Legionella Sensors - common questions
Overview
This guide covers the most frequently asked questions about Invisible Systems legionella (pipe temperature) monitoring sensors — including how to identify each sensor, correct installation, and common issues.
Important: these sensors measure pipe surface temperature, not the actual water temperature inside the pipe. A small temperature difference between the pipe surface and the water is normal. If you need your readings to reflect actual water temperature, contact support to discuss offset configuration.
Identifying Your Sensors
Legionella monitoring kits include up to four temperature sensors. Each sensor has a coloured cable and a corresponding number:
| Cable Colour | Sensor Number |
|---|---|
| Blue | 1 |
| Red | 2 |
| Green | 3 |
| Black | 4 |
The colours are identifiers only — they do not indicate hot or cold pipes. Blue does not need to go on a cold water pipe and red does not need to go on a hot water pipe. They can be placed on any pipe in any order.
Installation Requirements
- Sensors must be mounted on top of the pipe — not underneath — to avoid damage if there is a water leak or condensation.
- Sensors must not get wet. A wet sensor will stop working and will need replacing.
- Each sensor must have a unique number. You cannot have two sensors with the same number connected at the same time — they will not work correctly.
- Sensors can be connected in any order (1, 3, 4 is fine — you do not have to use them in sequence).
- Cable extensions are available if you need a longer run. The maximum available cable length is 2 metres. Contact support if you need to discuss cable routing.
At installation, confirm that each sensor is reading correctly in Live before the engineer leaves site. This ensures the setup is correct and makes it much easier to spot any issues later.
Common Issues
A sensor has stopped reading or gone offline
- Check for moisture: the sensors cannot get wet — a wet PCB will cause them to stop working. Ask whether there has been any leak or condensation near the sensor. If so, dry it thoroughly. If it does not recover after drying, it will need replacing — contact support.
- Check the cable connections: check that the cable is secure at both ends. If a cable connection appears loose or damaged, contact support before attempting to disconnect it yourself as the connectors require a specific tool.
- Check for duplicate sensor numbers: if a spare sensor was added on site later and given a number already in use, both sensors will stop working. Each sensor must have a unique number.
Readings appear wrong or inconsistent
- Remember that readings reflect pipe surface temperature, not water temperature. Some difference is normal.
- If readings seem significantly off, contact support — offsets may need to be configured to bring readings closer to actual water temperature.
- Check that the sensor is making good thermal contact with the pipe and is securely mounted.
When to Contact Support
- A sensor has been exposed to water and is not recovering.
- You need to change or extend a sensor cable.
- Readings are consistently incorrect after checking the above.
- You need guidance on offset configuration to show actual water temperature.