3
Roof-to-floor Extensometers
The roof-to-floor extensometer, used to measure conver-
gence, utilizes a cable-actuated position sensor (string
potentiometer) to measure movement (See Figure 2). This
instrument is typically installed in a mine opening where
standing support is present (See Figure 3). Its purpose is to
accurately measure the convergence between the roof and
floor of the mine during active mining or long-term entry
stability studies such as those involving bleeder entries
(Klemetti et al., 2018)
Installation of the roof-to-floor extensometer is usu-
ally accomplished by mounting a cable-actuated position
sensor to a standing support (See Figure 3). Although in
some circumstances, with the use of strong magnets, the
extensometer can be mounted between a roof bolt plate and
an anchor point on the mine floor or on metal CAN type
standing supports. The installation procedure of the roof-
to-floor extensometer at specific field sites is as follows:
• Locate a standing support for installation.
• Find a location on the standing support that is
plumb, so the potentiometer’s string can be run roof
to floor without any obstructions.
• Position the instrument at the top of the standing
support with the string and eyelet pointing down.
• Secure the instrument to the top of the standing
support using nails/screws through the mounting
bracket attached.
• Find a location near the bottom of the standing sup-
port to drive a nail or screw into for the correspond-
ing anchor point.
• Carefully pull the string out until it stops and then
let it retract a few inches. DO NOT pull hard or it
will damage the instrument. DO NOT let go and
allow the string to snap back into the instrument as
it can cause damage.
• Roughly determine the extra length of wire that will
be needed to connect it to the corresponding anchor
point at the bottom of the support.
• Thread the wire through the eyelet on the instru-
ment making a loop and securing it with a metal
wire crimp.
• Carefully pull the wire extension down towards the
bottom anchor point until it stops.
• Let the wire retract a few inches and make a loop
using a metal crimp at that location of the bottom
anchor point.
• Hook the wire over the corresponding anchor point
• Installation is complete and an initial zero reading
should be recorded.
Multipoint Roof Extensometers
The multipoint roof extensometer in current use by NIOSH
is a product of our own innovation (see Figure 4). Crafted
and assembled in-house by skilled technicians, this exten-
someter offers the flexibility to be adjusted and modified
as needed according to specific requirements. The enclo-
sure contains slide potentiometers with a range of 100 mm
that attach to varying anchor depths in the mine roof. The
predetermined anchor locations allow for the monitoring
Figure 2. Example of a string potentiometer
Figure 3. Complete extensometer installed on a can-type
support in conjunction with a standing support load cell
Roof-to-floor Extensometers
The roof-to-floor extensometer, used to measure conver-
gence, utilizes a cable-actuated position sensor (string
potentiometer) to measure movement (See Figure 2). This
instrument is typically installed in a mine opening where
standing support is present (See Figure 3). Its purpose is to
accurately measure the convergence between the roof and
floor of the mine during active mining or long-term entry
stability studies such as those involving bleeder entries
(Klemetti et al., 2018)
Installation of the roof-to-floor extensometer is usu-
ally accomplished by mounting a cable-actuated position
sensor to a standing support (See Figure 3). Although in
some circumstances, with the use of strong magnets, the
extensometer can be mounted between a roof bolt plate and
an anchor point on the mine floor or on metal CAN type
standing supports. The installation procedure of the roof-
to-floor extensometer at specific field sites is as follows:
• Locate a standing support for installation.
• Find a location on the standing support that is
plumb, so the potentiometer’s string can be run roof
to floor without any obstructions.
• Position the instrument at the top of the standing
support with the string and eyelet pointing down.
• Secure the instrument to the top of the standing
support using nails/screws through the mounting
bracket attached.
• Find a location near the bottom of the standing sup-
port to drive a nail or screw into for the correspond-
ing anchor point.
• Carefully pull the string out until it stops and then
let it retract a few inches. DO NOT pull hard or it
will damage the instrument. DO NOT let go and
allow the string to snap back into the instrument as
it can cause damage.
• Roughly determine the extra length of wire that will
be needed to connect it to the corresponding anchor
point at the bottom of the support.
• Thread the wire through the eyelet on the instru-
ment making a loop and securing it with a metal
wire crimp.
• Carefully pull the wire extension down towards the
bottom anchor point until it stops.
• Let the wire retract a few inches and make a loop
using a metal crimp at that location of the bottom
anchor point.
• Hook the wire over the corresponding anchor point
• Installation is complete and an initial zero reading
should be recorded.
Multipoint Roof Extensometers
The multipoint roof extensometer in current use by NIOSH
is a product of our own innovation (see Figure 4). Crafted
and assembled in-house by skilled technicians, this exten-
someter offers the flexibility to be adjusted and modified
as needed according to specific requirements. The enclo-
sure contains slide potentiometers with a range of 100 mm
that attach to varying anchor depths in the mine roof. The
predetermined anchor locations allow for the monitoring
Figure 2. Example of a string potentiometer
Figure 3. Complete extensometer installed on a can-type
support in conjunction with a standing support load cell