Pressure Sensor

(Order Code PS-DIN)

 

The Pressure Sensor can be used to monitor pressure changes in gas-law experiments in chemistry and physics, such as Boyle' s law (pressure vs. volume) and Gay-Lussac' s law (pressure vs. absolute temperature). You can determine the value of absolute zero on the Celsius temperature scale. The Pressure Sensor can also be used to monitor reaction rates as a gas is produced in a chemical reaction. Vapor pressure of various liquids and solutions can be monitored and you can show the relationship between vapor pressure and absolute temperature. Biology teachers can use the Pressure Sensor to monitor changes in the partial pressure of oxygen or carbon dioxide gases in an enclosed atmosphere.

 

The Pressure Sensor can be used with any of the following laboratory interfaces available from Vernier Software:

 

• MultiPurpose Lab Interface (MPLI)

• Voltage Input Unit (VIU)

• Universal Lab Interface

• Serial Box Interface

• Texas Instruments Calculator-Based Laboratory (CBL) System

 

How the Pressure Sensor Works

The heart of this circuit is the SenSym SCX1OOANC. It has a membrane which flexes as pressure changes. This sensor is set up for absolute pressure measurement, so one side of the membrane is a vacuum. The sensor produces an output voltage which varies in a linear way with absolute pressure. It includes special circuitry to minimize errors caused by changes in temperature. The 100 in the part number indicates that the range of this sensor is 0 to 100 psi. The "A" in the part number indicates that it measures absolute pressure.

 

Pressure Units Pressure can be measured in many different units. We quote values here in several of the units shown below. Some equivalent values for 1 atmosphere are:

 

1 atmosphere = 760 mm of Hg

        = 101.325 kilopascals

        = 29.92 in. of Hg (at 0°C)

        = 1.013 bar

        = 14.696 psi

 

We provide an amplifier circuit that conditions the signal from the SCXI00 sensor. With this circuit, the output voltage from the Pressure Sensor will be linear with respect to pressure with 0.00 volts corresponding to a complete vacuum, 0.454 volts corresponding to 14.7 psi (one atmosphere), and 3.09 volts corresponding to 100 psi. The output voltage increases 0.031 volts for each psi increase. This allows the sensor to cover the range of 0 to 100 psi (6.8 atm).

 

There is a plastic tube on the Pressure Sensor running from a port inside the box to a three-way valve on the outside of the box. The two openings (or stems) on the three-way valve have a small threaded end called a suer lock. You may attach plastic or rubber tubing to one of the ports using the supplied adapter (already mounted on extra tubing). The adapter accepts tubing with an inside diameter of 0.125 inches (3.2 mm), and commonly available 3/16-inch plastic tubing can be connected to it. You can also attach the 20-ml plastic syringe included with the Pressure Sensor to this stem. The other port (pointing upward in the figures below) of the three-way valve opens to the atmosphere and can serve as a pressure release. As you set up your experiments, you can always return the pressure to atmospheric pressure by opening this side stem. When the blue control (or "off") handle is aligned with one of the stems, it closes off this stem. Note: Since the side stem also has a luer lock, it is possible to connect the syringe or the adapter with tubing in this side position.

 

To perform pressure-volume (Boyle's law) experiments, connect the plastic syringe to one of the valve stems with a gentle 1/2 turn. Close the stem that leads into the Pressure Sensor box using the blue control handle, and set the syringe to any volume setting you like. Then close the open stem (see the figure below). The pressure inside the syringe is now equal to atmospheric pressure at the volume you selected. You are now set to collect pressure-volume data.

 

 

The white, threaded adapter end of the long piece of plastic tubing can also be connected to one of the valve stems with a full turn.  The other end can be connected to an experimental apparatus. For example, when  doing a pressure-temperature experiment (shown in the figure), the tubing can be connected to a piece of glass tubing that is inserted into a one-hole rubber stopper.  This stopper can in turn be inserted into an Erlenmeyer flask to provide a constant-volume gas sample. Pressure-temperature data (Gay-Lussac's law) can be collected using a temperature probe or thermometer, along with the Pressure Sensor.

The SCX 100ANC pressure sensor is fairly durable, but it is designed only for use with non corrosive gases such as air, helium, nitrogen, etc. Do not get it wet.

 

Vernier Software also manufactures a Barometer (order code BAR-DIN) that covers a pressure range of 24 to 32 inches of mercury (0.8 to 1.05 atmospheres) absolute pressure. It is designed primarily for monitoring atmospheric pressure as part of weather studies. It has a much higher sensitivity over its limited pressure range.

 

Calibration

There are several ways to handle calibration with this sensor. In some cases, the circuit can be used without calibration. Often you are interested in only relative pressures, and you can use the raw voltage output of the Pressure Sensor. With the Pressure Sensor as shipped, voltage is proportional to pressure. In most cases, you can simply load the appropriate calibration file that was included on your laboratory interfacing program from Vernier Software. This calibration will be satisfactory for most experiments using the Pressure Sensor.

