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136  Using Arduino / General Electronics / Re: 109.3035 ohm resistor on: March 03, 2013, 10:07:56 pm
Where did you find that a PT100 is 109.3035   It's 109.73 at 25C could you show how you did the math for Fahrenheit

Well:

 25 * 9 / 5 + 32 = 77 degrees F

(75 - 32) * 5 / 9 = 23.9 degrees C

From the chart at http://www.tnp-instruments.com/sitebuildercontent/sitebuilderfiles/pt100_385f_table.pdf :

77 degrees F = 109.735 ohms
75 degrees F = 109.304 ohms

It all looks right to me.



I think your misreading the chart. You look at the 70F row and move 6 columns (-5) right to reach 65 degrees = 109.304 ohms.

Lefty

I see what you are saying but it looks to me like the minus numbers just apply to the minus temperatures at the top of the chart. Look 8 rows up from the 70 degree row.
137  Using Arduino / General Electronics / Re: 109.3035 ohm resistor on: March 03, 2013, 09:54:25 pm
Where did you find that a PT100 is 109.3035   It's 109.73 at 25C could you show how you did the math for Fahrenheit

Well:

 25 * 9 / 5 + 32 = 77 degrees F

(75 - 32) * 5 / 9 = 23.9 degrees C

From the chart at http://www.tnp-instruments.com/sitebuildercontent/sitebuilderfiles/pt100_385f_table.pdf :

77 degrees F = 109.735 ohms
75 degrees F = 109.304 ohms

It all looks right to me.

138  Using Arduino / Sensors / Re: Analog pressure gauge / potentiometer coupling? on: March 03, 2013, 11:39:50 am
If it doesn't have a dedicated data output, it could be difficult to interface to the Arduino. Many devices like that use a SOC that drives the LCD segments directly.
139  Using Arduino / General Electronics / Re: Serial interface with an older motorola phone on: March 03, 2013, 10:48:38 am
Quote
I had read elsewhere that the C168i uses on 3.3v TTL, so I ran the tx from the FTDI through a 5 ohm/10ohm voltage divider to get me to 3.3v, and left the FTDI rx connected straight to the FTDI, since 3.3V should still be a logic 1. Still nothing.

If that is truly your voltage divider, you are asking the FTDI chip to supply 333mA to drive the divider, which it cannot do. If you haven't blown up the FTDI chip, you could try a higher value divider, e.g. 510/1K.
140  Using Arduino / Project Guidance / Re: Taking a shot at specific gravity measurements on: March 03, 2013, 09:51:57 am
There have been many ideas expressed here, but I think the simplest and most robust solution is using a submerged mass suspended by a string as suggested by liudr. If I was making my own beer (I wish I was) it's the method I would use. Here are a few of the benefits:

1. If you want to leave it there all the time, you can.
2. It's easy to move to another tank if desired.
3. It will be immune to foam on the surface or submerged particles.
4. If you suspect bubbles have formed on it, just lower it until it hits bottom to knock them off.
5. If you suspect the liquid is stratified, simply take measurements at different levels to verify.
6. The weight and volume of the mass is not important, as long as it's large compared to the string.
7. It requires exactly 1 calibration ever - by taking 1 measurement in plain water.

I noticed no one posted any equations to calculate specific gravity. Not even those who claimed to have an understanding...  so I'll post equations for liudr's method only:

Code:
float mAir;       // mass suspended in air (load cell measurement)
float mWater;     // mass suspended in water (load cell measurement)
float mBeer;      // mass suspended in beer (load cell measuremet)
float bfWater;    // buoyant force in water
float bfBeer;     // buoyant force in beer
float sgBeer;     // specific gravity of beer

...

bfBeer = mAir - mBeer;
bfWater = mAir - mWater;

sgBeer = bfBeer / bfWater;

This thread, http://arduino.cc/forum/index.php/topic,66603.0.html, from the old forum, discusses the hanging weight approach.
141  Using Arduino / General Electronics / Re: 109.3035 ohm resistor on: March 03, 2013, 08:23:33 am
Quote
Too strange a value to be a calibration standard, IMO. I guess I can email Vishay and ask.

Ah but grasshopper you must free your mind from such thinking. A resistance standard is anything that you have belief in it's stated accuracy, and willing to pay to own such. Such a resistor wired to your hot shot ohm meter should tell you if your meter needs calibration or not, that is all that is involved in validation. If your ohm meter doesn't agree, you don't go buy another resistor standard to adjudicate the discrepancy, you (or someone) adjusts the meter to agree with your standard. A resistance standard is a standard no matter what it's specific value is.

