Homebrew Temp/RH meter

Jason_WI · August 25, 2005, 11:50:55 PM

I have about 80 bux in this Temp/RH meter that is NIST traceable. I have it running on a 9 volt battery with low power 5 volt regulator.The sensor is a Senserion SHT71 temp/RH sensor. It comes fully calibrated from the factory. Here are the specs:

2 sensors for relative humidity & temperature
Precise dewpoint calculation possible
Measurement range: 0-100% RH
Absol. RH accuracy: +/- 3% RH (20-80% RH)
Repeatability RH: +/- 0.1% RH
Temp. accuracy: +/- 0.4°C @ 25°C
Calibrated & digital output (2-wire interface)
Fast response time < 3 sec.
Low power consumption (typ. 30 µW)
High precision sensor
Fully interchangeable without recalibration!
Leading CMOSens® Technology for superior long term stability

I have it jumper selectable to display degrees F or C. I could also calculate EMC and Dew Point but the code space on this verson PIC processor I used is small. I may add other features as to be able to store min and max and display them back. Also to be able to capture and store reading manually. Add a serial output so datalogging would be possible. Here is a link to the schematic in its current form:

http://my.vbe.com/~jbirnsch/Temp_RH_Meter/Temperature_RH_Meter.bmp

Jason

Jason_WI · August 26, 2005, 10:58:19 AM

I have uploaded the HEX file and the source code for the meter. I realize this project may be challenging for most here. If there is interest I can do a small production run for those interested at cost.
Here is the link to all the files:

http://my.vbe.com/34/EF/jbirnsch/Temp_RH_Meter/

Jason

Modat22 · December 14, 2005, 04:38:17 PM

I like it when people build their own electronics. Its fun, its educational, and gives you a sense of a job well done.

Good job man!

getoverit · December 14, 2005, 07:02:59 PM

that link required a username and password :(

Jason_WI · December 14, 2005, 07:14:59 PM

getoverit,

Sorry about that. I fixed the link so that everyone should be able to see it now.

Jason

getoverit · December 14, 2005, 07:52:13 PM

Good Job!!

Looks like you're good at assembly language aslo? ;D

I have actually been looking at the digital moisture meters, and wondering about what resistance they are measuring, and the formula by which they arrive at their reading. I already have a digital multimeter with temperature probe, however it doesnt do RH. What I'm more interested in right now is turning this multimeter into a moisture meter. I think I'm actually going to do some testing with brass brads driven into a couple of sample pieces and see if I can come up with a poor man's way to use a multimeter in this manner.

any ideas?

Jason_WI · December 14, 2005, 08:40:30 PM

getoverit,

Most DMM's will over range at 20 Meg ohm. My Fluke 87 will measure to 100 Meg ohm. You need a Gig ohm meter to measure wood resistance. You then need to correlate that reading to a meaningful RH reading that is temperature compensated. Not so simple but can be done.

The program for the PIC microprocessor is written in BASIC with PICBasicPro from meLabs. The .hex file is used by the PIC programmer to program the part.

Jason

Max sawdust · December 15, 2005, 06:15:41 AM

Cool project

I too have scratched my head a few times thinking hmmm how can I use my good Fluke as a moisture meter ???
Max

GregS · December 17, 2005, 11:57:06 AM

Jason,
Considering the micro-controller has other I/O can you do a kiln controller with it also? I don't know much about kiln controls but I would think it is control over humidity and temperature (of course air movement fits in there as well).

I do some embedded programming at work and will need a kiln controller sometime in the near future.

BTW that RH sensor is nice...thanks!

Greg S

shopteacher · December 17, 2005, 02:20:31 PM

Hey Jason,
Whats a linear transducer do?

beenthere · December 17, 2005, 02:29:24 PM

I'll let Jason answer that, but wonder 'for what' you are using or thinking of using it, or in reference to what? Measuring position, movement, ??

shopteacher · December 17, 2005, 06:38:18 PM

I have a couple I picked up with some other electronic paraphernalia and was wondering what they are and what their used for. ???

Modat22 · December 17, 2005, 07:14:08 PM

I think.. linear transducers are used to generate a signal in proportion to its extention. works kinda like an electronic micrometer.

Den Socling · December 17, 2005, 08:20:48 PM

They can be used for jobs like positioning saw blades.

beenthere · December 17, 2005, 08:22:24 PM

Do you have a picture? A 'model' # ?

Jason_WI · December 19, 2005, 07:09:18 PM

shopteacher,

Linear transducers can be resistive, capacitive or even inductive. Some are linear encoders with lines etched on a clear piece of glass / plastic. Depending on the type you have a different interface for displaying the output is required. Digital calipers use a linear capacitor as its measurement medium. They are very accurate and can easly be interfaced to.

Please send pics with part numbers/manufactorer.

GregS,

Yes, depending on what you want to control. If you want to switch on/off fans, heater or pump then isolated relay interface is required. As far as control algo, PID can be implemented but is cost prohibitive. For the most part a simple on/off control with band gap is sufficent to maintain temp or RH within a few degrees F or RH.

Jason