This topic covers Temperature and Temperature/Humidity (T/H) sensors used in VP2 stations manufactured since 2006 and is relevant also to Vue stations. [Please see the topic Older Davis Stations for T/H issues relating to Monitor II, Original Vantage Pro (VP1) and VP2 stations manufactured prior to 2006.]
While the general description, symptoms and troubleshooting procedures are similar are similar for T/H sensors between VP2 and Vue stations, replacement parts are of course different for the two station types. Please refer to the relevant page for details of how to replace the sensor.
Temp/hum sensor types
There are two kinds of temperature and temperature/humidity sensors used with modern Davis stations:
Combined temperature/humidity sensors
These are found in the Vue and VP2 ISS units and in supplementary T/H stations. The T/H sensor is placed as standard inside a radiation shield. Humidity sensing is always implemented using a combined T/H sensor; in other words there are no standalone humidity sensors in the modern Davis range. A sensor fault with either temperature or humidity readings will always require that the combined sensor element be replaced. Different versions of the combined T/H sensor have been used over the lifetime of the VP2 stations and are described in more detail on a separate page. Troubleshooting faulty T/H readings is discussed further below.
Temperature-only sensors
These are essentially just an encapsulated sensor bead at the end of a cable and not supplied as standard with a radiation shield. These sensors are Davis parts 6470, 6475 and 6477 and typically used with supplementary transmitters such as 6372 and 6345 to provide additional temperature readings. 6470 and 6475 have a robust stainless steel encapsulation and is therefore useful for locating almost anywhere, eg in soil or under water, while 6477 is cheaper but has only plastic encapsulation. 6470 has bare wire connections suitable for attaching to the terminals of the 6345 multi-sensor transmitter, while 6475 and 6477 have an RJ11 connector for connection to the 6372 Supplementary Temperature Station.
Temperature-only sensors are very simple and will typically have excellent reliability. Any issues with readings from these sensors is likely to be associated with the transmitter board to which they’re connected or a cable fault and not with the sensor bead itself.
Accuracy issues for temperature/humidity sensors
A detailed discussion of accuracy specifications for the combined temp/hum sensors used in the Davis stations will be found within the sensor topic. But there are four specific questions about temp/hum accuracy and resolution that regularly get asked:
Troubleshooting faulty T/H readings
Service life
The combined T/H sensors do tend to have a finite service life (typically of eg 5 years or so, but can certainly vary) and therefore needing to replace this sensor is one of the commoner maintenance tasks. The reason for the limited life seems likely to be related to the humidity measurement, which relies on having an active surface – through which the humidity can be sensed – exposed to the ambient air. Almost inevitably, this active surface is vulnerable to degrading over time, especially if there is pollution or corrosive agents such as tiny droplets of salt spray in the air.
It’s therefore to be expected that T/H sensors at exposed coastal sites or near major industrial plants may have a shorter life expectancy (for example, 2-4 years, depending on circumstances) than those at relatively benign inland sites. (Strong winds at exposed locations will obviously tend to drive rain and any dissolved contaminants on to the active surface of the sensor and so will contribute to shortening the sensor life.) Cold and persistent water-logging of the sensor are other contributory factors and it’s common to find that the onset of cool damp/foggy conditions in the Northern hemisphere autumn is when deteriorating T/H sensors will finally give out.
Symptoms
The symptoms of a failing T/H sensor can be quite variable, but probably the two commonest signs are:
- Temperature reads -90°F or -67.8°C! These temperature values, while seemingly very odd, are actually the lowest values that the Vue and VP2 consoles can read and so the values just imply that the ISS circuit is not receiving a meaningful signal from the T/H sensor. The temperature value may also simply be dashed out;
- RH reading is zero or absent or seems consistently wrong, eg it may be ‘stuck’ in the high 90’s % or fails to exceed 80-90% even in very wet conditions;
It is very rare for the temperature to be wrong by eg just a few degrees – it’s typically either correct or completely wrong or simply missing altogether. Temperature values that are just a little ‘wrong’ usually have other explanations, eg the comparison value may itself be wrong or the ISS is sited at a height or in a location that is genuinely giving different readings. Or maybe a temperature offset has inadvertently into the console reading – always worth checking that no offset is present.
If the temperature reading is missing then obviously another required check is that the transmitter is still working correctly, eg are wind readings still being correctly received from an ISS? NB Sometimes the console seems to retain old temperature readings, especially from supplementary stations, and it’s especially important to check that the value is still updating rather than the console just displaying a stale value.
But if the troubleshooting concludes that the T/H sensor is indeed likely to be faulty then the only long-term solution is to replace the sensor. Occasionally a failing sensor will recover, especially if it’s simply become waterlogged after a storm, but if you spot signs of incorrect readings then it’s often the case that the sensor will need to be replaced sooner rather than later. There are suggested procedures online for reviving a sensor by washing it thoroughly with distilled water before allowing it to dry out fully, but we’ve never found these to be more than a short-term palliative.
Note: The T/H sensor has a combined sensor element – there is no way to repair either temperature or RH independently.
Replacing the T/H sensor
On the VP2 stations replacing the T/H sensor in the ISS is relatively straightforward and certainly something that can be done by most users. However, the process does require disassembling the radiation shield. It is therefore much easier to do with the ISS on a table-top and so is best done by demounting the ISS and bringing it indoors.
On Vue stations after the F revision the T/H sensor is an integral part of the complete wiring harness and cannot be replaced separately. While a T/H sensor board is available separately for older Vue stations, availability of this part is becoming difficult and for a Vue of this age it’s probably more sensible to consider replacing the complete Vue ISS. Refer to the Vue ISS topic for more details.
The basic VP2 process is as follows:
- Remove the rain cone;
- Unplug the T/H sensor from its socket inside the SIM board compartment and feed the cable back out of the access port at the back;
- Carefully remove the 3 long screws that hold the shield elements together. The shield elements will now be loose!
- Try to keep the shield elements mostly together and in the right order and orientation;
- Identify the element with the T/H sensor attached and replace with the new sensor;
- Reverse the steps above to reassemble;
This process is also described in a helpful (but rather old and long-winded) 2-part video on the Davis website;
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