 

For the very best results, you can calibrate this unit yourself using one of the methods described below. You will need to know the local air pressure, and/or have a reliable pressure measurement instrument.

 

 

 

Two Point Calibration

This is the calibration procedure we use with all of our sensors. You simply do a two-point calibration as described in the program manual using two different pressures, both measured with another pressure gauge. One pressure is easy to get in this case— simply use atmospheric pressure as measured by a barometer. For the other pressure, you can:

 

• Use the syringe provided with the Pressure Sensor to produce a pressure very near zero. We have found that this method works very well. Connect the syringe to the Pressure Sensor and open the unused stem of the three-way valve by aligning the blue valve control with the stem that leads into the Pressure Sensor box. Push the plunger on the syringe all the way in and close the unused stem leading to the atmosphere by aligning the blue valve control with it. To produce near zero pressure, pull the plunger out to the 20-cc position. If your syringe and valve have a tight seal, the pressure will be <0.01 atm, and you can call it zero for calibration.

 

• Apply pressure with a pump, measuring it at the same time with a pressure gauge.

Whatever method you use, the input should be named "Pressure," and appropriate units should be entered as part of the calibration.

 

One-Point Calibration

This calibration method uses the predetermined sensitivity of the Pressure Sensor to simulate a two-point calibration. It cannot he used if you have changed the gain potentiometer inside the box. Follow the Instructions on the screen and in your program manual to begin calibration. The input should be named "Pressure" and appropriate units should be entered. For the first of the two calibration points, open the unused stem of the three-way valve to expose the Pressure Sensor port to atmospheric pressure. Do this by aligning the blue valve control with the stem leading to the inside of the Pressure Sensor. Read a laboratory barometer and enter its pressure reading as the known pressure. Also, note the voltage reading.

 

For the second point, close the unused stem by aligning the blue valve control with it. Blow on the tubing connected to the three-way valve of the Pressure Sensor, so that the voltage reported increases. Now press <Return>, while noting the voltage. Take this as the second calibration point and calculate the change in voltage between the two calibration points. Calculate what the pressure was using the known sensitivity of the Pressure Sensor. Use the equation:

 

Pressure (in. psi) = first pressure reading + voltage change /0.031 volts/psi.

 

If you use different pressure units, make the appropriate conversion. This calculation gives you the pressure at the second calibration point. It uses the known sensitivity of the unit.

 

Adjusting the Pressure Sensor

There is an offset potentiometer that you can use to increase or decrease the output signal. This potentiometer can be adjusted without opening the box. Place a small jeweler's screwdriver through the hole in the end of the box and you will notice a slotted screw that can be turned. This is a l 5-turn potentiometer, so the adjustment can be made very gradually. One way to adjust the calibration of the Pressure Sensor is to adjust this potentiometer. There is also a gain control potentiometer inside the box. Adjusting this potentiometer requires opening the box, and is not recommended for normal use. When this unit is shipped, we set the gain potentiometer in the fully clockwise position. This sets the gain at its lowest value. This potentiometer can be turned counter-clockwise to increase the sensitivity. The maximum sensitivity is about 290 mV per psi. If you adjust the potentiometers, test the response of the unit by attaching a plastic tube to the input port, and blow and suck on it. Make sure that the voltage increases when you blow on the tube and decreases when you suck on the tube.

 

A complete schematic of the Pressure Sensor is available. Contact Vernier Software.

 

Specifications

• Sensing element: SenSym SCX100ANC

• Pressure range (as shipped): 0 to 6.8 atm (0 to 100 psi)

• Maximum pressure that the sensor can tolerate without permanent damage:

10.2 atm or 150 psi

• Sensitivity (as shipped, minimum) 454 mV/atm or 31 mV /psi

(with gain adjusted to maximum) 4.2 V/atm or 290 mV /psi

• Resolution (as shipped) using a 12-bit, 5 volt A/D converter (ULI II, Serial Box):

0.00275 atm

• Resolution (as shipped)using a 10-bit, 5 volt A/D converter (CBL, original ULI):

0.011 atm

• Combined linearity and hysteresis: typical to 1% full scale, maximum to 0.5% full scale

• Response time: 100 microseconds

 

Suggested Experiments

 

Boyle's Law (Pressure vs. Volume)

Boyle's law is a classic physics/chemistry concept that can be easily demonstrated using the Pressure Sensor. One easy way to do this is to use the plastic syringe included with the Pressure Sensor. Connect the plastic syringe to one of the valve stems with a gentle 1/2 turn. Close the stem that leads into the Pressure Sensor box using the blue control handle and set the syringe to any volume setting you like. Then close the open stem (see the figure on page 2). The pressure inside the syringe is now equal to atmospheric pressure at the volume you selected. You are now set to collect pressure-volume data. Take data as you change the volume. The syringe is marked in volume units (cc or ml). You can both increase and decrease the volume. Sample data collected with this sensor and the syringe are graphed below:

 

Sample Boyle's Law Graphs

 

Gay-Lussac's Law (Pressure vs. Absolute Temperature)

Gay-Lussac's law states that if the volume remains constant, the pressure of a container of a gas is directly proportional to its absolute temperature. The white, threaded adapter end of the long piece of plastic tubing can be connected to one of the valve stems with a full turn. The other end can be connected to a piece of glass tubing that is inserted into a one-hole rubber stopper. This stopper can in turn be inserted into an Erlenmeyer flask to provide a constant-volume gas sample. Pressure-temperature data can be collected using a temperature probe or thermometer, along with the Pressure Sensor. Place the flask in water baths of different temperatures. Take data on how the pressure changes with temperature changes. remember that all temperatures should be measured using the Kelvin temperature scale. Using the same apparatus, pressure and temperature data may be extrapolated to determine a Celsius temperature value for absolute zero.

