Warming up

Warming up is a passive process, consisting of stopping the PTR and letting the LN2 tank run empty. The heat from the outside environment will then warm up the instrument. As a lot of effort has been put into thermal insulation and the mass of the cold instrument is quite high, heating to room temperature is a very slow process.

A temperature and pressure profile made while the instrument was warming up is shown in figure 3. As a rough estimate, one should wait at least a week from the LN2 is used up until it is safe to open the dewar. The actual time required may depend on the ambient air temperature and ventilation. Before opening, the lowest temperature inside must be above the dew point of the ambient air. A crude estimate of the dew point is:

The formula will give a dew point somewhat too high, so you should be quite safe when using it.

Several methods of accelerating the rate of heating exist, but only one have been tried out. Possible methods could be:

• Blowing dry air into the LN2 tank - But our experience is that supposedly dry air still forms ice in the tank.
• Electrical heaters could be installed inside - The heaters must not outgass when warm, and the necessarily thick electrical connectors might create a heat conduction problem.
• N2 gas could be let into the dewar - But the gas must be very clean and the heating rate may be hard to control.
• It has been tried to aim a spotlight at the entrance window. A 60W bulb was placed about 10cm in front of the window. The acceleration of heating is modest: No strong temperature differences are created, as all three temperature sensors showed practically identical temperatures during the heating. Four days after turning on the lamp, or five days after the last LN2 evaporated, the internal temperature was +22C and the pressure 0.6mBar. The T,P log for the heating is shown in figure 4. To avoid damage to the detector from rapid heating, the procedure below should be followed:

  1) Check that the shutter is closed.

  2) Move the aperture wheel to a low transmittance position, e.g. pinhole.

  3) Disconnect power to motors.

  4) Start heating.