I have an oil tank with a Sensor Systems Watchman level monitor. It transmits the oil level to a display in the kitchen. All was well for the last two years. A couple of day's ago the display went from 2 to r, with a flashing light. This is, according to the instructions, the display going into listening mode (i.e no signal from the transmitter). There was no indication of a low transmitter battery, but the remedy is to replace the battery at the transmitter. Now the battery, or Powertube, is an engineer's delight. It is a length of 15mm copper tube with a s/s screwed insert at one end with the electrical connections. At the other end is a permanently pressed-on copper cap. Inside is a plastic sleeve holding four Duracell Procell AAA batteries costing around £1.20 in total. The Powertube is £25+ and you have to throw the old one away. So I took off the Powertube and checked it with my amateur's multimeter. Voltage 5.75, and current (meter set to 10A) 0.16. I then knocked off the end cap and took out the batteries, covered in silicon grease. I tested the batteries out of the tube all in line and the voltage was 5.25 and the current was 3.6 (I assume this is amps). I bought a set of Duracell Ultra batteries and fitted them in the sleeve. On the end of the tube I fitted a 15mm comp cap (and the little spring that preloads the batteries). Voltage is now 6.33, but current 0.18A (sorry, can't remember what the current was out of the tube, but it was far, far more than this). So why does the current fall when the batteries are in the tube? Does this matter? (It must use milliamps in use). I refitted the Powertube and the display worked, for an hour or so, then it reverted to the irritating r. I took off the tube, checked it, all OK, refitted it, display worked. For another hour or so. Any clues why the thing isn't working? What more can it need than volts and some current? What else could be causing this problem? The sender is inside the tank bund, clean and dry and undamaged.