The Spiking Method in Bomb Calorimetry

WHAT IS SPIKING IN CALORIMETRY?

What is spiking? If a sample does not ignite easily or not at all in a calorimeter, then the spiking method of ignition can be used to promote complete combustion of the sample. Only once the sample is completely burned can an accurate determination of the sample be ascertained. A combustion aid is added to the sample, like a Benzoic Acid Tablet, which will easily ignite, and in turn ignite the non-combustible sample. The heat produced by the combustion aid must be subtracted from the total energy release. The DDS Calorimeters range does this automatically. So, when do you need to use spiking?

TOTAL COMBUSTION

As a principle, when a sample is fired in a bomb calorimeter, you are looking for the total combustion of the sample for an accurate result. A simple visual inspection of the bomb inside and the crucible is in most cases sufficient. Getting to understand your sample, like coal, animal feed or ammunition, is key to discovering incomplete burning with a simple visual inspection. Should you find small solid bits of the sample still remaining in the bomb; then you possibly need to spike your sample.

NON-COMBUSTION

Non-combustion is probably the easiest spiking problem to solve. Should your sample not combust, or partly combust, after a determination, then you will need to spike it. This can be achieved in multiple ways. Certified Benzoic Acid tablets have a consistent burning property and are ideal for spiking. Depending on your sample, Technical Oil can also be used as a spike or combustion aid. If your sample grain is small enough, you could even use Gelatin Capsules. It really depends on your sample consistency and laboratory preference. We have successfully fired wood chip with a moisture content of over 75% using Benzoic Acid as well as Technical Oil.

RATE OF BURN

Some samples, like fine wood chips, burn extremely fast in a bomb calorimeter. So fast in fact that an explosion occurs, throwing out the rest of the sample from the crucible. So again you don't have total combustion, not because it can't burn. To solve this problem is to either compress the sample into a pellet or to use a Gelatin Capsule (3K-4-068). Creating a pellet of your sample will work in most cases to reduce the "Rate of Burn", but not always. In some instances you want to reduce the burning surface area of your sample, much like a fuse. The Gelatin Capsule created a fuse-like environment for your sample, because only one end of the capsule will ignite, causing your sample to burn evenly and slowly. Using a Gelatin Capsule is considered Spiking.

LIGHT SAMPLE

This is a light powder type sample. It is both light as a mass and also prone to become airborne with the slightest breeze. Just by filling the bomb calorimeter with oxygen, it will disperse the sample out of the crucible. The problem arises when the sample makes contact with the inside bomb walls as it will no longer combust. In this instance, you can either make a pellet out of the sample, depending on the bonding properties of the sample or use a Gelatin Capsule. A Gelatin Capsule tends to be more reliable as it will also regulate the burning speed of the sample as these types of samples tend to have a high burning speed.

VOLATILE SAMPLES

A volatile sample is typically a fuel that evaporates before your eyes. The problem is in the weight of the sample as it will reduce from the time of weighing to the time of firing. This is easily solved by using a crucible cover disc. The cover disc is a clear vinyl round disc, the same size as the top rim of the crucible. The cover disc (CAL2K-FLDS) is considered a spike as it will contribute to the energy of the sample. Stick the cover disc to the top of the crucible and then TARE the balance. Now, inject the fuel sample through a small hole into the crucible. This technique works well for all volatile samples, even for fuel samples such as diesel.

SMALL VOLUME AND LOW CV

In some instances, a small sample quantity with a weight less than 0.1g is only available. Our DDS Calorimeters are able to burn a sample with a mass of less than 0.1g. However, it becomes a challenge to fill the bottom of the crucible in such a way so that the firing cotton makes contact with the sample consistently. A Gelatin Capsule will allow the sample to appear bigger in the crucible and provide a better firing repeatability. A low calorific value (CV) can similarly be solved by using a Gelatin Capsule. Our DDS Calorimeters range is able to burn a low CV of around 1MJ and lower, but the problem comes from the low heat amount generated by the burning sample. To increase the amount of heat generated by the sample, either increase the sample weight, but not more than what the crucible can accommodate, or spike it with a gelatin capsule. A Gelatin Capsule will create more heat for the bomb to register with a relatively small sample weight.

EASY SPIKING

The spiking requires a detailed operating procedure in which the spiking mass is measured in addition to the sample mass. If this is too complex then the EASY SPIKING method can be used which relies on a constant spiking mass. The gelatin capsules and cover disc have a near constant mass and are ideal for the EASY SPIKING method.