 

Studying Chemical Reactions by Monitoring Pressure

Many chemical reactions produce gases that can cause a pressure increase in a sealed container. The pressure change can be used as a way of monitoring the rate of reaction. You can change concentrations or temperatures, or add catalysts, to see how the reaction rate changes. As a very simple example, the graph below was made using the Pressure Sensor to monitor {he reaction of Alka-Seltzers and water. We made a hole in the top of a plastic 35-mm film container. We then ran a tube from the Pressure Sensor through the hole and sealed it with silicone sealant. We then placed a piece of an Alka-Seltzer tablet and 10 ml of water in the container and sealed it. The graph below shows the pressure increase as the reaction takes place. Note the sudden drop in pressure when the lid of the container pops off.

 

Using the Pressure Sensor to Monitor the Rate of Reaction

 

Pressure in Liquids

 If you measure (he pressure al the end of a long plastic tube forced underwater, you can indirectly measure depth. Connect the tubing to the input port of the Pressure Sensor and then put the end of the tube under water. The pressure reading will increase 0.0965 atmospheres for every meter below the surface of the water. If you use different pressure units, you will need so make the appropriate conversion.

 

Note: if you measure depth in this way, the depth you are measuring is to the top of the air, which extends up the tube for a short distance If this measurement error bothers you, you can simply calibrate your depth measurement system when the end of the tube is al known depths and automatically correct for this.

 

Additional experiments are available in our Chemistry with Computers or Chemistry with CBL books. Contact Vernier Software for details.

 

 

Flinn QuickStartsTM

 

Pressure-Pressure Probe ()

 

The Connections:

1. Connect the Pressure Probe to Port 1 of the Serial Box Interface.

2. Connect the Serial Box Interface to the COM I or COM2 port of the computer using the cable provided. (Use the 9-pin to 25-pin adapter if necessary.)

3. Plug the Serial Box Interface into an electrical outlet using the AC adapter.

 

The Set-up/Customizing Data Logger:

1. Open Data Logger program; Click on Start, Program, Vernier, Logger Pro.

Note: If a message box reading “Warning! Cannot find the interface box” appears:

a) Check all connections,

b) Determine which port (COM1 or COM2) is accepting the cable from the Serial Box Interface, and

c) Click on the appropriate button (“COM1” or “COM2”).

A default graph call untitled- 1” will appear.

 

The right mouse button Is used only to access Data logger menus. Use the left mouse button for all other operations.

 

2. Under the “File” menu, choose “Open... ”. A new window will appear, listing the various experiment files for Data Logger.

 

3. From the list, double-click on  “Pressure-pressure Probes” choose “Pressure Probe” to open the file for the direct temperature measurements. You may need to scroll the list by clicking on the left mouse button and dragging the cursor across the list to find these files.

 

5. The “Setup” menu can be used to specify

Sensor  [Sensor setup, Calibration, and Details(units can be specified here)]

Data Collection [Mode, Sampling (Rate)]

and Interface details (Com port).

 

6. You do not need to customize the axes for the introductory activity. However, if you would like to customize the axis:

 

a) Double-click anywhere on the graph. This cases the “Axes” dialog box to appear.

 

b) Click and hold the cursor on the y-axis label (which currently reads

“Temp 1 & Temp 2”). Move the cursor to “Temperature 1” to collect data using Port 1 only. If you are using temperature probes in both ports, you do not have to do this.

 

c) To change the y-axis temperature range, simply Double-click inside the upper and/or lower limit value box and enter the desired value.

 

d) The x-axis should already be in units of “Time.” You may change the values in the upper and/or lower limit value boxes in the same manner as you did for the y-axis above.

 

 

You are now ready to collect data.  Simply click “Collect”  when you are ready to begin; click “Stop” to discontinue data collection.

 

Introductory Activity:  Calibration:

1. Open the “Pressure probe” file and load the calibration file, as described above.

2. Evacuate system with isoteniscope connected determine pressure inside system.

3. Place the Standard Temperature Probe in the water bath.

4. Click on ”Collect”.

5. Click “Stop” to discontinue data collection.

6. Heat a water bath to boiling!

7. Place the Standard Temperature Probe in the water bath.

8. Click on ”Collect”.

9. Use Calibration in the “Experiment” menu to calibrate the sensor using the water baths at 0 °C and 100 °C.