make the reading say it is actually 1,000 lbs.
Because all truck scales get used regularly, their readings will begin to drift and will have to go through a routine scale calibration. Even the DOT has a routine calibration process they must follow at weigh stations.
How do you perform an onboard truck scale calibration?
Onboard truck scale calibrations are typically done a couple of ways
- Using known calibration weights
- Using an object that was weighed on a calibrated scale
Since most people don't have calibration weights available, using a scale that has already been calibrated is the most popular approach. The calibrated scale that you use for reference should have been calibrated by a professional certification body who used known calibration weights during its calibration process.
You should tell you onboard scale system what your set up feels like when it is empty and what it is like when it is loaded. For some onboard truck scale systems the more data you give it the smarter and more accurate it becomes. Under perfect conditions one empty weight and one loaded weight is all you need to fully calibrate your onboard truck scale but more entries can help get the accuracy tighter.
You should scale your truck and trailer multiple times with varying loads to get a good outline on how your equipment changes under different loads. Your loaded weights should be no less than 12.5% of the axles max legal weight.
How do onboard truck scale calibrations work?
Truck scales use a simple property of physics to determine the weight of the truck when it is on a platform or suspension system. The formula deals with pressure and it is:
P = F/A
P = pressure
F = force (the weight you are measuring)
A = Area (what the truck is sitting on)
The sensors on a truck scale are designed to measure pressure and using this math the system is able to determine how much force (or weight) a truck is applying to the sensors. This is the basic concept that truck scales use to give the driver their gross or axle weights.
When a calibration is being performed it is critical that only known weights are used and calculations are done immediately after measuring. The output of your measuring service is only as good as the inputs given during the calibration process. This is where the phrase "garbage in = garbage out" usually gets used and can leave you thinking that you have a bad truck scale when the real problem is bad data.
During a calibration process the measurement device (truck scale) takes the weight from the user (the known weight) and associates that value with the reading that it is taking (pressure). This process is done a couple times to associate multiple weights with multiple measurements and a function is produced to associate what all readouts will be for all of the loads in the future. The result can be seen in the image below.
What do bad truck scale calibrations look like?
Bad truck scale calibrations come in a lot of different shapes and sizes. Most bad calibrations are done when the user enters bad data during the calibration process. Below is an example of what a bad scale calibration can look like:
In this example you can see that most of the entries made kind of form the straight line of a good calibration. However, on one entry the measurement device that the user had was giving a pressure reading of 54.099 psi but the user entered a weight of 8,400 lbs. This obviously bad entry can throw everything off thousands of pounds and give the carrier or driver very wrong readings from their onboard truck scales moving forward.
Most bad truck scale calibrations happen when user enters weights into their onboard truck scale system that is not a true representation of their actual current weight. Some of the most common mistakes during calibration are:
- Entering in empty weights from old scale tickets (your truck/trailer does not always weigh the same empty)
- Entering in loaded weights after driving a couple hours (fuel has burned and loads can shift which means axles weigh different than they did during scaling)
- Only calibrating the system while empty (not giving it enough data to work with to give accurate results)
Making sure that you always enter weights into your onboard truck scale as soon as you get them will set you up for the best chance of success for the most accurate results.
How often should you calibrate truck scales?
If you drive your truck every day you should get in the habit of calibrating your truck scales at least once per month. This will help keep your readings accurate and prevent your readings from drifting to far the actual weight. A good way to keep up with this is to make it part of your routine maintenance for the truck.