The EASY SPIKING method is enabled once and then the CAL3K will subtract the temperature of the spike capsule with every sample. The weight of the capsule is not required. It is replaced with the temperature rise.

Here is the sequence of operations for the EASY SPIKING method.

Enable the Easy Spiking method
Tare the empty capsule together with the crucible
Fill the capsule with the sample material and weigh it.
Enter this weight as sample mass into the CAL3K
Prepare the vessel and place it into the CAL3K, close the lid.
After the determination the CAL3K will display the sample CV (without the spike!).

The above procedure is not as accurate as the normal spiking method but it is very convenient and easy to operate.

CALORIMETER SPIKE MASS

This is the mass of the substance to be used for spiking, i.e. the gelatin capsule or benzoic acid tablet. The procedure to follow is to place the crucible on the balance and press TARE. Then add the substance to be used for spiking to the crucible. This mass value is known as the "Spike Mass" and must be entered manually into the calorimeter. The sample is then added. This mass is the spike item's mass plus the sample's mass and must be entered in as the total mass. This can be read directly from the balance if a balance is connected to the calorimeter.

STEP BY STEP - HOW TO ENTER THE SPIKE VALUE

Place the empty crucible on the balance.
Press "TARE" to clear the weight of the crucible and get a 0.0000g reading.
Place one Benzoic Acid Tablet and take note of the new mass.
Enter the mass value into the calorimeter as a "spike mass".
Add the sample to the crucible and take note of the total mass.
Enter the total mass into the calorimeter as "sample mass".
Using the Up/Down Arrow (Navigation Keys) on the keyboard, go to the next item "Spike Value C".
Press "=" and enter the energy value of the Benzoic Acid Tablet that you are using. This can be found on each bottle of DDS Certified Benzoic Acid bottle of tablets, e.g. 26.454 (MJ/g). This value needs to be entered each time you do a determination.
Continue through the menu items, until you get to "Enter MASS, SID".
Now place the prepared crucible (holding the benzoic acid tablet with sample) in the lid assembly of the bomb vessel, close the vessel and fill it with the required 3000Kpa of oxygen.
Place the vessel inside the calorimeter and close the calorimeter's lid.
The calorimeter will now perform some standard checks and if all is okay, it will begin with the "Initial Phase", then "Fire", and finally the "Main" phase and then indicate that the determination has been completed.

Once the determination is complete, the result will display the calculated calorific value of the sample. The calorimeter will automatically take the end result, and subtract the spike energy and present you with the final result of your sample.

WHY IS BENZOIC ACID USED IN CALORIMETRY?

Benzoic Acid is a colourless crystalline solid and a simple carboxylic acid. The name is derived from gum benzoin, which was for a long time its only known source. Benzoic Acid occurs naturally in many plants and it serves as an intermediate in the biosynthesis of many secondary metabolites.

Although benzoic acid has many uses outside of bomb calorimetry, we will touch on the subject of its use in calorimetry. Benzoic acid is relatively non-toxic and occurs naturally in plants. Appreciable amounts of it has been found in most berries (around 0.05%) and is formed in apples after infection with a specific fungus.

Due to its constant value it is the ideal substance for calibration in bomb calorimeters. For this purpose we press the powdery substance into tablets of approximately 0.5 grams and 1.0 grams, with a calorific value of approximately 26.454KJ/g.

CAL3K CONSUMABLES AND ACCESSORIES

Crucible Cover Discs (CAL2K-FLDS)

DDS supplies Crucible Cover discs in packs of 10 sheets. These cover discs are used to cover the crucible during the firing of combustible substances like fuel and oil and can be used with any of our combustion calorimeter systems.

Gelatin Capsules (3K-4-068)

DDS supplies gelatin capsules in packs of 100 capsules. The gelatin capsule holds the sample in the crucible during firing to prevent splattering i.e. powder samples, sawdust etc. and can be used with any of our combustion calorimeter systems.

Certified Benzoic Acid Tablets (3K-4-084)

DDS supplies benzoic acid tablets in bottles of 50 x 0.5g tablets. 2 bottles are included with our installation kits when purchasing a complete system and can be used with any of our combustion calorimeter systems.

For more information on any of the above consumables visit our consumables page or contact your nearest agent for a free quote.

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