Several types of Humboldt testing machines control the normal load on a sample through a manually controlled regulator. Among these are the HM-5755, HM-2560, and the HM-2432. In order to correctly calculate the correct applied load to the test sample the following steps must be taken.
You should have received a loading chart with the equipment. The loading chart will help you calculate the psi or kpa value to dial into the equipment to achieve the desired vertical pressure.
If you did not or have lost the loading chart please contact Humboldt support at email@example.com and include the serial number and size of sample you are testing with.
Each loading chart is machine specific and is for a specific size sample.
On the loading chart you will have a Low Load Chart and a High Load Chart with each chart containing a formula. You will use the formula to compute the pressure to dial into the equipment.
In either formula (low or high load), you will be solving for x which will be a pressure.
For example, if formula is y = 17.708 x - 6.5348 then to solve for x, rearrange the formula as follows:
y = 17.708 x - 6.5348 --> (y + 6.5348) / 17.708 = x
The formula does not have to be rearranged but it does make it easier to compute the pressure.
Determine Load Required
The desired vertical loading value (ksf, tsf, kg/cm^3, kpa, etc.) will need to be converted into the X axis units of the loading chart. For example if my loading chart X axis units are in psi and my desired vertical pressure is tsf, I will convert my tsf to psi.
The area of the sample will need to be converted to inches^2 if the X axis is in psi and cm^2 if the X axis is in kpa.
The force will need to be calculated by multiplying the converted pressure and converted area.
The force is the y value. Insert the force into the y value of the formula and solve for x. The result is the psi or kpa value that will be dialed into the equipment.
Vertical pressure is 2 tsf with a sample diameter of 2.42 inches. I have a loading chart for a 2.42 inch specimen and will be using the high range formula. The loading chart formula for High Load 2.42 inch diameter High Load is y = 17.708 x - 6.5348.
- Rearranged the loading chart formula is x = (y + 6.5348) / 17.708 where x is in psi and y is in lbs. It is not necessary to rearrange the formula, we are solving for x.
- 2 tsf converted to psi = 27.78 psi.
- My area of a 2.42 inch sample is 4.6 inches^2.
- Determine the lbs of the psi by multiplying out the area 27.78 psi * 4.6 inches^2 = 127.79 lbs
- This poundage can now be substituted into the rearranged formula in step 1 which will yield a psi value of (127.79 + 6.5348) / 17.708 = y = 7.6 psi.
- This 7.6 psi value will be dialed into the equipment and the applied vertical pressure to the sample will be 2 tsf.
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