Where to put your SAMHE Monitor
Here are 6 simple things to bear in mind when choosing where to put your SAMHE monitor. Monitor placement is important because it can affect the readings you get. Please also refer to our standard operating procedures for advice on the safe usage of your SAMHE monitor.
| Only use it indoors | SAMHE Monitors are designed to measure indoor air quality. |
| Remember the wire | Your monitor needs to be plugged in. When choosing a spot ensure that the cable does not present a trip hazard. |
| Find a flat surface | Seek out a flat surface for your monitor to sit on and make sure that the monitor is placed away from its edges. Your monitor should always be placed vertically, making use of the stands (if supplied). |
| Away from windows and doors | Place your monitor away from windows and doors if possible. Windows and doors help ventilate your room. If you put your monitor close to them this could distort the readings you get, giving you an inaccurate picture of air quality in your classroom. Ideally the monitor should also be out of direct sunlight to avoid the sensors within overheating. |
| Monitor height | Put your SAMHE monitor somewhere you can easily see it. Ideally your monitor should be placed at head height. Avoid placing your monitor any higher than this and always ensure it's positioned at least 50 cm from the floor. |
| Distance from occupants | Exhaled breath contains CO2. If your monitor is somewhere where it is often breathed on directly this will affect the readings. Although we may encourage you to explore this in experiments we'd recommend that your monitor normally lives somewhere at least 50 cm away from the nearest person. |
SAMHE Monitor Accuracy
The accuracy of a measuring device is the smallest change in the measurement that the device can confidently measure. For example, if a temperature sensor is accurate to within ±0.3°C, if the true temperature is 20°C, the sensor will display a reading between 19.7°C and 20.3°C. All measuring devices have a limit on how accurate they are. The SAMHE monitors have the following accuracy (according to their manufacturer):
- Temperature: ±0.3°C
- Humidity: ± 2%
- TVOC: ±15%
- M: ±10%
- CO2: ±40 ppm
Two monitors placed next to each other should show similar values but there might be a difference up to 100 ppm.
Whether you've just unboxed and connected your monitor, or you're moving it at some point thereafter, it's likely that people are breathing on or near it. This can affect the readings because exhaled breath contains CO2 which may lead your CO2 levels to appear high at first.
If your monitor consistently shows high CO2 values despite actions you've taken to improve ventilation perform the following check to ensure that it is working as expected.
First thing in the morning check your SAMHE monitor CO2 reading. If it is not below 500 ppm, find a suitable sheltered spot close to an open window and leave it plugged in there undisturbed for 15 minutes.
If after 15 minutes the readings are still above 500 ppm, please report this using our technical queries form and provide your monitor number.
Why does this check work?
First thing in the morning, providing the room has been unoccupied overnight, indoor CO2 levels should be at 500 ppm or below. This is because with no-one breathing in the room (and therefore exhaling CO2) until you arrive, we'd expect the CO2 levels in your classroom to drop towards the background level of CO2 in the atmosphere - see our key definitions page for more information on this.
Sensor drift is the tendency for monitors to gradually become less accurate over time. This can be caused by physical changes that affect the sensor. For example, airborne pollutants such as dust and chemicals may build up on the sensor and affect readings over time, and so may mechanical damage, such as if the sensor is dropped. Prolonged exposure to extreme conditions, such as very high temperatures or humidity, may also result in sensor drift.
Calibration is the process of adjusting a sensor to give a more accurate reading. Usually, this is done by comparing the sensor reading to a known value (possibly from another sensor which is known to be accurate). The sensor reading can then be adjusted to account for the difference between the known value and the inaccurate measured value. Calibration is a continuous process. If you think a monitor is providing inaccurate readings, try looking at your past data in the SAMHE Web App to see if your measurements have changed.