After drilling out the initial holes in your substrate in preparation for the attachment of the grab bar on the wall, it is wise to gently probe into the hole with a short rigid wire to assess the amount of depth available, potential obstacles, and by listening and feeling - the presence of metal studs, ducts, pipes and etc. Important - stay away from electrical panels, outlets and etc. These can usually be identified on the opposite side of the wall prior to deciding on the location for the installation of grab bars.
I usually bend back the end of the metal wire and flatten it against the remaining portion of the wire which then allows me to assess the wall thickness as well. The information on wall thickness, depth and obstacles help me make a decision with regards to anchoring method and screw length.
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