Simplest way to measure humidity & temperature

Hi guys,

I’m trying to create a dyno correction factor for engine dyno runs taken. For this I need to measure air density, which is calculated from the actual air pressure (aka absolute pressure, total air pressure, or station pressure), the water vapor pressure, and the temperature. I could just buy a weather station off Ebay and type the values into my software each time I do a run, but the Arduino is there doing the engine speed timing so I think it would be much cooler if it could measure the ambient weather as well.

So my question is is there a simple way of doing this with the Arduino? It would be great if I could just buy the discrete components (temp sensor, humidity sensor etc.) but there seems to be a lot of work in getting these to work from threads I’ve read on here and I have enough on my hands with the rest of the code. So what are my options? Are there any sensors that I can buy that are pre-calibrated and I can just stick onto my breadboard and feed into the analogue inputs of the Arduino? I don’t mind dealing with non-linear outputs etc. in code, just don’t want to get into calibration or mad timing/RC circuits etc. as I’m not hectic at electronics :).

Cheers!

The SHT75 from Sensirion is a relatively inexpensive and easy-to-interface temperature and humidity sensor, but it may not have the precision you need.

Read some pretty poor reviews of that Sensirion sensors on here so I might steer clear of that unless it's the only option.

After posting last night I came across the SCP1000 on Sparkfun. It can do temperature and absolute pressure (the two values I need really), I just wonder about the ease of interfacing as it looks pretty damn complicated! Is SPI going to take up any pins I need on the Arduino? I need both interrupt pins for other uses for a start.

Thanks!

I use an intersema ms5534c with good results so far. I only use the baro pressure, but it also provides temperature. http://www.intersema.ch/products/guide/calibrated/ms5534c/ Iterfacing is a bit more complicated than I first thought, but I can provide the code if you're interested.

In the meantime they also have an i2c sensor, although with less precison, which would be easier to interface.

Read some pretty poor reviews of that Sensirion sensors on here so I might steer clear of that unless it's the only option.

Not sure what your concerns are regarding Sensirion - I've got one (SHT11) working well in a board of mine using the library from the Playground. I'm adding non-blocking calls to the library to free up more CPU cycles for my application (ie, the calls don't stall waiting for the measurements to complete). I hope to post my changes soon.

For air humidity I've used Honeywell HIH-4000-001 and it's dead simple to read with the Arduino. It gives a linear voltage 0-5V out, corresponding to the relative humidity.

On the other hand it costs almost as much the arduino itself :(

There's a 1-Wire module for humidity (with other models doing humidity plus some other things) from....

http://www.hobby-boards.com/catalog/product_info.php?cPath=22&products_id=46

$55 assembled. Quote from the page:

We now use the HIH-4021 humidity sensor which has a cover for better sunlight resistance and a condensation filter to help keep the sensor from getting wet from condensation.

1-Wire isn't a "big deal".... you CAN do it with an Arduino. There are several possible routes to solution. Once you've got past that bump in the learning curve, all sorts of things become possible.

HobbyBoards has been around for many years, produce good units, most of them less expensive, but the sensor at the heart of the humidity board is expensive.

For more on 1-Wire:

http://www.arunet.co.uk/tkboyd/e1didx.htm

Thanks for the replies guys.

The price isn't really that big a deal, it's a once off project that has already cost $1500+ so a few more quid to do it right is not a big issue. I don't mind a bit of a learning curve, I just want to avoid spending days and days figuring out how to interface each of these sensors when the time would be better spent coding something else :). What would be sweet would be a 'weather sheild' that I could just clip on to the Arduino and get pressure, temperature and humidity but I don't think there's one of those available to do the job. Failing that sensors that are pre-calibrated and give off a simple output (linear or not is no big deal, once I can figure it out in software) would be great. Any more thoughts?