ToughSonic sensor products from Senix are very flexible due to their programmability. Sensors can be adjusted to work in most applications, but this same flexibility can make startup a challenge for first time or occasional users. This note deals with important programmable parameters for sensors in tanks.
Sensors measure the distance to targets by sending out a sound pulse and timing how long it takes the pulse to come back. This time is proportional to the distance as sound travels at 1128 feet per second or about 1 millisecond (ms) per foot. If a target is located 20 feet away, the sonic pulse takes 20 mS to reach the target, and 20mS for the reflection to return to the sensor. The total round trip time is 40 ms.
Measurement Repetition Rate and Tank Dynamics:
The rep rate is the rate at which the sensor sends pulses and takes measurements. The further the distance, the more time required for the sound wave to travel to the target and back. In a 20 foot tank it will take 20 ms for the sound to get to the bottom of the tank and 20 ms for the sound to travel back to the sensor for a total trip time of 40 ms. If our sensor is firing every 100 mS (TSPC 15S default value), for example, there should be ample time for a sound pulse to die out, before the next measurement pulse is launched.
Tanks tend to contain the sound energy. In closed tanks, the tops can act as sonic reflectors. If we set a TSPC -15S sensor to measure every 50mS, in our 20’ tank, we could run into difficulty. The pulse travels down and back in 40mS, bounces off the tank top, and heads back to the bottom. Partway into that secondary return path, the sensor launches another measurement. The FIRST pulse bounces off the bottom target again, and heads back to the sensor, arriving first. Weak, by that time, but first. Depending on gain and power settings, it stands a good chance of being detected. The result is an incorrect measurement, and possibly a very jumpy set of measurements.
In closed top tanks this can happen 2, 3 or 4 times so the sensor must wait for all these reflections to dampen out. To do this we would set the rep rate to 3 or 4 times the travel time of the sound depending on how much the sound echo’s. So in this example we would try 3 times or 20*3= 60 ms or more rep rate. It is always safe to pick a slow rep rate to start with (500ms) and work your way down.
Turbulence and Filters:
In general, tank levels change slowly. SenixVIEW software allows you to invoke filters, which are stored in sensor firmware. These filters allow you to deal with waves due to filling or stirring. In general, taking a running average of 50 or 100 readings will provide very stable indications of tank level, every 5 or 10 seconds. The “slow-fast” filter will ignore repetitive turbulence from stirrers, while the input filters permit selecting maximum or minimum readings in a sequence, to deal with waves. Once the sensor configuration is determined in SenixVIEW and stored in the sensor, the computer is no longer necessary. Data can be logged, or analog signals metered, with the full effect of filtering.
The default factory configuration for all ToughSonic Sensors sends out a burst of 10 pulses. This can be adjusted in SenixVIEW, with two impacts. First, by increasing or decreasing the number of pulses, you change the overall power level of the measurement. For longer distances and smaller targets, more pulses may be needed. Shorter distances and larger targets may not need as much energy for reliable measurement. The second effect has to do with the short range behavior, or deadband of the sensor. A TSPC-30S1, for example, is rated with a 4” deadband. If you’re measuring a short distance… say one or two feet, you can shorten the deadband effect by reducing the number of pulses to 1 or 2. Smaller targets at that distance may require more power. Another option is to use the Senix 90 Degree Adaptor, which offers both a lower profile mounting and the deadband distance is inside the adaptor allowing for measurement to the top of the tank.
The gain is the second variable in configuring the transducer. Sensitivity is set in SenixVIEW, in the same quadrant as the transmit power. There are several standard settings for free air and stilling tube use. In addition, Custom Gain settings permit you to optimize the setting for your particular distance.
Dealing with foam:
Ultrasonic sensors will respond to the first-surface reflection, where there is foam on top of a liquid level. Using a stilling tube will allow you to avoid the impact of the foam.