The sensors embedded in your RuuviTag are ready to deliver reliable measurements right out of the box. In certain cases, adding an offset may enhance the accuracy of your results even further.
In this article, we will delve into the offset correction feature within sensor settings, guiding you on how to adjust your sensor measurements in Ruuvi Station utilising the tools at your disposal.
Inconsistencies in Ruuvi sensor measurements within the same room
This could be perfectly normal – don’t panic! First of all, only compare differences between the same RuuviTag models: the RuuviTag Pro is shipped with a superior Texas Instruments TMP117 temperature sensor, so temperature results between standard and Pro models are not directly comparable. RuuviTag Pro can be used as a starting point reference for temperature when comparing with different models.
Next, remove all possible variables that may affect your measurement results. When making a comparison between two distinct RuuviTags, first move them out from their protective covers and place them in a controlled environment such as a tightly sealed box, that is shielded from direct sunlight, drafts, or temperature influences transmitted from surfaces.
Remember that sensor components require time to acclimate to their surroundings so it’s advisable to maintain them in the same controlled environment for an extended duration, ideally at least 12 hours, before doing any comparisons.
Personally, I prefer enclosing my RuuviTags within an airtight box, then placing this box inside a darkened wardrobe cabinet and coming back for the results the following day. Any remaining differences between sensors of the same model should be minimal and linearly comparable.
When do you need to use offset correction in Ruuvi Station?
The short answer is that offset correction is not typically required. From a technical standpoint, the sensor components within your RuuviTag are quite accurate, when considering typical absolute accuracy tolerances outlined in the RuuviTag technical datasheet. Consequently, these minor variations are unlikely to substantially impact your measurement accuracy when comparing readings between multiple RuuviTag sensors of the same model.
Offset correction can be useful when your RuuviTag reports a slightly higher or lower value compared with other Ruuvi sensors of the same type when placed in the same conditions. In typical cases, users want these minor differences to be ironed out by using the offset correction feature in Ruuvi Station.
Good reference points for correcting your measurements
A useful offset correction can only be done when a well known reference point is known.
For this reason, we will resort to some good old science to find reliable enough real world reference points with simple tools that you already have in your home.
Temperature reference
An appropriate temperature reference is the triple point of water, where water coexists in liquid, ice, and vapor states, precisely at 0.01°C (273.16 K). This for us serves as a convenient and easily accessible reference point for a target of 0°C.
Humidity reference
To set a humidity reference, we will utilize the Salt Test method, a widely adopted easy-to-use approach for calibrating hygrometers.
This involves creating a sealed environment within a container, achieving a 75% humidity target by introducing water and sea salt, and allowing it to stand undisturbed for 24 hours.
Pressure reference
Finding a highly accurate reference point for air pressure is a relatively difficult task, since atmospheric pressure changes due to altitude and current weather conditions in your location.
In practical terms, the Meteorological Aviation Report (METAR) often serves as the closest and most readily accessible local reference point. METARs are commonly sourced from airports or permanent weather observation stations, with reports generated approximately once every hour or half-hour at most stations. These reports are utilized by aircraft to set altimeters and are generally accessible to the public. Please be aware that the accuracy of the reference may be influenced by your altitude and distance from the local weather observation station when using local weather information. It is advisable to compare data from multiple weather stations in your vicinity, particularly when they are reporting consistent pressure readings.
Find and set offset for your Ruuvi sensor
Before we begin, be aware that offset correction settings are available to you in Ruuvi Station sensor settings page in two different scenarios: when you are not signed in to the app and want to use your sensors with no ability to restore your sensor cards from Ruuvi Cloud, and when you are signed in to the app and sensor ownership is marked to your account. If offset correction settings are not shown to you, contact the owner of the sensor to do the configuration for you. More information about sensor ownership can be found here.
Now, let’s roll up our sleeves and conduct these experiments to configure our Ruuvi sensors in Ruuvi Station with above mentioned reference points.
Find temperature reference and correct offset in Ruuvi Station
For this experiment you will need an airtight container, crushed ice and water.
To achieve the 0°C temperature target, place crushed ice and water in a sealed container that is placed in a controlled environment (shielded from direct sunlight, drafts or temperature influences transmitted from surfaces). Make sure to place the ice to the bottom of the container to avoid getting temperature differences in water.
Ensure that your Ruuvi sensor is encased in its protective covering before placing it inside the container. Subsequently, seal the container, taking care to prevent the sensor from coming into contact with water. Wait until the temperature reading from your sensor falls close to 0°C in Ruuvi Station.
Next, go to the Sensor Settings page in Ruuvi Station, select Temperature under Offset Correction, click on Calibrate, input the current known value of 0°C, and then confirm your selection.
Find humidity reference and correct offset in Ruuvi Station
For this experiment, you will need an airtight container that comfortably fits a cap of a jar or a lid filled with sea salt and water, and your Ruuvi sensors.
Begin by placing sea salt into a small container (such as a cap or lid) and then add water until the salt becomes wet without fully dissolving. Move the small container inside the bigger airtight container, place your Ruuvi sensors inside the container and seal it with an airtight lid.
After 24 hours have passed, check the relative humidity reading of your sensor in the Ruuvi Station app.
Next, go to the Sensor Settings page in Ruuvi Station, select Humidity under Offset Correction, click on Calibrate, input the current known value of 75% RH, and then confirm your selection.
Notice that in Ruuvi Station, the humidity offset is set using relative humidity values that are reported in percentage. It’s not possible to set an offset using another unit.
Find pressure reference and correct offset in Ruuvi Station
Finding this reference point only requires a bit of Googling. Open Google.com and search for METAR + your current location, for example
“METAR Kuopio”
When conducting your search, you should find results for the nearest airport or weather station to your location, complete with weather information. Click on the top link in the search results. On the opened page, you should discover comprehensive local weather conditions, along with the observation time, and crucially, pressure information typically reported in hPa. Then find a possible second or even third weather observation station in your area to check if the reported pressure is comparable in the current weather conditions in your region.
Next, go to the Sensor Settings page in Ruuvi Station, select Pressure under Offset Correction, click on Calibrate, input the current known value in hPa, and then confirm your selection.
Conclusion
Offset Correction in Ruuvi Station provides an easy way to set your Ruuvi sensors for a known reference point and to remove differences from measurements between Ruuvi sensors of the same model.
The reference points mentioned in this article can also be useful when using third party solutions (other than Ruuvi Station), where calibration or offset settings are available.