(4) Anode-cooling chart. Figure 3-17 illustrates a typical anode-cooling
chart. From the curve on this chart, you will note that the maximum HU permissible is
72,000. When this maximum has been reached, you must wait 6 minutes before this
same quantity of heat may be applied to the anode again. Exposures may be made
before the six-minute period has elapsed, but the total HU must not exceed 72,000. For
example, if four series of exposures are needed and each series generates 24,000 HU,
you would have to wait after the third series until the tube had cooled. To find the
shortest possible cooling time, consult the chart. You will see that cooling from 72,000
HU to 48,000 HU will require about 1 1/4 minutes.
NOTE:
To find how many exposures per minute are possible with a tube already
heated to capacity, divide the cooling rate per minute as the chart by the heat
units per exposure:
Cooling rate in HU per minute/HU per exposure = Exposures per minute
The cooling rate (CR) as shown for a full-wave rectified machine on the chart
(figure 3-17) is 6 minutes divided into 72,000 HU. Thus, the CR is 12,000 HU
per minute. If exposures producing 1,000 HU per exposure are to be made
with an already heated tube, divided 12,000 HU (cooling rate, or CR) by 1,000
HU (HU per exposure) to find how many evenly divided exposures can be
made per minute. Cooling rate (CR) here, as you will note on the chart,
means average CR since the rate of cooling is not the same at every
temperature.
MD0950
3-24