Are Ultrasonic Height Stadiometers accurate?
Height is most commonly measured either using a physical height rod or ultrasonic waves. In the more common height rod method, the stadiometer's rods are adjusted until it touches the subject, at which point results can be read via printed markings on the rod, on a digital screen, or via direct transfer to receiving device.
In comparison, Ultrasonic Height Stadiometers work using a different technique. They consist of a transmitter and receiver of ultrasonic sound, and the device measures the time it takes for the ultrasonic sound wave to fly from the sensor to the object and back again to calculate distance. A shorter time means the object is closer, and a longer time means the object is farther.
Image credit: Distance Measurement via Using of Ultrasonic Sensor, Journal of Automation and Control, 2015, Vol. 3, No. 3, 71-74
A well known example of ultrasonic sensor utilization is collision detection in vehicles! The concept and technology is fairly straightforward and well-established, though of course there can still be a noticeable difference between high and low quality devices.
During design and certification, Charder's ultrasonic height stadiometers are validated by conducting measurements for known lengths and actual persons, comparing measurement results between test devices and other certified height stadiometers.
What are some factors that may affect the accuracy of an ultrasonic height stadiometer?
There are generally two types of ultrasonic height stadiometers, and while they both use the same principle, there are differences in implementation which create different variables affecting measurement accuracy.
(1) Portable version
Portable height stadiometers like the Charder HM250U measure distance from the ground to calculate height.
The most common issue potentially affecting accuracy of measurement is tilt. While ultrasonic sensors can receive sound waves even at an angle, the distance to the ground is farther away when the device is tilted. Thus, a tilt detection system needs to be built into a handheld device, only allowing height measurement to complete if tilt is minimal, thus ensuring results fall within acceptable accuracy range. For example, the HM250U has 4 tilt warning lights that tell you if the device is tilted, and will not measure until the device is straight, thus minimizing potential inaccuracy.
(2) Mounted Version
Mounted ultrasonic height stadiometers like the Charder HM200U measure distance from top of the subject's head.
Mounted devices don't need to worry about tilt, but can be affected by other variables such as hairstyles that extend out from the head - such as an Afro or a high bun - as the ultrasonic waves may bounce back after reaching the hair, rather than the top of the head.
Benefits of using an Ultrasonic Height Stadiometer
All in all, the general expected accuracy for ultrasonic devices tends to be slightly lower than traditional devices due to such variables. If so, then why would you choose an ultrasonic device?
(1) Convenience and speed
In addition to quicker measurement, ultrasonic measurement allows for safe contact-free usage.
For traveling practitioners - especially in remote locations - setting up and breaking down a traditional height stadiometer can take a significant amount of time, compared to measuring using a handheld device.
(2) The difference may be immaterial
Measuring every centimeter accurately may be important when tracking childhood growth! However, in other usage applications, such as calculation of BMI for determination of appropriate medication dosage, this is less clear cut.
As an example, imagine a person whose weight is 70 kg, and their height is 180 cm. In this case, their BMI would be calculated to be 21.6.
The standard accuracy acceptability range is ±1 cm. For this person, they might receive a result of 179 cm (BMI 21.85) or 181 cm (BMI 21.37) at worst when it comes to inaccuracy, which is essentially negligible for BMI purposes. Even if we use a larger inaccuracy of ±2 cm to see if it makes a difference, we would get 178 cm (BMI 22.09) on the low end, and 182 cm (BMI 21.13) on the high end.
This level of variation in BMI is largely immaterial when it comes to usage such as obesity assessment or determining medication dosage.
Is an ultrasonic height stadiometer accurate enough? It depends on your specific use scenario, but for most purposes, the answer is yes! Unless you need very precise height measurement for assessment of growth (such as for children), the minimal margin of error even when there are slight inaccuracies largely does not outweigh the improved portability, speed, or convenience offered by this measurement technology.