Determining Pressure for Devices Using Loading Charts

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 [e-mail] 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.

1. 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. 2 tsf converted to psi = 27.78 psi.
3. My area of a 2.42 inch sample is 4.6 inches^2.
4. Determine the lbs of the psi by multiplying out the area 27.78 psi * 4.6 inches^2 = 127.79 lbs
5. 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.
6. This 7.6 psi value will be dialed into the equipment and the applied vertical pressure to the sample will be 2 tsf.