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Precision thermometer: question

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Sulaiman

Fri Feb 07 2014, 10:25PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

The A/D process can be as accurate as required,
I was suggesting a look at the accuracy,
or more precisely repeatability of the probe measurements.

If you calibrate your probe at say 0.000C and 100.000C
then put the equipment away.
Next day check the reference temperatures,
can you get 0.1C repeatability, 0.001C etc.
Not much point having 0.0015% resolution electronics with 0.1% repeatability sensor/connections/ambient temp. etc..
(o.k. there are some cases where careful calibration during an experiment allows for precision interpolation, but rare cases)

Then there's linearity, perfect calibration at for example 0C and 100C
does not mean perfect at 50C

What temp range and precision are you considering ?

IamSmooth

Sat Feb 08 2014, 02:23AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Avalanche wrote ...

You can get a lot more than 10 bits of resolution from a 10 bit ADC if you oversample.

The basic method is to add a small amount of random noise to your signal, then (for example) 16 samples averaged will give you a 12 bit result, or 256 samples averaged will give you a 14 bit ADC result.

Obviously the drawback is that your effective sampling rate goes down rapidly with an increase in bit depth

I'm actually do this. It is called oversampling decimation

BigBad wrote ...

fwiw I built an arduino based sous vide controller using a thermistor.

All I did was put it in series with a resistor put the 5v/0v from the arduino across them, and plugged the middle straight into the arduino's analogue input. I think the resolution was about 0.2 degrees.

I just took the data from the manufacturer and interpolated it; it seems to be within a degree accuracy without calibration. I'd advise you to ditch the pots; you can always calibrate it with software if you have some other way to measure temperature more accurately.

I would like to do this, but I won't get the full 5v range if I just put one series resistor in there. I would like to have a range from -325 to 100F. With decimation oversampling I can get 0.1F resolution (12 bits from a 10bit ADC).

BigBad

Sat Feb 08 2014, 04:57PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600

Not sure whether the Arduino is the best tool for this; the ADC may be a bit marginal.

IamSmooth

Fri Apr 04 2014, 06:21PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I've finished my temperature project. I am able to accurately measure LN2, ice water and boiling water. I am curious if anyone thinks there is a market for me to make these and sell them at a competitive price with other accurate RTD measurement devices. I could even sell it as a kit for someone to put together. I did the project for fun, but I think I have something that is better for the price than what is out there.

While I have a nice product, I don't think many people want a cryogenic temperature measuring device to make it worth my while mass producing it. Any thoughts? Has anyone ever gone down the road to sell a kit or finished product that has been successful?

Andy

Sun Apr 06 2014, 04:35AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Your market could be quite large,if I were you I would setup a company and get a accountant to do a cashflow(take as much info as possible), search alot to find a supply that you get on good terms with, and has a really low price, you might need multiable suppliers, if you could offer a product that is cheap in price, or at-least have the ability when starting up helps, if for no more than negations. Maybe look at electronic suppliers like mouser etc to sell it, thats were the cheap price comes in, you should beable to build the thing mentioned for atleast 3/4-1/2 cheaper than what you built it first time around, and let mouser do all the leg work. They will want a 33-200% markup, can you sell it that cheaply(see first point). Little things can stuff up a perfectly good thing, budgeted $15 for something that comes to $30 out of a product that might be $400-$450 that you sell for $50 profit can stop it.

KEEP your day job, and work on this part time. Your will need the money

Word of mouth is the best advertising, but peoples main reason to select a purchase is price, then widgets, of unknown products general. Companys like dealing with other company, it should take about $100-500 to setup, and about $20-40/week for its running.
Workout when to contact certain parts in the chain, if you contact someone and you don't need there service or supply something to them within a week, you might run into problems.Contact mouser(EXAMPLE) last, and a temp senses near-last(guess the price, and knock off 30-70% bulk order)

Shrad

Mon Apr 07 2014, 07:22AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

if you want the advice of someone with field experience in automation, here are the things you want to have if you really want to hit the market :

- RS485 communication with R, W, R/W functions and distant configuration
- Local configuration through simple menus
- Multiple configurable alarm levels (LL,L,H,HH) with local display
- RTD, thermocouple, and multiple topology sensor interface (a couple AOPs and instrumentation amplifiers)

such controllers also have a potentiometric input to measure level from 0 to 100% of a configurable scale

for instance :

I used to work with a brand selling fully compliant models at approximately 300â‚¬ a piece

Dr. Slack

Mon Apr 07 2014, 08:05AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

... and so the feature-creep starts.

The hardest part of doing a successful product is figuring out where it fits in the market. Do you do cheap and cheerful (lots of competition from China), no compromise performance, and price (lots of competition from the likes of Fluke), or somewhere in the middle (what do you leave in and remove, where on the slippery slope do you draw a line in the sand?) The hardware is to some extent the easy bit.

Bored Chemist

Mon Apr 07 2014, 10:19AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

How do you plan to calibrate it?
The ice point is pretty reliable but the boiling point of water varies day by day.

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