Swiftlet farmers having various equipments that provide readings of variable data such as temperature and humidity tend to find that the readings varies and therefore made them doubt the accuracies of the readings which are important to understand the state of a farm in whether its suitable for swiftlets to stay.
Some types of digital hygrostats are also not very accurate as I had also did an experiment in which I placed two of them side by side and found them to give different readings. However, the placement of such meters or the sensors of hygrostats must be correct; we must place them at the plank area at an area that is not directly atop a humidifier which should skew its readings.
In any case, there are alternatives to the common digital meters. For example, there is a German made mechanical meter that do not require battery to operate and is said to provide accurate data. The downside is that it cannot provide historical data as in a digital meter that tells us the lowest and highest temperature and humidity recorded over a period. A possible solution to this problem is the use of a data logger that is use to gather temperature and humidity inside a farm over an extended period.
As to the generally inaccurate hygrostat, there is an alternative German made mechanical hygrostat that is said to provide accurate result so as to off or on the humidifiers at the right time. Having said that, even not so accurate meters and hygrostats can still be useful to serve as a rough guide to farmers who already own such equipments and is in no mood to replace them.
However, if the birds are just visiting and not staying to make their nests; then they should try to find out whether the design of their farm or the microclimate is far from ideal. And the way to remedy the microclimate of their farm is to get hold of reliable meters and hygrostat devices that can help them to turn around their farms.