HHV experiment with bomb calorimeter

[Will]

This question refers to a lab that we did for determining the HHV value of bio diesel fuel. We are asked if the experiment was done using a higher initial temperature than the bomb was calibrated for, how would this affect the HHV determined? The room temperature is assumed to be the same in both cases. Our logic says that if the bomb is not 100% insulated, then more heat would be lost to the surroundings, and thus a lower value for HHV would result. Is this correct? Looking at some results online, it seems that a higher value was found with higher initial temperatures. So I need some input from the experts here; it would be greatly appreciated.

 

[FredGarvin]

Think about it this way...you are looking at how much energy has to go into the fuel to find it's HHV. If the bomb is not insulted, you will lose energy to the surroundings which means that it will appear that you have to add more to the fuel to overcome that loss. As an example, let's say it takes 10 units of energy in a perfectly insulated machine. Now, there is a slight loss of 2 units to the atmosphere. You now have to provide 12 units of energy to get the same result. So, with the imperfections, it would tend to show an increase in HHV.

 

[Will]

So, with the imperfections, it would tend to show an increase in HHV.

Why higher? Wouldn't more heat lost to the system mean less change in temperature in the bomb, thus a lower calculated HHV? Also how does this relate to changing the initial temperature of the bomb?

 

[FredGarvin]

You do not want the bomb to be insulated. It's the fact that heat transfers from the bomb to the surrounding water that gives you the temperature rise to calculate the HHV. You do not want any heat transfer from the bucket to the surroundings. In fact, the more precise machines will have a water jacket around the bucket with heated water to try to match the rate of temperature rise of the water in the bucket in an attempt to make the entire bucket adiabatic.

I am trying to remember the whole process of doing this. We should look at the equations you use to calculate the HHV. I haven't read the ASTM spec on this in quite a while. Let me do some research back into the calcs before we go any further.