How does one measure moisture distribution in a circuit board?
To determine the distribution of moisture in a circuit board, you need to use a pin-type meter with insulated pins (non-insulated pins can be used, but insulated pins make this easier).
Firstly, remove the pins of the meter and gently push them into the material to be tested. Read the "shell" or outer layer of the board. After the reading is taken, push the pins in 1/16" increments, removing a new reading at any time. By recording moisture readings at different depths of the board, you can get a good indication of the moisture distribution in the board.
Why not use a pinless meter? Because pinless meters read the humidity of the entire area and the depth of individual digital scans, they cannot isolate the depth at which moisture is present.
Does temperature affect the accuracy of meter readings?
The short answer is yes, but usually to a very small degree in pin meters. As the temperature of the wood increases, its resistance decreases, resulting in an increase in the indicated %MC for that sample. For field conditions, there is no need to correct the temperature reading results if the temperature of the wood sample being tested is between 50°F (10°C) and 90°F (32.2°C).
When measuring wood materials beyond these temperature values and the meter itself does not have built-in temperature corrections, you can use the temperature correction chart to find the corrections (Delmhorst provides these corrections for each hygrometer).
Needleless meter readings do not measure resistance, so unless the sample is completely frozen, their readings are unaffected by the temperature of the wood.
How do I correct readings for different types of wood?
With both pin and pinless moisture meters, the species of wood will affect the readings you get. For pin-type meters, the difference in readings is due to the fact that different species of wood have different inherent resistance characteristics. For pinless meters, the specific gravity (SG) of the wood will affect the readings.
Each hygrometer is usually calibrated using a specific species of wood, such as Douglas fir or oak, which makes the meter accurate for that type of wood, but less accurate for other woods.
There are usually two ways to calibrate the readings for a given species of wood:
Use the species correction chart provided by the meter manufacturer.
Use a meter with built-in species correction.
