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1  Using Arduino / Sensors / Re: DS18B20 - offset increases with temperature on: December 21, 2012, 02:30:30 pm
Does the DS18B20 itself have the same temperature as the boiler's sensor? That means, have you ensured that there's a good heat transfer from the copper pipes to the DS18B20?
The copper pipe is wrapped in foam insulation and the DS is tucked under that insulation. It is either touching the pipe, or is within 1mm of it (if I move the insulation to see, then the sensor moves around). So the reaction time may not be fast, but since copper is a much worse insulator than foam, then the temperature under the insulation should be very close to the temp of the water after a few minutes.

I checked around with the IR meter and when the boiler claims its supply is 55°C, then the pipes (many types, sizes, etc.) in the basement were all above 51°C, while the ds18b20 was still registering 42-44°C.

Since this seems to be only my problem I'll try to reinstall the sensors, maybe I'll tape them to the pipes to ensure contact, and add insulation around it.
2  Using Arduino / Sensors / Re: DS18B20 - offset increases with temperature on: December 21, 2012, 10:40:55 am
You can't measure an accurate temperature with an IR temperature meter. The material you are pointing at influences the temperature.
I know that copper is good at reflecting heat, so I put black electrical tape on the spot where I check with the IR. Didn't change the reading much, though, if at all.

That's assuming that the other measurements are more accurate than the DS. How do you know that?
I don't know for sure, but I admit that I assumed that the sensors on the boiler are more or less accurate. Even if they aren't, and the ds18b20 is, then could they be off by more than 10° at the temperatures they were designed/installed for (showing 55°C when the ds18b20 shows only 44°C)? I woundn't expect that on a not-too-cheap boiler.
3  Using Arduino / Sensors / DS18B20 - offset increases with temperature on: December 21, 2012, 05:35:30 am
When I got my first ds18b20 digital temperature sensors I was surprised at how accurate and stable they were compared to the analog sensors I had been using. Now after a few months I've noticed that this is only in a certain temperature range. It appears that the offset from real temperature increases as the temp rises. Two examples:
1) Monitoring room temperature. At 16.5°C the sensor is off by less than 0.2°, but at 17.5°C it is off by up to 0.5°. I check this against the room thermostat, which is ~50cm away from the sensor, and against an IR thermometer. Kinda sucks since I'm trying to build my own thermostat, so I'd like the offset to at least be constant.
2) I have sensors monitoring the heating system's supply and return water temperature. The sensors are inside the boiler, on the copper pipes where the boiler's temp sensors are, covered with foam insulation. When the boiler is off and the plumbing has cooled down to ~20°C then the ds18b20's show about the same temp as the boiler's sensors, but when the boiler is on and the supply is at 50°C, then the ds18b20 shows no more than 42°C. On a few occasions I've checked the temps with the IR thermometer and it is closer to the boiler's sensors than the ds18b20.

All 4 of my ds18b20's have these discrepancies, while the data sheet claims ±0.5°C accuracy from -10°C to +85°C.
4  Using Arduino / Displays / LCD quirk after long run time? (Duemilanove w/ Winstar WH1602B) on: December 14, 2012, 04:21:09 pm
I have a 16x2 LCD (Winstar WH1602B-yyh-ct) displaying some stuff. No problems for almost a year, when it would be on for days or weeks at a time. Recently I had it going for over a month without turning the Arduino off or uploading anything new to it, or even reseting it, when the LCD started acting up: It started showing weird symbols, mostly '?' but also black squares etc. I thought that maybe there is a problem with the connection - the display is wired through a breadboard and the whole bundle ended up on the floor once when my 6 mo old son started exploring the room;) (This happened many weeks before the LCD problems started). So I checked the wires between breadboard and Arduino, then took out the display and pressed it into the breadboard again. The latter changed what was being displayed, but it still didn't show what it was supposed to. The next day I played around with it some more thinking that maybe one of the pins isn't contacting inside the bboard, and since then only the first row would show, the second is blank, even the pixels don't show up when adjusting the contrast pot. And, of course, the first row was still showing nonsense - mostly '?'s with other junk, sometimes moving left->right. I hit the Arduino's reset button a few times, but that didn't help.
A few days later I resoldered the LCD's pins, but that didn't fix anything.
Finally, I uploaded via USB the same program that the Arduino was running until now and THE DISPLAY WORKS AGAIN!!

