Photoswitches in liquid speed detection
Photoswitches in liquid speed detection
(OP)
Hi,
I am in need to measure the velocity of a liquid running inside a plastic tube, so I set up a system of photoswitches and related control circuits. However, a wird thing is happening.
When there is no obstacle (just air) between the LED and the phototransistor, there is a current of about 2.25mA flowing in the transistor. When i place the empty plastic tube, the current falls to 1.5mA due to the loss of light (the tube is semi-transparent). So far so good, but when the liquid flows in front of the slit, the current rises to about 3mA, implying that the tube+liquid system conveys more light to the transistor than even the unobstructed photoswitch. Anybody know what could be going on? I tried several liquids of different density and color (just to try: water, red water, blue water, oil ecc) and there was absolutely no difference. The tube covers the slit completely, so this is not an effect of partial slit obscuring. Also, I tried in a completely dark environment, so it is not due to weird refraction of external light. I am using a Omron EE-SX1070 switch.
Thanks in advance,
Giovanni
I am in need to measure the velocity of a liquid running inside a plastic tube, so I set up a system of photoswitches and related control circuits. However, a wird thing is happening.
When there is no obstacle (just air) between the LED and the phototransistor, there is a current of about 2.25mA flowing in the transistor. When i place the empty plastic tube, the current falls to 1.5mA due to the loss of light (the tube is semi-transparent). So far so good, but when the liquid flows in front of the slit, the current rises to about 3mA, implying that the tube+liquid system conveys more light to the transistor than even the unobstructed photoswitch. Anybody know what could be going on? I tried several liquids of different density and color (just to try: water, red water, blue water, oil ecc) and there was absolutely no difference. The tube covers the slit completely, so this is not an effect of partial slit obscuring. Also, I tried in a completely dark environment, so it is not due to weird refraction of external light. I am using a Omron EE-SX1070 switch.
Thanks in advance,
Giovanni





RE: Photoswitches in liquid speed detection
Benta.
RE: Photoswitches in liquid speed detection
RE: Photoswitches in liquid speed detection
Also, this is to detect the speed of the air/liquid interface using two widely separated sensors, not the speed of the liquid when the tube is full, right?
RE: Photoswitches in liquid speed detection
But given that this is an IR device, you don't really know how your dye works.
Benta.
RE: Photoswitches in liquid speed detection
RE: Photoswitches in liquid speed detection
RE: Photoswitches in liquid speed detection
I'm not clear what you thought should have happened. You place a transparent liquid in a transparent tube, you should still expect a transparent object.
Aside from that, measuring time of travel can be highly affected by how the leading and trailing boundaries of the liquid behave during actual flow conditions.
TTFN
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RE: Photoswitches in liquid speed detection
If I place a semi-transparent liquid in a semi-transparent tube I would expect a semi-transparent object. What puzzles me is that tube+liquid is more trasparent than both tube only and no tube at all.
RE: Photoswitches in liquid speed detection
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RE: Photoswitches in liquid speed detection
RE: Photoswitches in liquid speed detection
But sticking with the PE concept, have you tried using a polarized lense or is this an option with this Omron product?
have you tried diffrent covers for the send receive like slits vertical or horizontal?
Different sensor
use a different type of sensor like a radar type and looking at the doppler effect then you would get a phase shift and use this for velocity.
RE: Photoswitches in liquid speed detection
Unfortunatly I don't have visible-light coupled LED-sensors so at the moment I can't try this. The other switches I have come with an integrated comparator, so maybe I won't be able to see any difference between no-tube and tube-with-liquid comparator. The refractive index of the tube is 1.54 (PVC), water is 1.3. Both hypothesis (focusing and "fiber-optic" effect) seem to be reasonable, unfortunately as by now I can't try anything meaningful to sort things out. I attach the output of the phototransistor, which has a 1k resistor connected to the emitter. X axis is time in seconds (fluid runs at 100um/sec set by a siringe pump) and Y is volts. The initial 2 V refers to switch obstructed partially by the empty PVC tube (unobstructed gives 2.3 V). When the water approaches the sensor there is a current decrease, then it increases as the water covers the entire slit, until it stabilizes at about 3V. These values differ from the opening post since I slightly changed the setup. Maybe something can be inferred from the shape of this curve
Thanks again for everyone's answer!
RE: Photoswitches in liquid speed detection
RE: Photoswitches in liquid speed detection
RE: Photoswitches in liquid speed detection
RE: Photoswitches in liquid speed detection
TTFN
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RE: Photoswitches in liquid speed detection
Right now, he's got a cylindrical sight glass, which has a dark line between the clear and less clear regions, or the regions where a scale placed behind the tube will be magnified differently depending on whether the lumen contains air or liquid.
You might think he'd get a little closer with the tube squashed flat, so all of the optical surfaces have zero curvature. ... but there will still be a dark meniscus, and the clear and less clear signals will be affected only by optical density, not refractive index, so he'd be worse off.
It's not a linear problem, so it doesn't matter if your photodiode is linear across five decades (as an old analog artificer bragged to me) or not.
The only way I know of to get a sharp repeatable transition _in_volume_ is to make the lumen at the detectors as small as is reasonably practical, so the meniscus passes through very rapidly even for low flow, and round, so the clear and less clear signals (again, really different magnifications) are more distinct from each other.
Mike Halloran
Pembroke Pines, FL, USA