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How To Measure Low CV Samples

Introduction

Low Calorific materials can be measured in our Calorimeters. They typically are flame retardant material that can be found the airline, vehicle and building industries. Low calorific results can also be from materials that have a high water content, like fresh wood chips. Other low Calorific Value tests measure the useable energy contained in a substance like low grade coal and industrial byproducts. In todays environmental climate change, companies are looking at their normally disposable byproducts to utilize them for their burning energy in a safe and environmentally friendly way.

The low CV materials have one thing in common: They burn reluctantly or not at all.

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The CAL3K measures the temperature RISE when the sample is burned and relating the rise to a calibration to derive the energy from it. The temperature resolution is 0.000’001 C but the uncertainty is 0.000’010. This is the noise floor. To achieve a reasonable accuracy the sample rise must be 1000 times higher (0.01C). This is the error or uncertainty of the instrument. The error relates to approx. 0.01Kj/g (or 0.002Kcal/g, or 2.5Cal/g).

The instrument can measure a CV rise value as low as 0.1Mj/g with 10% uncertainty or 1Kj/g with 1%. That is not a problem with low CV values. The problem comes with igniting (and burning) the low CV sample. Once the sample burns a sufficient temperature rise must be achieved.

Solutions | DDS Calorimeters

We discount hardware, weighing and preparation errors. In all instances the result is lower than expected. This is so because the CAL3K can lose energy but cannot gain it. Three possible scenarios can cause a lower result.

Lowering the Temperature rise limit
The minimum default temperature rise warning limit is 8C. It can be adjusted down but then requires a recalibration. Very time consuming. Not recommended.
Increasing the low CV sample mass to get more temperature rise
The default max mass limit is 1.5g and can be increases easily. But the sample may not burn and rather smoulder. The sample must fit into the crucible. A possible solution. Not recommended.
Igniting the low CV sample with a higher CV substance
This method is called SPIKING. For details see: Spiking in the operational manual. Without getting in to the details of the method a sample in burned together with a known substance (e.g. Benzoic Acid) and after the burning the cocktail the known substance CV is subtracted from the result yielding the low CV. This solves two problems:
  • A sufficient temperature rise is obtained
  • The sample was ignited by the spike material
  • Place the sample in to a gelatin capsule. This is called ‘spiking’ and the CAL3K can handle it.

Summary

The spiking method should be used whenever the sample substance is low and reluctant to burn. It introduces some operational complexity (see: SPIKING) and a neglectable determination error. The 0.5g Benzoic Acid tablets are ideal for spiking. Then adjust the unknown sample mass so that a 2-4C temperature rise results.

The following table shows the approx. sample mass for a 3C rise and anticipated low CV value:

Low 3 °C Rise
CV (Kj/g) Mass(g)
2 1.5
3 1.1
4 0.8
5 0.7
6 0.55
7 0.5
8 0.4
10 0.3
12 0.3
16 0.2
20 0.15

The 3C rise was chosen because the calibration is valid up to 16C rise and the 0.5g Benzoic Acid spike raises the temperature to approx. 12C which leaves 4C head space. The instrument will issue a warning when the 16C rise is exceeded. The warning can be ignored or the limit can be raised or the sample mass can be reduced.

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