Band Math to convert from linear to db [Snappy]

I have performed radiometric calibration and lee speckle filtering on a Sentinel-1 SAR data. Now i want to use Band Math with a threshold value of 2.18 Γ— 10βˆ’2 𝑑𝐡 mask the water surface in order to distinguish from the non-water surface. How should i solve the problem?
@marpet @ABraun @Ciaran_Evans @kedziorm

first you right-click on the Sigma0 product and select β€œlinear to/from dB”
the output product is the log-scaled raster.

You can then apply the threshold in the band maths by simply using a condition

Sigma0db > x

where x is your value. But according to your example, 2.18 Γ— 10βˆ’2 is 0.0218. Are you sure this does not refer to the linear-scaled data?