Feasbility of temperature measurement in a huge building


I’m working for an event agency and sometimes we have special conditions in our contracts with our customers that the temperature in the different locations inside the venue has to be in a specific range (eg. between 18.5 - 21°C).

Most of the time, the venues do not have the option to take automatic measurements in all areas. So currently we have a dedicated member of our team walking around the whole day taking measurements and noting them in an Excel sheet, which is very time consuming - talking about 60-100 different locations, depending on the type of event/venue.

I was looking for a more automated solution and came across OpenEnergyMonitor. As we normally have WLAN across all of the venue, I was thinking of distributing a number emonTH V2 -Temperature & Humidity Nodes and some emon Base with on board Wifi

Did anyone do something that huge, do you think this is feasible?

I’m especially wondering about how long the batteries of the emonTH V2 would last and if it is possible to power the emon Base with batteries (which I doubt as Wifi is quite “expensive” regarding energy consumption)

Since a conference venue can be quite large I’m also concerned about the range of the emonTH V2 and also about how many sensors I can connect to a single emon Base?


Hi Thomas

I think it’s feasible, but it requires carefull planning since all sensors use the same radio frequency.

How often should each sensor measure the temperature?

Will you allow a delay before the temperature is posted to EmonCMS (or whereever you want the data)?
If so, how long?

Best regards


I missed this post earlier. Unfortunately, there’s a limit in JeeLib, the RF library that we use, of 31 discrete IDs. You could bury your own device ID within the data packet, or you could use different group IDs, but I think I’m right in saying that there’s no provision in emonHub to receive from more than one group, nor to manage a message format that is special to the extent of including the sending ID in the data. (@pb66 will put me right on those points.) So apart from the channel usage question - which may give rise to the occasional collision of data - the firmware might need some significant work to go past the limit of 31 nodes.

Range is highly dependent on the building construction, as well as distance, and of course on interference levels generally. It’s almost impossible to predict, and the only suggestion I can make would be to try it and see. You might need, as @kirkholt seems to infer, several base stations to give you complete coverage. That would not remove the problem with collisions.

Have you looked at Wireless Tags? You’d have one tag manager device at each customer location along with wireless sensor tags placed in various locations around your customer’s site. They have tags that do temp only, temp and humidity and they have one that stores data in memory in case you lose internet or the tag manager goes down. The tags have long range radios as well (700+ Feet line of sight). I use them at my beach house to monitor temp and humidity in some rooms as well as the refrigerator along with EmonTX and Emoncms to collect energy usage.

Wireless Tags also has KumoApp which is their cloud based scripting API that allows you to perform actions based on data from you tags. It also has IFTTT integration so you can take action there as well.

Along with OpenEnergyMonitor products and software, Wireless Tags, WeMo, SmartThings and some others, I’m able to fully monitor and automate my houses.

Thanks Ole and Robert,

thanks for clarifying this - I think the main problem is that we can’t actually test this until the event is actually set up and running.

At events of this size, we normally have our own IT and WLAN infrastructure and lots of additional equipment from 3rd parties (mainly exhibitors) that might as well use some kind of radio equipment - plus our own walkie-talkies :frowning:

So probably we would have to test this while also collecting measurements with conventional methods (aka running around with an infrared thermometer :slight_smile: )


Sorry for the slow response I’ve been bogged down with work and other stuff.

Actually, there is a mechanism in emonhub that allows for multiple RFM receiver devices, those devices will only operate on on one freq and for one group, but emonhub could in theory have 255 groups per frequency if you could attach enough devices, the airwaves would become somewhat congested way before you got anywhere close to those limits though.

depending on the floor plan, ie whether stands are grouped or not, it is possible to use several temp sensors per emonTH radiating upto 10m from a centrally located emonTH (upto 6 in the default FW but upto 128 by the 1w library) those emonTh’s could number up to 30 on any group reporting to one receiver if the RF is strong enough. That’s 30 blocks of 7 (6 external and 1 internal) without any coding changes of additional devices.

To cover a greater area or a more congested area you could use more receivers on different groups, if you use JeeLinks on 5m USB leads (or better still USB over Ethernet or even build a few RFM-to-esp8266 wifi repeaters) there should be more than adequate channels.

Sending data every 60s might not be necessary for you purpose so you could reduce the send frequency to reduce traffic, but 30 devices sending every 60 secs shouldn’t be an issue IF you fit the batteries (therefore initializing) with an equal spread over the 60s, 1 send every 2secs shouldn’t drift much over the course of a single event.

If neighboring group devices are causing RF clashes you could drop the send power or decrease the receive threshold so only the strongest (hopefully most local) signals hit each receiver.