[AllanG]

Hi folks.

I'm in the process of getting together some of the recommended components. I've been looking at the options for external temp & humidity sensors and ideally would like to go for the most accurate. (I know sensor accuracy isn't the most important thing though - location etc. is probably even more critical)

As I understand it, the most accurate sensor the WeatherDuino will accept is the Sensiron SHT25 although it's very expensive and usually only available as a SMD chip. While searching yesterday though, I came across the following on ebay:

http://www.ebay.co.uk/itm/SHT25-Breakout...2c9487d5c7

Looking a bit closer, I went to the Amaryllis Development website which I thought had some interesting information:

http://www.amaryllisdevelopment.co.uk/sh...ut-boards/

I don't know if anyone else has come across these modules or if they will be suitable. If they are, there's not a lot of difference between the GB Pounds 19.99 cost of these and the cheaper alternatives. Any thoughts or comments gratefully received!

Allan.

[werk_ag]

Just one thought about the the SHT2X sensors family...
They are I2C sensors, so you must be very careful with the length of the SDA, SCL lines, and cable capacitance. In any case, the cable between the sensor and the TX board must be very short, with a very good cable 1 meter máx, which in most cases is not enough.
In my modest opinion, for outside Temp / Hum readings, using a SHT11 is a very balanced choice between price and performance, and it can safely be used with cables up to 6 to 7 meters.

For testing and evaluation purposes, I'm using one SHT21 and one SHT11, side by side on the same RS (each TX unit can have both connected at the same time, and will send data from both. In the software you select wich one will be your main outside temp / hum sersor, and the other is treated as a Extra Sensor) and the readings of both are very similar.
Just as a note, the Davis weather stations are using the SHT11 sensor.

If you want to go with any of the SHT2X sensors, and need a long cable to connect it, there are on eBay some small modules, called I2C extenders, that can handle SDA / SCL lines, up to ten metters.

[AllanG]

Yes, I know the SHT25 is an IC2 device which can only be connected by a very short cable. I read the Sensiron datasheet which says "Carrying the SCL and SDA signal parallel and in close proximity (e.g. in wires) for more than 10cm may result in cross talk and loss of communication."

It goes on to say, though, "This may be resolved by routing VDD and/or VSS between the two SDA signals and/or using shielded cables." Even so, I really wouldn't want to use even a metre long cable.

So before constructing anything, I think I should begin planning my whole setup and carefully consider exactly how and where I place the Tx unit and sensors - and how they are connected too!

Thanks for the advice and information, I appreciate it.

Allan.

[werk_ag]

It goes on to say, though, "This may be resolved by routing VDD and/or VSS between the two SDA signals and/or using shielded cables." Even so, I really wouldn't want to use even a metre long cable.

I followed that advice, and connected my sensor with CAT5e cable, using the twisted pairs. It's working for months with a cable length of around 1.20 meters.

Sorry for the alert, but those considerations may also help others that are now starting planning their building.
You are right, planning the placement is an important keyword for successfully build am WeatherDuino Pro2 weather station (as with any other weather station).

[AllanG]

It's very interesting that your 1.2 metres of twisted pair is working well. Maybe in the future, once everything is working well, it will be worth experimenting to see what is the maximum length that can safely be used.

I'm glad you did alert me. I am very happy to be told something that I already know, but what I don't like is making a mistake and then somebody saying to me "but I thought you would know that"

Allan.

[werk_ag]

It's very interesting that your 1.2 metres of twisted pair is working well. Maybe in the future, once everything is working well, it will be worth experimenting to see what is the maximum length that can safely be used.

Yes, it is. I was also surprised.
I'm using this SHT21 module:



If you look at the module schematic you will see that they are using some MOSFet transistors on the SDA / SCL lines, maybe this explains why it's working with SDA/SCL lines with 1.2 meters length.

This one:
http://www.ebay.com/itm/1pc-New-Standard...3cdfc85ac2
costs three times less and it seems to have the same schematic.

...I am very happy to be told something that I already know, but what I don't like is making a mistake and then somebody saying to me "but I thought you would know that"

Can't agree more.

[AllanG]

Thank you for that information, Werk_AG, it's very helpful. The Elechouse module that you're using is the one I found on ebay and was probably going to use until I discovered the relatively inexpensive SHT25 from Amaryllis that I mentioned in a previous post.

I *think* the air temperature sensor will be mounted on a pole close to the wind speed and direction sensors with the Tx unit only half a metre or so away, similar to the existing Fine Offset setup so I should be able to use the SHT2# device. As I said before though, a lot of planning and research to be done yet. The alternative SHT21 device costing three times less than Elechome is certainly worth testing at that price. It's always good to have options

Allan.

[werk_ag]

An interesting setup, where is possible, it's having one TX unit for roof level instruments (wind speed and direction) and another TX unit for ground level instruments ( outside temp / hum, rain gauge). This allows, the best possible placement for weather instruments.

[zitoune]

Hello,

My external DHT22 used to correctly display the humidity until now where the readings are very low (about 10% - whereas it should be much higher where I am). This is what Werk_AG mentioned in different threads: the DHT22 tends to go haywire in high humidity environments.

So I decided to replace it with a SHT21. I read the correct humidity level while the temperature is ... -312 degrees heheh.
Do you think is is due to the above problem? I use un-shielded cable, about 2m long.

Thanks!

[zitoune]

Interestingly I made a shorter cable (<10cm) and the result is the same. The temperature reading also never changes - unlike the humidity.
So I think I will try with another SHT21..