In coating pharmaceutical dosage forms such as tablets, control of the temperature of the product bed is very important. Overheating can harm the product, and underheating can result in poor quality of the coating due, for example, to inadequate drying, or require substantially longer coating times.
In coating, the temperature of the product bed in a coating pan is typically controlled by monitoring an exhaust temperature and using the measured temperature value to control various coating units. It is well known that the measured exhaust temperature can vary widely depending on the position of the exhaust temperature probe relative to the coating pan, the design of the coater, and the sizes of the pipes that are routed to the exhaust ducts. Erroneous temperature measurement can occur especially when the portion of the piping leading to the temperature probe is long or of a large diameter, because, in either case, the large area of the piping can dissipate a significant amount of heat. The local environment can also have an effect on temperature measurements at the exhaust port because less heat is dissipated through the pipe on a warm day than on a cooler day. Because of the effects of heat dissipation, the use of exhaust temperature as an indication of coating conditions is not sufficiently reliable. Moreover, when an exhaust temperature probe is used, it is difficult to use temperature information derived from the operation of one coater to predict conditions within a different coater. A significant amount of experimentation is needed in order to correlate temperature measurements with coating conditions in any coater.
To avoid the problems with exhaust temperature measurement, some coating pans have temperature probes installed on an arm that holds the spray nozzle inside the coating pan. Although an arm-mounted temperature probe is much better than an exhaust temperature probe for process control, the probe does not adequately measure the product bed temperature. Because the probe is suspended in the air, it measures the temperature of the incoming air, but is not able to measure directly how the incoming air is affecting the tablets being coated. As in the case of an exhaust temperature probe, it is difficult to use temperature information derived from an arm mounted probe in one coater to predict conditions within a different coater.
Another method of measuring product bed temperature is to use an infrared thermometer. An infrared thermometer typically provides very accurate measurements of product bed temperature. However, because an infrared thermometer is affected by conducted heat, it cannot be installed permanently in the interior of a coating pan. Therefore, when an infrared thermometer is used, a door to the coating pan must be opened, and opening of the door results in a drop in the air temperature that can cool the product bed and adversely affect the coating operation. Furthermore, the temperature as measured by an infrared thermometer is an average temperature over an area, and does not necessarily correspond to the actual temperature of an individual tablet.
There has been a need for an accurate process for product bed temperature measurement that does not have the drawbacks of an exhaust temperature probe, a spray arm-mounted probe, or an infrared thermometer.
Although only a coating process has been mentioned so far, similar problems arise in measuring temperature in other processes such as drying, freeze drying, and production of low dosage pharmaceuticals by liquid dispensing technology (LDT).