Accuracy specification (or a good thermometer for calibrating DS18b20's) - Page 1
Forum section: electronics / beginners

author: foufee
author_rank: Newbie
timestamp: June 27, 2016, 10:36:46 pm
content: Hi,I'm looking at getting a calibrated thermometer to provide a reference whilst calibrating DS18B20's, and was looking athttp://nz.rs-online.com/web/p/digital-thermometers/4109066(manual:http://www.chauvin-arnoux.com/sites/default/files/D00AZW86_24.PDF) but the line about accuracy is throwing me off a bit ( ±0.1% of reading +1°C from -40°C to 1350°C) , is this saying that there is a 1°C offset error and a 0.1% measurement error, or is it more likely 0.1% + 1 least significant digit (so 0.1°C when at the highest accuracy setting).Or does any have any suggestions of a good (and reasonable priced) reference thermometer?Cheers

author: DTJ
author_rank: Super Contributor
timestamp: June 27, 2016, 11:23:20 pm
content: It's a thermocouple meter so unless you calibrate the meter plus the thermocouple as a pair you will have an error of >1 C.Look for a unit with an integrated thermistor or RTD probe. You mentioned the 18B20’s so I assume you are working with temps say 0 to 100C? A possible option is a certified thermometer (they used to be around $100). Alternatively build something using one of the more expensive calibrated digital sensors or buy one of their dev kits.

author: uncle_bob
author_rank: Unknown
timestamp: June 28, 2016, 12:42:53 am
content: Quote from: foufee on June 27, 2016, 10:36:46 pmHiHow close are you trying to get?Do you simply want all the product to be the same or is there a real accuracy spec?What temperature range are you trying to cover?Does your budget cover $100K, $10K, $1K or $100? There are very real reasons why you would spend at each of those levels depending on what your real needs were.Temperature is one area where you *can* afford to buy a primary standard. It's good at one temperature (0C) but it's a start. It's also the basis for a lot of systems. You can also *make* a fairly good triple point cell without a massive lot of effort. It will not be cal lab rated, but it will be a lot more accurate than you probably need.Bob

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: June 28, 2016, 12:57:49 am
content: Quote from: uncle_bob on June 28, 2016, 12:42:53 amDo you have more on that? I tried to figure out what would be involved in making one, but I couldn't invent anything that was practical for me to do on my own.

author: foufee
author_rank: Newbie
timestamp: June 28, 2016, 01:15:59 am
content: We're trying to get to less than 0.5°C, and prepared to spend a few hundred dollars on kit to help achieve it. The temperature range we need to cover is -10°C to about 40°CCheers.Quote from: uncle_bob on June 28, 2016, 12:42:53 am

author: DTJ
author_rank: Super Contributor
timestamp: June 28, 2016, 02:10:26 am
content: Perhaps look at Sensirions development kits for temp or humidity (does temp as well).https://www.sensirion.com/products/humidity-sensors/evaluation-kits/I have a EK-H5 kit which I think I paid $40 and is good for +/-0.3°CIt has the sensor, USB adapter & software to display temp / RH.They have other sensors @ +/-0.2°C.I got mine from RS components.

author: uncle_bob
author_rank: Unknown
timestamp: June 28, 2016, 10:20:50 pm
content: Quote from: Galaxyrise on June 28, 2016, 12:57:49 amHiGoogle will turn up a number of pages on doing triple point cells. The typically approach is to use test tubes as part of the structure and a bit of glass blowing to fuse things. You fill the empty cell with as good distilled water as you can get your hands on. Heat it up to boiling for a while and then melt the pinch off tube. As it cools you get a vacuum since the headspace was filled with steam rather than air. You can also do the enclosure with a variety of materials. Whatever you use needs to be "boil compatible" unless you have a good vacuum pump.Bob

author: uncle_bob
author_rank: Unknown
timestamp: June 28, 2016, 10:24:04 pm
content: Quote from: foufee on June 28, 2016, 01:15:59 amHiThere are a lot of ways to go for something like that. I'd look at Fluke.Bob

author: Paul Moir
author_rank: Frequent Contributor
timestamp: June 29, 2016, 12:18:57 am
content: Something to keep in the back of your head is that you can do this with a higher accuracy RTD and a good multimeter which can do 4 wire measurements. Some bench multimeters will read in degrees if you tell them you have an RTD plugged in. So if you need or have a good multimeter anyway, this might be a solution.