VSP2 Pressure Transducer Calibration & Maintenance Guide

Calibration

The calibration system within the VSP2 control box requires the user to adjust a zero and gain reading based on a separate calibrated source.  In order to calibrate the VSP2 pressure transducer, perform the following steps:

1. Secure the pressure transducers that will be used for the experiment (with a diaphragm that will cover the pressure range of interest) to the calibration tree.  The calibration tree should be connected to a high-pressure inert gas (i.e. nitrogen) source, a vent line, and a calibrated source.  The FAI calibration kit comes with a pressure gauge that can be separately calibrated.  This gauge acts as the “calibrated source” that the transducers are calibrated against.  See Figure 1 for the FAI calibration tree with pressure gauge and transducers installed. Note; do not place the pressure transducers on the Super Magnetic Stirrer when it is enabled.

2. Within the VSP2 software, select the button that says “Calibrate Temperature and Pressure” to open up the calibration window (see Figure 2).

3. The full output of the transducers is 10V. In order for the VSP2 software to correlate the measured voltage from the deflection of the diaphragm to a pressure, the user must input the full range of the diaphragm divided by the total voltage (10V).  For example, if a 2,000 psi transducer is used, the VSP2 user should input 200 psi/V inside the “Pressure Sensitivities” window for each transducer that is used.  Figure 3 includes a table indicating the pressure range based on the number imprinted on the diaphragm used.  Figure 4 shows where the pressure range divided by the total voltage needs to be specified.

4. With the pressure transducers at atmospheric pressure, adjust the “zero” on the VSP2 potentiometer screw for each transducer (note, P1 measures the sample pressure and P2 typically measures the primary containment vessel pressure for pressure balancing), shown in Figure 5, until the display on the VSP2 screen (Figure 6) reads 0 psig. The manufacturer specification for accuracy of the pressure transducers is ±0.5% of the full scale for pressure ranges less than 2,000 psig.  For diaphragms 2,000 psi and above, the stated accuracy is ±1% of the full scale.  Note, adjusting the screws clockwise will increase the pressure reading on the VSP2 control box, and adjusting the screws counterclockwise will decrease the pressure reading.  Be careful not to apply pressure when adjusting the potentiometer screws as they can be pushed past the control box surface.

Figure 5: VSP2 potentiometer screws where the user can adjust, the “zero” and “gain” for any pressure transducers that are being used to match the pressure reading from the gauge in Figure 1

Figure 6: The display from the VSP2 software.

5. adjust the “gain”, pressurize the transducers to near the peak pressure of interest (e.g. potentially 800 psig for a 2,000 psi transducer or 600 psig of an 800 psi transducer; these are recommended ranges, but it depends on the experiment and what pressure one expects). Be careful not to overpressurize the diaphragms above their measurement range, because this can permanently damage them.  Adjust the “gain” potentiometer screws until the pressure readings (Figure 6) matches the reading on the pressure gauge (Figure 1).

6. Depressurize the transducers back to 0 psig, check the reading, and adjust the “zero” potentiometer until the VSP2 screen reads zero.  Repeat with the “gain”.  This may take several iterations before the transducers have been fully calibrated.

7. When the calibration is complete, it is suggested to incrementally increase the pressure from 0 psig to the peak tested pressure and check that the pressure of all transducers are within the accuracy range for each increment.

MAINTENANCE

General Cleaning

The most important thing for properly maintaining the VSP2 pressure transducers is to promptly clean them after every experiment.  The simplest way to clean them is to remove the bleed screw and flush a suitable solvent through the channel.  To remove the bleed screw, use a small hex tool, and carefully twist to loosen it. Figure 7 shows where the bleed screw is located.

• Be careful to not lose the orange washer inside the bleed screw port.

• Flush the channel using a non-residue solvent from the negative side of the transducer, and out through the positive side to flush out any remnants of the sample.

• Utilizing low pressure compressed air or other inert gas, dry out the channel.

• Replace the bleed screw and tighten.

Thorough Cleaning

Periodically it is important to completely disassemble and clean the pressure transducer.  This can be for a few reasons:

A. It is expected that the transducer was exposed to a chemical that warrants a thorough cleaning (such as a sticky or viscous difficult to clean material or something that is toxic or severely corrosive).

B. The pressure range of the diaphragm needs to be changed.

C. The pressure transducer is not performing as it is expected, for example:

• The pressure increments from full scale to zero are non-linear and deviate causing excessive hysteresis.

• The zero reading shifts after being calibrated.

• The outputs are not within the stated accuracy.

• The outputs are not balanced despite after making multiple iterations from full scale to zero.

Disassembly

1. Carefully secure the pressure transducer within a vise (see Figure 7, note we have brass on the vise faces to prevent damage to the transducer body).

2. Using a hex or star key wrench (depending on the screws used), loosen the bolts, by slowly loosing and alternating the bolts incrementally until they are removed.

3. Gently pull open one side of the transducer. Be careful when opening the transducer because thin gauge wires are secured near the top of the transducer and can be snapped off. Now, the transducer is in a position that it can be cleaned with a suitable solvent, or parts can be replaced as needed. 

Figure 8: A vise clamp holding a transducer, so the user can unscrew the bolts to remove the diaphragms or the O‑rings.

There are few key features of the transducer to note:

1. Diaphragms

The adaptability of the transducer is due to its economical feature allowing the user to replace the sensing diaphragms without purchasing multiple transducer bodies. This allows for numerous pressure ranges to be looked at accurately.  For example, closed cell VSP2 experiments can be performed to high pressures by implementing a high pressure diaphragm.  To run experiments in lower pressure ranges (like in an open cell orientation) with improved accuracy, lower range diaphragms can be implemented.  A benefit of this transducer and diaphragm type is they can withstand extreme shock, vibrations, and generally reasonable laboratory abuse from the stainless steel construction. Further, the stainless steel construction is ideal for use with the VSP2 due to the protection against corrosion and oxidation.  Some notes on the transducer diaphragms (shown in Figure 9):

• Check for signs of warping or damage to the diaphragms, and periodically replace the them. They can become damaged over time from overpressure, or long exposure to numerous chemicals without prompt cleanup. 
• Make sure to utilize the correct diaphragm that correlates with the desired testing range (see Figure 3). The accuracy of the transducers is based on the full scale, and thus the lower the pressure range, the more accurate the pressure readings will be.
• Tip: label what diaphragm is being used on the outside of transducer, so it is clear what the range is without disassembling it.

Figure 9: Each diaphragm has a dash number and this case, the diaphragm being cleaned has a dash number of 60, indicating that this diaphragm has a maximum pressure limit of 1250 psig. The diaphragms are compressed by the coiled pads inside of the transducer bodies where the applied pressure compresses internal coils.

2. O-rings

Besides the diaphragm, there is also an o-ring on either side of the body of the transducers between the diaphragm and transducer body (Figure 10):

• Replace the o-rings when they swell-up, appear damaged, or have been inside the transducer of a long period of time, they do not have to be replaced every time the transducer is opened.

• Ensure the o-rings are compressed securely in their grooves.

• Avoid pinching the o-rings (this can damage them).

• It is suggested to use Teflon o-rings instead of Viton-A, which are the default for when purchasing a pressure transducer. Teflon can resist corrosives better than Viton-A.