Extech instruments tm20 tm25 waterproof wall mountable temperature indicators extech tm20 the extech instruments tm20 is a waterproof portable wall mountable temperature indicator this indicator has been designed and manufactured for measuring the temperature of air liquid paste or semi solid materials the tm20 uses a standard temperature probethe tm20 comes with a removable foot and magnetic back it can easily be used on your desktop or mounted on any metal surface this device is