Ultrasonic sensors for level measurement come in a variety of ranges; from a few inches to 50 ft. Picking the right range for your application is very important as it has a direct effect on the overall performance of the sensor.
For example, you wouldn’t want to use an ultrasonic sensor with a 50 ft. range on a tank that is only 6 ft. tall. This is overkill, and you probably spent more than needed. In addition, with a 1.5 ft. blanking distance, the sensor would not be able to detect the liquid above ¾ full.
So, when you are deciding which range will be best for your application, please consider the following factors:
Obviously you will need to know the maximum distance you will require the ultrasonic sensor to detect. A general rule to follow is to use a sensor that will cover the max distance plus 25% after that. This will ensure that the pulses generated will be strong enough to reach the target, and return to the sensor. So for a 20 ft. water tank, a 25 ft. ultrasonic sensor would be ideal.
The blanking distance is the area within the first few inches (or feet) of the transducer where targets are unable to be measured. Typically, the longer the range of an ultrasonic sensor, the longer the blanking distance. For example, our 50 ft. ultrasonic sensors, the IRU-3430 series, have a 1.5 ft. blanking distance as mentioned earlier. While our IRU-5000 series, on the other hand, have a range of 79 inches, and only a 4 inch blanking distance.
Sometimes we have customers that require both a longer range ultrasonic sensor, and a short blanking distance. In cases like this, standpipes, special programming, and other adjustments may be required.
Soft vs. Hard Targets
Finally, you will need to consider the type of target you will be sensing. Ultrasonic sensors work best with what we describe as hard targets, or targets with good reflective characteristics. These are targets that are flat and smooth like most liquids. Soft targets meanwhile have poor reflective characteristics. Examples of soft targets may include solids like sand or grains.
We have a customer that uses our ultrasonic sensors to calculate the volume of rocks in a gravel pit. With a soft target like this, the ultrasonic waves will often get deflected or scattered. A way to overcome this is to generate a stronger sound wave so that enough of the return echo will be detected by the sensor.
To do this, the general rule is to use a sensor with a range that is double the required sensing distance. For our customer, the rocks in the gravel pit were about 25 ft. away so we recommended that they purchase our 50 ft. ultrasonic sensor. With the stronger pulses being generated by the IRU-3433 our customer was able to successfully measure the distance so they had the information they needed to perform their calculations.
Using ultrasonics on soft targets present a plethora of challenges, so before you purchase an ultrasonic sensor for this application type, it is always recommended that you speak with an application engineer first.
Don’t hesitate to contact us if you have any questions about selecting an ultrasonic sensor.