**Summary**

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.

**Requirements**

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.

**Note 1:**

If you did not or have lost the loading chart please contact Humboldt support at support@humboldtmfg.com and include the serial number and size of sample you are testing with.

**Note 2:**

Each loading chart is machine specific and is for a specific size sample.

**Solution**

**Loading Chart**

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

**Note 3:**

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.

**Example**

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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