This topic covers issues and faults with readings of wind speed or wind direction. These are often also referred to as anemometer issues. (Strictly speaking, an anemometer measures wind speed only, but a sensor that measures both speed and direction readings is also often referred to as an anemometer.)
The look of the wind sensors on Vue and VP2 models is obviously fairly different with the Vue’s all-in-one sensor design contrasting with the separate anemometer of the VP2 station. At a technical level, however, the sensing of wind speed and direction is broadly similar on both Vue and VP2 and much the same troubleshooting approach applies to both models, with specific comments on differences in fixing wind issues included below where appropriate.
There are actually three types of VP2-type anemometers that have been used in Davis stations:
- The original 6410 VP1/VP2 unit that was used for many years from about 2000-2013 with only minor changes during that time;
- The revised 6410 anemometer introduced in 2013 (though not necessarily widely supplied until 2014) and incorporating a replaceable speed bearing and solid-state speed sensor;
- The 7911 anemometer that was used for the WMII and Wizard III stations;
Troubleshooting for all of these three anemometer types is much the same, but technical differences and how to tell the difference between them is described on the separate anemometer hardware page.
Note: If wind data is being received from a 6332 Anemometer Transmitter rather than direct from the ISS then the possibility of a transmitter issue rather than any anemometer fault should be borne in mind.
Issues and faults with readings of wind speed and direction tend to fall into one of three categories, each of which is discussed in more detail below:
- Zero wind speed readings
- Wind speed lower than expected
- Wind direction issues
Wind speed & direction update every 2.5 seconds on all Davis stations and therefore watching the console display for 2-3 minutes should quickly clarify the symptoms; obviously this is easier on a breezy day than a relatively calm one!
Wind speed and wind direction are measured by separate sensors and it is very rare for both speed and direction to fail simultaneously – usually the fault is with one or the other of speed or direction. There are only two scenarios where both speed and direction are likely to go faulty together:
- With VP2 systems where the anemometer cable has been cut or damaged.
- With stations using the 6332 Anemometer Transmitter where the unit has stopped transmitting for some reason;
So if speed and direction readings have both failed (and excluding scenarios such as a recent nearby lightning strike or use of the Anemometer Transmitter) then the anemometer cable needs careful inspection to identify the cause of the problem.If the anemometer cable includes a joint or extension then this is very likely to be the problem. The further troubleshooting notes below assume that the issue is only one of speed or direction and not both readings.
Zero wind speed
Note: On older 6410 (VP2) and 7911 (VP1 and WMII/WWIII) anemometers it is not uncommon for the wind speed to fail intermittently as an early symptom. This is usually a sign that the reed switch used as a sensor in these older units is starting to fail. A sharp tap to the anemometer body above the cups will sometimes restore correct operation and if this works then it tends to confirm a reed switch issue, but which will need addressing sooner or later. But another possible cause of intermittent speed readings is a damaged cable with the speed conductor making intermittent contact.
Simple causes: There are certain easily-remedied causes of low or zero wind speed, as follows:
- If a visual check shows missing or damaged wind cups then these are readily available as spares;
- On older pre-revision 6410 (VP2) anemometers it was possible for the cups to slip down their shaft and then break magnetic contact with the speed switch. The remedy is simply to loosen the cups, push firmly up on the shaft and re-tighten securely;
- Don’t underestimate how strong even a small spider’s web can be around the cups, which obviously do need to be able to spin freely;
- While the drip rings above the cups help make the anemometer fairly resistant to winter icing, there will come a point in severe conditions where the cups will stop due to ice accretion – there’s no remedy for this other than thawing the ice;
- The cups do need to be able to rotate freely when spun by hand. If much resistance is felt then the bearing is likely to be worn or rusty. Bearings can be replaced (see wind speed cartridges in the spares list) on Vue and revised-pattern 6410 anemometers, but older 6410 units are not repairable commercially and need replacing completely;
- The speed signal is carried by a single wire in the VP2 anemometer cable. If this particular wire is nicked or damaged then the result can be intermittent or zero wind speed;
Speed switch failure: If the simpler causes above have all been checked and ruled out then the only other likely cause of zero wind speed readings is that the speed switch has failed. Regrettably this usually requires buying a new anemometer or Vue harness/ISS, although in older Vue models (rev A-F) the wind reed switch was mounted on the temp/hum PCBA (part 7345.283) and could be replaced separately at about half of the cost of a new harness/ISS.
The reed switch in pre-revision 6410 and 7911 anemometer can in principle be replaced with a new reed switch component, but it is quite an intricate operation to complete successfully and is not recommended for anyone who is not a skilled technician. (And for the same reason it is also no longer practicable or cost-effective for service centres to do this job commercially.) For anyone who is interested in doing it themselves, details are given in wxtech’s online anemometer guide, but be aware that this guide does not appear to have been updated for a few years and some details are out of date, eg the wind cups are now a single-piece moulding and not comprised of individual cups. Also, the revised 6410 anemometer with its different design, separate bearing and solid-state speed sensor is not described.
Additional note: The wireless diagnostic screens nominally provide some information about how often the anemometer speed switch contacts are seen open and closed. Use of this information is explained a little more in the console manuals. But, to be honest, we’ve never found this to be of much extra help in troubleshooting wind issues.