So, could this be a problem with Arduino, for example if running for a long time something screws up and (in my case) starts to send junk to the display?
5  Using Arduino / Displays / Re: 4x8 character LCD, or two 2x8's? on: May 07, 2012, 12:59:22 am
OK, so it seems that the 2x16 I measured happened to be one of the larger units, at least 10mm wider than most "normal" 2x16's. I looked at OLEDs, as liudr suggested, and they are slightly narrower, closer to what I'm looking for (though twice as expensive). Maybe I will be able to find a 2x16 that will just barely fit, without having to play with 4x8's, etc. Any ideas where I can look for narrow 2x16 lcds?

I am surprised that 4x8 character displays aren't available. I would imagine that it could even be used with a typical 2x16 library, just the 2x16's first row would spill over into the 4x8's second row, and so on.

Thanks everyone for your input:)
6  Using Arduino / Displays / 4x8 character LCD, or two 2x8's? on: May 05, 2012, 08:42:29 am
I've got an application which sends four parameters to the display. For testing I'm using a standard 2x16 character LCD, but space limitations won't allow a display that wide. The display can't be wider than 35-45mm, while 2x16's are twice that. I did a quick search for 4x8 LCDs, but no luck. Are they not produced?

What about using two 2x8 character LCDs? Could they be wired so that one displays both parameters from the 2x16's first row, and the other displays the second row? (In fact, they can display the four parameters in any combination.) Can the 2x16 be replaced by a 4x8 or two 2x8's by pure rewiring of the display leads, assuming that the device cannot be reprogrammed?

Thanks in advance.
7  Using Arduino / Sensors / Re: HCH-1000 Humidity sensor and AREF on: January 14, 2012, 02:49:40 am
What if I used a voltage divider? One resistor between the humidity sensor and the analog input, a second resistor between analog and ground? What resistors would I need so that analog input reads 3.3V when the sensor is charged to 5V.
(Yes, I could just do the math to know the proportions between the two resistors, but I'm a n00b and don't know what order of magnitude they should be.)
8  Using Arduino / Sensors / HCH-1000 Humidity sensor and AREF on: January 12, 2012, 12:34:56 pm
I have a humidity sensor (Honeywell HCH-1000), hooked up as described here. In short: First the capacitor (sensor) is discharged, then charged to a certain percent of full charge (5V). Once the charge times for 0% and 100% are known, the relative humidity can be calculated depending on the time needed to charge. The state of charge is measured through one of the analog pins.

Here is my problem: Other than the humidity sensor, I also have a few temperature sensors (MCP9700-E/TO) using the analog inputs, and I've increased their accuracy by feeding the 3.3V power supply to AREF. Only after a while did I realise that re-scaling the analog inputs from 0-5V to 0-3.3V also effects the readings from the humidity sensor: If it is set to wait until the capacitor gets to 99.3% of full charge, then it trips when the cap charges to 99.3% of 3.3V, not 99.3% of 5V.
I'll add that in my set-up the charge time to 99.3% is 4302 microseconds for 0% RH and 4900 microseconds for 100% RH. Charge time for the open circuit (with the sensor itself removed) is 700 microsecs with 3.3V fed to AREF. But the first time I calibrated was without AREF and then I got 12257 microsec (0% RH) and 27417 microsec (100% RH).

My question is: Should I be worried about the humidity sensor charging only to 3.3V instead of 5V? What could the effect of this be on the sensor's accuracy? Is it possible to have the temp sensor analog inputs scaled a lower AREF voltage while the humidity inputs stay tuned to 5V?
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