Introduction
Powerful samples have a very high CV. The highest (nongaseous) is Kerosene at 48Kj/g. All the fuels are not far behind and fat is 37Kj/g. Note that Dynamite is not powerful but fast burning. The vessel and apparatus can handle powerful samples from a safety aspect (Not dynamite!) but in order to maintain the instruments accuracy, the temperature rise of the sample should be within limits of the calibration rise.
The calibration rise of 0.5g Benzoic Acid is approximately 12 °C. So, the sample rise should be approximately the same.
The rise limits defaults are:
| 104 | LOW RISE LIMIT | 8 °C |
| 105 | HIGH RISE LIMIT | 16 °C |
These limits are 50% down and 33% up from the calibration rise of 12 °C.
The limits are un-equally spaced because the heat losses increase more when going up.
The Rise limits are parameters and can be changed from the keyboard.
The temperature rise is proportional to:
- The mass of the sample
- The energy in the sample (to be measured)
Burning Powerful Samples
The sample mass should be adjusted so that it produces a temperature rise which is approx. the same as the calibration rise. If the rise limits are violated then the CAL3K issues a warning. The warning can be ignored! Violating the high-rise limit should be avoided because it requires longer cooling time.
Here are the sample masses for calibration with 0.5g Benzoic Acid and the limits:
| Expected CV (Kj/g) | Expected CV(Kcal/g) | Std Mass(12°C) | High Mass(16°C) | 50 | 11.9 | 0.26 | 0.36 |
|---|---|---|---|
| 45 | 10.7 | 0.44 | 0.6 |
| 40 | 9.6 | 0.33 | 0.44 |
| 35 | 8.4 | 0.38 | 0.51 |
| 30 | 7.2 | 0.44 | 0.59 |
| 26.5 (BA) | 6.4 | 0.5 | 0.67 |
The above table assumes a standard calibration with 0.5g Benzoic Acid which yields approx. 12 °C rise. When a high CV sample is expected then the sample mass should be reduced according to the above table.
Conclusion
When the CAL3K issues a Rise limit warning then adjust the mass accordingly. The adjustment is done to operate at the best accuracy. In most application the expected CV range is known before a determination.
