A consultant, looking at a project where sandstorms and rain are prevalent, asked how WirelessHART performs whilst under dust storm and heavy rain conditions in locations with clear line of sight up to 150m apart.
The environment does impact radio communications, however, at the 2.4GHz frequency that WirelessHART uses, the reduction in signal strength is not going to be noticed in a properly running system. Emerson Process management has documented the impact on WirelessHART for rain, fog, snow and sand storms and has made the information available in the following document.
http://www2.emersonprocess.com/siteadmincenter/PM%20Central%20Web%20Documents/Weather-Wireless-Networks.pdf
Ideally, a WirelessHART transmitter should receive the radio signal from its neighbours with a strength greater than -75db. In the WirelessHART snap on tool, this is shown as the Received Signal Strength Indicator, or RSSI. With no interference, transmitting for 150m over clear air, there would be a loss of signal, termed "Free Space Loss", of 84db. Given the transmitted power of 10db and some amplification in the receiving antenna, you get an expected RSSI of approximately -72db for the proposed application. Anything above -75db is good, above -80db is usable and I have seen systems ticking along happily at even lower signal strength. At -72db the weather could have a 5 to 10db impact and it would likely go unnoticed.
To summarise the document referred to above, the impact of weather over 150m would be as follows:
It stands that with losses less than 1db, for the proposed application, with an RSSI estimated to be -72db, none of these weather phenomena will disrupt the WirelessHART network.
Craig Abbott - Wireless Specialist (South Eastern Australia)