Lefty



Ah, so true, wizened old monk.  smiley
142  Using Arduino / General Electronics / Re: 109.3035 ohm resistor on: March 02, 2013, 08:08:32 pm
Now that's interesting. Anything we should be worried about if everyone was wrong?  smiley

Most certainly!  The analyzers are used to deliver ultra-pure nitrogen to Texas Instruments for chip fabrication.  Millions of little lives are on the line!


Now that IS serious! I'm crossing my fingers too!
143  Using Arduino / Sensors / Re: Analog pressure gauge / potentiometer coupling? on: March 02, 2013, 07:54:40 pm
Sounds like a fun project!
144  Using Arduino / General Electronics / Re: 109.3035 ohm resistor on: March 02, 2013, 07:35:13 pm
Are you saying that they were shooting for, say a 110 ohm and got this value?

No.  I'm saying they were shooting for approximately 100 ohms.  After meticulous design work they determined that the final value would be precisely 109.3035.

 smiley-grin

I worked with a state-of-the-art gas analyzer where something similar was done.  Nothing existed that could be used to test or verify the accuracy of the meter.  The accuracy had to be determined entirely on paper (with some finger-crossing by the secular and praying by the faithful).

Now that's interesting. Anything we should be worried about if everyone was wrong?  smiley

145  Using Arduino / General Electronics / Re: 109.3035 ohm resistor on: March 02, 2013, 07:25:38 pm
Say on.

The resistance of a PT100 sensor at 75 degrees Fahrenheit is exactly 109.3035 ohms. So, if you want a handy way to calibrate your instrumentation, a very precise, very stable, resistor of that value is just the ticket!  smiley
146  Using Arduino / Sensors / Re: Analog pressure gauge / potentiometer coupling? on: March 02, 2013, 07:17:57 pm
Quote
I don't think the gear will be any issue at all.  In fact it's common to bend a needle before stripping the gears.

It's the backlash you need to be concerned about, a sort of mechanical hysteresis.
You might take a look at the pots they use in servos. They may be looser than normal.
147  Using Arduino / General Electronics / Re: 109.3035 ohm resistor on: March 02, 2013, 07:11:20 pm
Aha! I figured it out. Is anyone curious?  smiley
148  Using Arduino / General Electronics / Re: 109.3035 ohm resistor on: March 02, 2013, 07:02:15 pm

I suspect the value is a result of how the thing is engineered and manufactured.  In other words, they strive for a certain range but the precise value is actually a side-effect.


That's a stretch. smiley Are you saying that they were shooting for, say a 110 ohm and got this value? And, they are so good at producing this wrong value that they assigned it a part number and sell it for 5 times the rate of the 100 ohm? Great marketing! Not only that, it is guaranteed to be this weird value to 0.005% error and to have a 0.2ppm temperature coefficient and is supplied in a hermetically sealed package which is either, your choice, oil filled or air filled!? All its siblings in the series have values like 100, 130, 200, 400, 500, 1000, ... By the way, you can order a special value to the 0.005% tolerance but all the possible values are not listed in the Mouser catalog. No, I think a 109.3035 ohm, very precise, very stable resistor has some application that I don't know about.
149  Using Arduino / Sensors / Re: Analog pressure gauge / potentiometer coupling? on: March 02, 2013, 05:41:21 pm
I suspect it will be a challenge. One thing to pay attention to is the spring that is used to keep the gear teeth properly meshed to prevent backlash. You might want to consider mounting the pot from the back of the gauge in order to maintain a visible analog readout should your electronic one fail or go squirrelly. Good luck!
150  Using Arduino / General Electronics / Re: 109.3035 ohm resistor on: March 02, 2013, 04:18:34 pm
I was shopping for some precision resistors on Mouser's website and got distracted by the really high precision resistors. I found that there is a standard value in a Vishay part of 109.3035 ohms, 0.005% tolerance. That value seems to ring a bell some how but I can't dredge up any reasons for it. Does anyone know what the application of that value would be? It would be nice to have one but they are about $100 so I'm going to pass.  smiley More common values in that precision are around $15-$20 in case anyone needs one.

Well with a 0.005% tolerance ratings it's useful just as a bench standard for calibration validation and such. But no, 109.3035 ohms means nothing to me. I've a few 250 ohm .01% resistors squirreled away. They were used to convert 4-20ma current loops to 1-5vdc measurement systems used in the process control industry.

Lefty

Too strange a value to be a calibration standard, IMO. I guess I can email Vishay and ask. smiley
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