eTape readings depend on specific Arduino?

[wylbur]

hi Adafruit,

I've got a 12" eTape sensor for which I measure the resistance as 528 ohms when there's liquid at 30cm on the eTape, and 2200 ohms when there's no liquid on the eTape. It's wired with a 1.2K pullup resistor (measured at 1196 ohms). I used 1.2K because from my understanding of the estimate of resistance (res <- R1/((1023/ar)-1)), making R1 a little bigger might spread the analog readings a bit farther apart.

I've used the eTape with two different Arduinos. First with a Diavolino, and I got the following readings:

dp <- c(0, 10.3, 15.3, 20.3, 25.3, 30.0)
ar <- c(813, 742, 677, 603, 517, 396)

where dp is in cm and ar is the analog reading at the Arduino. Then with a stock Arduino Mega 2560:

dp2 <- c(0, 4.7, 9.3, 13.7, 18.2, 22.7, 27.1, 30)
ar2 <- c(702, 681, 648, 600, 553, 484, 389, 335)

These are pretty far apart. I have two questions:

a) is it possible that different Arduino boards will have substantially different analog readings from the same sensor? If so, does this have to do with unstable or wonky analog reference values? Is there anything I can do to stabilize this, or should I just resign myself to recalibrating for each board?

b) in terms of going from an analog reading to a depth estimate, you guys recommend linearizing the analog reading back to resistance, then using the proportion of measured resistance in the min, max range to find a proportional height, then multiplying by the sensor's length.

http://learn.adafruit.com/thermistor/using-a-thermistor

I didn't find that to be a good fit (sorry, I'm a statistician in my day job). A somewhat better fit can be found by a 4th degree polynomial directly from the analog reading to depth. Details here: http://wylbursinnergeek.net/2012/07/26/ ... el-sensor/ What do you guys think?

thanks -- wylbur.

[adafruit_support_bill]

a) Standard practice in the instrumentation world is to re-calibrate any time you change a sensor, (or any other component or condition that might affect the reading). On some of the medical instruments I work on, we perform calibrations before each sample.
b) Nice work!

[wylbur]

a) check, wilco.
b) thx!

-- wylbur.

[wylbur]

hi Adafruit,

I used the eTape in my application over the weekend, and it worked for a while, then mysteriously stopped working. When I got it back on my test bench, I find that it reads dry 507Ω, and at 26cm, 397Ω.

I have another eTape I've never used, and it reads 2374Ω dry, and 925Ω in 26cm of water.

Neither of these readings is anything like the specs in the app note, which are 700Ω empty, 85Ω full.

What would drop the resistance of the eTape so dramatically? This doesn't seem good. Thanks for insights -- wylbur.

[adafruit_support_bill]

We haven't had any problems like that reported with these sensors. It is odd that a new one out of the box is showing similarly bizarre readings. Have you double checked your meter against a known resistance?
What is the application? Can you show photos of your setup?

[wylbur]

hi Adafruit,

thanks for your reply.

First I just this second checked the meter against a 1.2KΩ resistor, and it came up as expected (1.196KΩ).

The application is an evapotron, see here:
http://wylbursinnergeek.net/2012/07/28/ ... prototype/

You can see the eTape in the video posted there. Also a discussion of how I mounted it on a piece of plastic using double-sided tape here:
http://wylbursinnergeek.net/2012/07/23/ ... el-sensor/

To make sure I didn't do anything obviously wrong last night, I just checked the eTape again. It was literally dry from sitting out all night, and registered 520Ω. Then I put it back in 25cm of water, and it says 400Ω. Weird.

[adafruit_support_bill]

Not quite sure what an Evapotron is used for, but it looks like your mounting of the sensor is fine. Just to double check, you only attached the double-sided tape at the very top of the sensor? Also, have you kept the vent hole from being submerged?

You might try contacting techsupport@milonetech.com. They might have a better idea of what is going on with your eTape.

[wylbur]

Uh oh, the vent hole might have been submerged briefly. Does that ruin the eTape permanently?

[adafruit_support_bill]

Does that ruin the eTape permanently?

Can't say that I've tried it myself. But the documentation does warn against it.

[wylbur]

hi Adafruit,

I emailed with Chris at Milone about my eTape, and his opinion is basically that I mangled the eTape, and that's the problem. He also suggests that water got into the envelope, which distorts (maybe permanently) the readings. He might be right, so I think the eTape is too fragile for me.

Would an IR sensor read the surface of water? Could I mount this:
https://www.adafruit.com/products/164
above the water, then beam it to find the surface and thereby calc the level?

Questions:

-- I've tried using a Ping))), and it seems to give pretty random readings in a lot of circumstances. In particular, it has to have a very clear view of a big space to get a good distance measure. Is an IR measure more focused?

-- IR measures seem to be dependent on the reflectivity of the subject. Would water (dirty water, anyway) reflect adequately, or is it likely to confuse the sensor?

-- What else can I do? Maybe a float of some kind?

thanks -- wylbur.

[adafruit_support_rick]

I worked once on a high-speed ion-deposition printer. Similar to a laser printer, but way bigger and faster. My piece of it was the toner hopper (same kind of toner used in a laser printer). This thing created a "fluidized bed" of toner powder by blowing air through it from a perforated bottom plate. Kind of like quicksand. So you had this container of floating powder. It looked and behaved just like a fluid - the surface was pretty clearly defined, waves would propagate through it, etc.

The problem was knowing what the level of this fluid was, so that we could replenish the toner as it was consumed.

The solution was a sonar rangefinder. This was quite a few years ago, and Polaroid was making industrial rangefinder units based on the sonar focusing rangefinder on the old SX-70 cameras.

We weren't interested in precise measurements - it was enough to know when to dump in some more toner.

So I guess I'm surprised the Ping))) isn't working for you. I wonder if you're getting multi-path reflections from the walls of your container. Sparkfun lists the Maxbotix, which claims to offer a range of beam angles. Maybe one of those would work better for you?

[adafruit_support_bill]

I've had pretty good luck sensing fluid level in containers as small as a wine glass ( ) using the Maxbotix ultrasonic sensor.
http://www.adafruit.com/products/172

[wylbur]

thanks, Adafruit and driverblock. A different ultrasonic device might well do better.

For now, I'm going in a different direction, using a weight sensor to assess how much liquid is in the reservoir:

http://www.meas-spec.com/product/t_product.aspx?id=2438

It brings it's own moisture-isolation challenges. This device comes with the amplification built in, no need for 2 (or 4!) of them, etc. Weight sensing seems to be an evolving art in hobbyist-land. Will post findings to my blog when time permits.

thanks again -- wylbur.

[haroldl]

I too have lost to much time to eTape, it is unreliable, I have seen readings float up and down, appear stable then for what ever reason suddenly change with Pro Mini's and Nano's using the 3.3 volts and REF. The units were setup on a test bench with no fluid and many tests with fluid. As I read the above post, Wylbur states he tried a brand new eTape from the box and he is still questioned about mangled eTape and the hole going under water. Mine has never been under water and was placed inside a 1 1/8 inch side diameter tube so it can not fall over nor any fluid within 4 inches of the hole. eTape is not reliable, waste of money and time..................................they should recall the product.

[adafruit]

we'll pass this feedback along to the manufacturer.