Humidity / temp / solar kit, bad temp

equinoxefr

New Member
Hello,

I have build an hobby-boards humidity kit, everything is ok but when i run digitemp, i think temp is wrong, ~ 4° higher than real temp. I have checked this with digitemp and another DS1820 with passive power. When i have 16.8°C with kit, i have 12.44°C with standalone DS1820. What's could be wrong ???

I have no CRC errors reported by digitemp.
I have a barometer kit on the same bus. I use homebuild serial passive adapter connected to barometer.

An idea ??

Thank's
Pierre
 
hello,

i have done more tests. When i remove barometer from bus, everything is ok. When i plug barometer, temp growing slowly... :p
 
Are you sure that DigiTemp is showing the temp for the DS18S20 or is it showing the temp for the DS2438? I have not seen heating problems with the DS18S20 but I have seen self heating in the DS2438.

Eric
 
Hello,

Thank's for your reply, DS2438 and DS1820 reports same temp. When i connect humidity kit to barometer with term block, i have no problem. When i connect with RJ45, temp grow up to 4°...

There is a LM 7812 for barometer and LM 7805 for humidity kit. normally Vin > Vout + 2V so you specify power barometer with 14V. I use a 18V power supply. Is it too much ?

I am looking for a 14V power supply to do some tests...
 
Are you saying that with the power applied that both the DS18S20 and DS2438 temps rise 4°?

+18v power supply won't hurt anything but it will cause the voltage regulators to get warmer than they would with +14v and could contribute to some of the rise in temp you are seeing.

Eric
 
Looks like my registration and reply got removed in last nights site move. I just re-registered and will repeat my post that got lost.

I have the same problem as well. I have been bench testing my weather station inside before installing it permanently outside. So far I have a TAI8515, Hobby Boards temp/humidity board, and a Hobby Boards barometer. Before adding the barometer, the temp board matched the TAI8515 and also a separate digital thermometer within less than 1 degree. After adding the barometer, the temp/humidity board started reading 4 - 5 degrees F too high. If I remove the barometer, the temperature eventually drops back to agree with the other thermometers. If I plug the barometer back in, the temperature rises back up.

Thinking that the 14V power from the barometer was causing some heating, I made a cable that doesn't pass along the 14V. Now everything works fine with the barometer attached. Since I'm currently using parasitic power for every device except the barometer it isn't a big probelm. But I still would like to have a fix in case I ever need to move away from parasitic power.
 
Which chip on the Humidity/Temp/Solar board was reading high. The DS18S20 or the DS2438?

The DS2438 has a know self heating problem but the DS18S20 shouldn't be affected since it is always parasitically powered.

I will do some experiments here and see if I can find anything answers.

Eric
 
It was the DS18S20, although now that you mention it, the humidity reading was affected as well. I was only using the temp reading from the DS2438 as part of the humidity calculation and not looking at it separately.

After looking at the schematic, I see what you mean about the DS18S20 being parasitically powered. Also, it also looks like the only parts close enough to affect it if they get warm are C1 and C2. They never got noticeably warm to the touch, but my fingers probably can't discern 4 or 5 degrees anyway. ;) And having my fingers that close to the DS18S20 raises the temperature reading by a lot more than 4 or 5 degrees.
 
Same problem here I am using the solar sensor with Weather Display had a few DS1820's around so I put one on the board (ordered it with solar only) I am powering it with a 12v supply and am getting a +4 degree reading from another temp sensor nearby....thought maybe it was a housing issue, but has the same difference even a night.
 
I know this post is a little old, but thought would put my experience in here where the sensors were not on the board or case.

I have the Humidity/Temp board and the Temp reading on the Humidity Sensor is about 3-4 degrees higher than the DS18S20 also. That was before took them out of the board. I needed my box behind the wall and am not running parasitic on the DS18S20. I extended them from the board to poke out behind the thermostat. Still read the same with the Humidity Sensor about 3-4 degrees higher.

Initially I was thinking about just getting the Humidity board and using it for Temp and Humidity, but read somewhere about it's accuracy or something, that caused me to decide to just use it for Humidity.

I just ordered another board and will see if my results are the same before modify it. Hmmm, I might not modfify this one. This one is for outside and can just poke the RJ45 out the hole from the attic and mount the case in the eave.
 
Hmmm. In the picture on the Hobby Boards web site, it looks like the LM7805 regulator is right next to the DS18S20. There's bound to be some heat when converting 15v to 5v. Put your finger on the regulator and see if its noticably warm.
 
