Hi, I purchased the RB-DFR-396 to monitor the liquid (water) level in a 1-1/2" clear plastic tube, the sensor would be used control a pump used to re-fill the tube. As a first test, I initially purchased a single sensor and tried it out by monitoring the liquid level in a plastic cup. I simply filled the cup half way and moved the sensor slowly up & down by hand along the surface of the cup (to simulate the fill/drain cycle) and I received the expected voltage levels, so I figured it would work in my final set-up so I purchased 3 more sensors, so I have now invested about $45. My problem comes when I attach the sensor to the plastic tube. Here is what happens. The tube starts empty (liquid level is well below position of the sensor) and when I turn on the the power, the sensor calls for the pump to fill (this is correct) and then when the liquid level reaches near the top of the sensor, the sensor signals for the the pump to shut off, this phase works properly at all times. The problem occurs when I start to drain liquid from the tube. To be specific, I am removing single drops at a time by accurately controlling a solenoid valve at the bottom of the tube, so the water level in the tube is lowered very slowly (literally, one drop at a time). After removing a series of drops, that the water level in the tube goes down about half of the sensor height (about 1/4"), the sensor then signals the pump to refill. I’m ok with this level of the sensitivity in the sensor, however, the problem comes when sensor the sends the “re-fill” signal to the pump but then never sends the “all full” signal and the pump remains on, and the tube then over fills and the water overflows out of the tube. I did a lot of experimenting and determined that the problem does not exist if, after the initial fill, the water level drops below the entire dimension (about 1/2" or more) of the sensor before the pump turns back on, then the refill and all-full signals seems to work properly (However, I manually forced this condition and this is not how my application works). However, in my case the water level drops slowly and the sensor triggers the “re-fill” signal at a point where the water level is still around mid level of the sensor. I understand sensors often have some level of hysteresis, but in this case, if the sensor is accurate enough to sense a small level of drop in the liquid level, it should have the same level of accuracy to determine the small level of addition to the liquid level and then change it’s voltage output to signal the all-full condition. I have tried a lot of different things, I have placed the sensor both vertical and horizontal, with and with out the small foam pad (per the DF-ROBOT WIKI site info) and it has the same problem. I can’t find any real technical info (I think it’s based on capacitance) on the sensitivity and hysteresis of the sensor. I think there is a problem with the sensor and how it’s presented on the web site, as no where is it stated that water level must drop a certain amount before it give you an accurate signal. I’m looking for any guidance on how to move forward. Thanks, John
Hi,
The Non-Contact Liquid Level Switch doesn’t have an analog interface, therefore it doesn’t provide the level of the liquid in the container. It only sends a digital signal low when triggered (when it detects the liquid).
One of the solutions you can try might be to create an “acceptable” liquid level range in the container to keep the liquid level always within this range.
To do so, you can place two sensors in different but close heights on the container. The distance between the sensors will depend how accurate you want the level in the container to be.
One sensor will detect the minimum level, so that the liquid level will always be higher. The second sensor will detect the maximum level when the liquid reaches it to shut off the pump.
Another suggestion might be to use an analog liquid level sensor as this one which will be immersed in the container and will output an analog signal proportional to the immersion depth.
Regards,
I’m aware the Non-Contact Liquid Level Switch does not have an analog output. I’m using it to drive a relay directly, as it is stated on the spec sheet, so in effect I’m just looking for a digital indication of the level (above or below the position of the sensor, as implied by the spec sheet). I am not looking for a tight range and can tolerate an acceptable level, however again, the sensor triggers when a small amount is remove but then does not trigger again when it hits the high level and then overflows. I already thought about using a second sensor and there are two problems
a) two sensors will cost about $31 for each tube (I’m actually building a system with four tubes thats why I’m already in for about $60, another $60 does not make sense for the overall cost of the system b) more importantly, a second sensor consisting of a Non-Contact Liquid Level Switch won’t work. As the level initially drops the first (lowest sensor) will trigger a re-fill and I know the it will not trigger an all-full trigger and I believe that’s why you suggest the second sensor to act as the all-full. That second sensor will trigger the all-full only ONCE and stop the pump (ok the first time), however, once the level drops again that small level again, the second will trigger a re-fill signal but then never trigger and all-full signal. Think about it, the second sensor is in the same condition as the first sensor so an all-full signal never is sent. So not matter how many sensors the all-fulll signal works only once. The problem with the Non-Contact Liquid Level Switch is that the spec sheet is not an accurate representation of the true characteristics as it does not indicate a “minimum level” so it won’t work in a small level change application. I would prefer to return the four sensors for a credit, could you arrange with customer service to make that happen please. Thanks John
Hi John,
Please contact us through our Support Center referring to this post for a return.
Best Regards,