I am very pleased with the Vantage Pro 2 24 hour aspirated weather station so far with excellent performance except for high temperature measurement. It has been reading high temperatures that are 0.5 F to 1.1 F higher than a nearby passive weather station from around 12:00 PM to 2:30 PM. All the other times day and night the temperature difference is under 0.5 F. This shows that an aspirated weather station is not necessarily more accurate than a passive one especially in areas where solar insulation is very high. This time of the year the solar insulation is near 1000 W/ms near solar noon and this is might be a problem for the Vantage Pro 2 aspirated weather station. The wind does not seem to help it much because today the average wind speed was 4 Knots and party cloudy weather conditions yet it still read a high temperature 0.8 F higher than a nearby passive weather station. After performing maintenance on the weather station recently it seems that the radiation shield itself is well protected from direct sunlight. The senor tube is surrounded by plates that shade it completely so it is not heated by sunlight. I do not think much solar radiation reflected from the ground that enters the air intake on bottom enters the sensor tube because it is blocked. It seems that the solar radiation causing these errors is reflected sunlight from the edge of the fan plate to the open cap plate above it that is then reflected down through the fan blades to the sensor and causing the errors. This problem possibly could be mitigated by painting the inside of the open cap plate flat black. I have not tried this yet but may try it when not busy. Another solution would be a more powerful fan. I am not sure how big this problem is because the sensors are different and do read temperatures at the same time and the Davis temperature sensor is much more sensitive and responsive. At first I thought this may explain the difference in temperature readings around solar noon, but now I am not so sure. For those who have tested this are your results similar to mine?
Anyone else experiencing these type of errors?
What exactly is the “nearby passive weather station”, how close is it to the VP2, and why do you assume it is accurate?
The other passive weather station which is a Davis 7714 radiation shield is only 5 to 10 feet from the Davis Vantage Pro 2 station. The sensors I am using in the Davis 7714 are Ambient Weather remote sensors for the WS-08 weather station. I have already compared the Davis Vantage Pro 2 sensor and the other sensors to a calibrated NIST traceable thermometer and adjusted the Ambient Weather sensors down 0.6 F. The Davis Vantage Pro 2 sensor
temperature readings were identical to the NIST traceable thermometer.
That’s interesting but contrary to the results published in this report which also compares the Davis FARS with the 7714 - makes no sense to me :?
My results make no sense to me either. I know that the Davis sensor tube is not heated much and it seems like it is a very well designed radiation shield.
My plan is to get another Davis Vantage Pro 2 sensor to test in the 7714 shield. The high temperatures from the Davis Vantage Pro 2 are close to some of the weather stations on Weather Underground, but they do differ from my passive weather station.
That’s good 
I know that the Davis sensor tube is not heated much and it seems like it is a very well designed radiation shield.
My plan is to get another Davis Vantage Pro 2 sensor to test in the 7714 shield. The high temperatures from the Davis Vantage Pro 2 are close to some of the weather stations on Weather Underground, but they do differ from my passive weather station.
That will be interesting. I have to say there is a lot of crappy data on WU these days (just my personal opinion of course).
Approximately where are you located?
I am south of Miami ,Florida near the Florida Keys.
On days with moderate or high winds the passive weather station reads high temperatures about 0.5 F to 1.0 F lower than the aspirated station with the greatest difference so far 1.1 F. For the few days when there is very light winds with average wind speed 1 Knots or lower and full sun with no clouds all day long, the passive weather station and the Davis Vantage Pro 2 aspirated weather station read nearly the same temperature and twice the exact same temperature. This is very odd! Most days the average wind speed is 3 to 5 Knots between 10 AM and 4 PM. Windy days with party cloudy conditions is when the difference between the aspirated and passive weather stations greatest.
I did more testing this time with unshielded but shaded thermometers and the measured temperatures from these thermometers were much closer to the Davis Vantage Pro 2 so the Davis 7714 sensor appears to have a cold bias in very warm weather. This cold bias is much less during light winds and long periods of high solar insulation. I still plan to compare the Davis 7714 with the Vantage Pro 2 weather station but this time with the same sensors, the SHT11, in each.