def report(self, name, ok, msg=None, deltat=20):
        '''report a sensor error'''
        r = self.reports[name]
        if time.time() < r.last_report + deltat:
            r.ok = ok
            return
        r.last_report = time.time()
        if ok and not r.ok:
            self.say("%s OK" % name)
        r.ok = ok
        if not r.ok:
            self.say(msg)
report a sensor error