Low wind speed readings
It is not uncommon for users to think that their anemometer is under-reading. There are a couple of genuine anemometer issues that could lead to low speed readings, most obviously a stiff bearing on the wind cups shaft, which can be remedied as described in the section above, or a failing reed switch which is missing some revolutions of the wind cups.
But much more likely is that the user is failing to appreciate the importance of anemometer height and exposure in recording higher wind speeds. If all of the potential causes of zero wind speed listed above can be ruled out then it is highly probable that the recorded wind speeds are essentially correct but anemometer exposure is suboptimal. This is not the place for a primer on optimum anemometer positioning, but one key point is that wind speeds increase strongly with height. The official height for measuring wind speed is 10m (33ft) and this is the height probably used at official stations such as airfields. So comparison of a privately-located anemometer at maybe 2-3m (say 6-10ft) height with an official one at 10m will always show significantly lower readings. Official readings are also typically made in wide open spaces such as airfields where there are few barriers to the full force of the wind. Private locations, by contrast, usually have other houses and buildings nearby, with bushes, hedges, trees etc around, all features combining to materially reduce the local wind speed.
If you wish to measure maximum wind speeds then you must get the anemometer up high and do so in as open a space as is available.
Comparison with reference anemometers
Sometimes users report that wind speeds from their Davis anemometer are reading consistently higher or lower than a reference anemometer. There are two technical reasons why this might happen:
First is the importance of identical exposure – as outlined above, measured speed will vary with exposure and it’s therefore important that two anemometers being compared are mounted at the same height and with the same exposure.
The second reason is less obvious: the wind does not blow at a steady speed, especially at inland locations, but typically blows in gusts with short-period highs and lows. Davis anemometers measure the average speed over a 2.25 second interval; it is this short-term mean speed which is used as the measure of gust or maximum speed and is a period which ties in well with the WMO definition of wind gusts which is mean speed over a period of 2-3 seconds. However, anemometers with a different measurement period will typically record different gust speeds. A longer period will give lower gust speeds while an anemometer that might measure over eg a 1-second period will measure consistently higher gust speeds. Neither reading is right or wrong, but it is a reminder that gust speeds are not absolute values but will depend to some extent on the characteristics of the anemometer being used.
High wind speed spikes
There is an uncommon issue where intermittent spikes of anomalously high wind speeds can be seen. This can have any of three causes:
- Moisture getting into the SIM board socket into which the anemometer cable is plugged and as detailed under the SIM board faults topic;
- A failing reed switch in pre-revision 6410 and 7911 anemometers, where the reed switch starts to bounce repeatedly rather than close cleanly. The result is that the SIM board sees many contact closures per cups revolution, which gets interpreted as a high wind speed;
- On cabled VP2 stations, especially those connected to a PC with earth/ground issues where a ground loop is the culprit. If the spikes disappear when the PC is temporarily disconnected then this is the likely cause. Solving the ground loop issue is more tricky, especially if the logger is a USB type (with a serial logger it should be easy to find an opto-isolator for the serial lead), but at least diagnosing the problem will be a step forwards.
Different wind speed readings on different wireless consoles
Davis Vantage wireless transmitters simply broadcast their data and therefore any number of wireless consoles can pick up data from a single ISS and hence a single anemometer. Occasionally and on careful inspection it may be noticed that two consoles may not be showing identical wind speed readings at the same time. There are two possible reasons for this:
- On a Vue console (which can receive data from VP2 ISS units as well as Vue ISS’s) there is the option to receive an anemometer as a Vue anemometer or VP2 anemometer. These have slightly different settings and a VP2 anemometer does need to be set to be received as a VP2 and not Vue anemometer for maximum accuracy.
- The raw speed readings received from an ISS or anemometer transmitter are actually processed and corrected in the console. This is to allow for the fact that wind blowing from different directions will be affected by passing around the anemometer body in a fairly predictable way and it is therefore possible to correct the raw wind speed to a value closer to the true wind speed. This seems to be done in a fairly simple way by the console whose firmware contains a table of raw and corrected wind speeds vs wind direction (what is known as a look-up table or LUT). The exact formula for making these corrections has evolved over the years and so the latest firmware allows a more accurate correction compared to older firmware. So if two consoles, one with new and one with older firmware are compared then it’s possible for speed readings on the two consoles to vary slightly, especially at higher wind speeds. This effect is not large but may amount to 1-2mph up to moderate speeds. Davis do not release details of exactly which LUT is in which firmware version, but it’s reasonable to think that the later the firmware version then the more accurate the wind speed correction is like to be.
Wind direction issues
In general, it’s rare for the wind direction reading to give problems. But occasionally a couple of issues may be seen:
- On VP2 systems, cable damage involving the direction conductor will cause the direction to show North, either constantly or, with intermittent wire contact, from time to time;
- Any other direction issue usually involves a faulty direction potentiometer in the VP2 anemometer. There is typically no fix for this other than a new anemometer;
- Again, as with high wind spikes and on rare occasions only, cabled VP2 stations, especially those connected to a PC with earth/ground issues, can show odd wind direction readings if a ground loop is a possibility. And, as above, the test is to see if the problem disappears when the logger cable is disconnected.
Note: If wind data is being received via a wireless repeater then a constant wind direction in the range 030-050 can be caused by the test DIP switch #4 on the repeater being inadvertently left in the ON position. (This causes the wind rose on the console display to show the wireless signal strength at the repeater, rather than the wind direction!)