Why choose a waterless calorimeter?
Most traditional bomb calorimeters make use of water to measure the temperature of the sample combustion within the calorimeter, but DDS Calorimeters have a different approach to this. We've removed the water component from the calorimeter, and here's why...
Traditionally water is used in adiabatic and isothermal combustion calorimeters. It is used as a heat sink, as a transfer medium, or as a transport medium.
In short : A calorimeter will combust a weighted sample in a steel cylinder filled with oxygen and measures the resulting temperature increase. From the temperature increase it calculates the calorific value. Somewhere in the process the apparatus must minimize/prevent or calculate the temperature losses to the environment.
Traditionally the steel cylinder was placed in a water bucket and the temperature sensor was placed in the water together with a stirrer. Then the bucket is placed in an apparatus which was designed to minimize the heat loss (adiabatic) or make it constant (isothermal). The water in the bucket acts as a transfer medium : it washes around the steel cylinder and transfers the heat to the sensor. After a determination the water acts as a transport medium : it is discarded, or recycled for cooling.
The water bucket method has one big advantage : It is simple. It also has a big disadvantage : it requires an exact weight/volume measurement. We, at DDS, have replaced the water with an aluminium sleeve pressed over the steel cylinder and we have placed the temperature sensors in a cavity between the steel cylinder and the sleeve. No water! No stirrer! The outside of the vessel is now equivalent to the outside of the bucket. This leads to the following considerations :
|Water is a very poor heat conductor. It is approximately 350 times worse than aluminium. The stirrer is increasing the conductivity to the temperature sensor, but it adds energy to the bucket.||0||1|
|The water quantity in the bucket must be measured precisely.||0||1|
|The heat capacity of water is approximately 4 times higher than that of aluminium. This property and the large water volume results in approximately 4 times less temperature increase per released energy||0||1|
|Once a determination is done the warm water can be discarded easily.||1||0|
|If a water cooler is used then the water is recycled.||0||1|
|A large engineering effort is required to place the temperature measuring electronics into a cavity between the stainless steel cylinder and the aluminium sleeve.||1||0|
|The cost of a water apparatus consists of a bucket, stirrer, and water quantity/weight measuring compared to an aluminium sleeve is excessive.||0||1|
|The water has one advantage : it equalizes the temperature (gradients). To minimize this effect we have placed 4 temperature sensors around the steel cylinder.||1||0|
|Determination speed : To heat a large water mass takes time||0||1|
We, at dds, use this method for the last 20 years, because it is better and faster and uncomplicated. The vessel machining is done by CNC equipment. The temperature sensors and the electronics are vacuum encapsulated as part of the complete vessel. Simple! The electronics contain a microprocessor for digitizing the temperature and since they are part of the vessel construction unparalleled precision and accuracy is achieved. The resolution is 0.000001C (1PPM).
Simple, and beautiful!