Just got my new boards yesterday. I didn't notice anything feeling warm on the board, but didn't leave it plugged in for long time either. But here is what I did get:

Original board plugged in:
DS18S20 - 69.58F
HIH-4000 - 71.26F & 39%

Uncovered:
DS18S20 - 68.56F
HIH-4000 - 70.92F & 39%

Removed and extended sensors from board, powered DS18S20 (owfs has trouble with parasitic thru hobby-board hub, thought was fixed, but temp was reading 185, so used Digitemp for all these readings):
DS18S20 - 62.60F
HIH-4000 - 69.41F & 46%

Now it did take a bit to remove and extend the sensors. It got dark by the time got it done, so temp might have dropped in basement. Had the others disconnected so couldn't tell from graphs.
My point is the temp difference between the two sensors.

Right now I have this in the attic and another separate DS18S20 up there also. Moving the extended sensors to outside house today thru a hole. But right now this is what they read, and the two DS18S20's read almost same and all 3 of these are in close prox. (these are with owfs and this is in the morning, above was about 5-7pm last night)

DS18S20 - 46.625F
HIH-4000 - 51.1812F & 81.4954%

The DS18S20 close to the board - 46.625F

The humidity is really hard to tell, yahoo weather says humidity here is 93%, not sure how much sensor being in attic right now on winter day matters, will have it outside later. Temp for the DS18S20's match yahoo though at 46F. Just watching the house humidity board I see it can vary quickly so humidity at my house may not match the yahoo weather as closely. That is just a guess though. My biggest gripe though is the temp difference between the two sensors. Doesn't seem to match the DS18S20 wether in case, out of case, extended. But the DS18S20's match with another.
 
After rereading the entire thread, I believe what you're saying is that the HIH-4000 is reporting a higher temperature than the DS18S20? I had misinterpreted your earlier posts to mean that the onboard DS18S20 was reporting the higher temp.

The HIH-4000 does not report the temperature. It only outputs a voltage that is proportional to the relative humidity. The DS2438 reads that voltage. I think the temp sensor you're referencing is actually onboard the DS2438.

In 12/2006, sdcohe tells an odd story about how adding a barometer board to the 1-wire bus affects the temp reading on the humidity board. Then reports that not powering the humidity board corrects the problem.

In 12/2006, Eric mentions that the DS2438 has a known heating problem.

I also found this unsubstantiated blurb:
"the DS2438 is a SMT device, it may be unduly affected by heating of your PCB by the operation of the circuits on it."

One would think so, but the DS2438 may not use the same temp sensing method as the DS18S20.
All it says in the spec sheet for the DS2438 is:
"The DS2438 measures temperatures through the use of an on-board temperature measurement technique."

You've said that even with moving the HIH-4000 and DS18S20 sensors off the board and you still get a higher reading from the DS2438(HIH-4000).

Looking at this info, it seems like the DS2438 reports a higher temp when its powered.
The board may be heating up and/or the DS2438 itself heats up with VDD power.
You could try running the humidity board unpowered (and pull JP1), and see what happens.

"not sure how much sensor being in attic right now on winter day matters"
Your attic should be drier than the outside.
Also, I read in the spec sheet that the humidity sensor is sensitive to light. Something to consider for your outside mounting.
 
After rereading the entire thread, I believe what you're saying is that the HIH-4000 is reporting a higher temperature than the DS18S20? I had misinterpreted your earlier posts to mean that the onboard DS18S20 was reporting the higher temp.

The HIH-4000 does not report the temperature. It only outputs a voltage that is proportional to the relative humidity. The DS2438 reads that voltage. I think the temp sensor you're referencing is actually onboard the DS2438.

My bad, sorry, yes it is the DS2438 that does the temperature, besides the DS18S20. But yes that is what I was talking about. The DS18S20's seem to be reporting correctly.

Looking at this info, it seems like the DS2438 reports a higher temp when its powered.
The board may be heating up and/or the DS2438 itself heats up with VDD power.
You could try running the humidity board unpowered (and pull JP1), and see what happens.
Owfs has a slight timing problem with hobby-boards hubs and parasitic sensors. It is suppose to be fixed in CVS, so will have to see. Before I powered my DS18S20 on the board it reported 185, nope not 85, 185. Anyway, works now since powered it. So I am going to leave it powered.

"not sure how much sensor being in attic right now on winter day matters"
Your attic should be drier than the outside.
Also, I read in the spec sheet that the humidity sensor is sensitive to light. Something to consider for your outside mounting.
Mine is under the roof, just outside of attic, out of the weather and sun. Back of my house don't get much direct sunlight except 1st part of morning. Between trees and orientation of house it will be in the shade all the time. I do believe I read that on the light sensitivity also, so good thing to point out.
 
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