TSI Model 8640 SUREFLOW Face Velocity Controller

Introduction

The Model 8640 User's Guide describes how to operate, calibrate, maintain, and troubleshoot the Model 8640 Fume Hood Face Velocity Controller. Please read this guide thoroughly before using the Model 8640.

This guide assumes that the Model 8640 has been properly installed. Refer to the Installation and Checkout Instructions if there is any question as to whether it has been installed properly.

The User's Guide has been divided into the following chapters to simplify its use:

• Chapter 1 describes how to operate the Model 8640.
• Chapter 2 describes how to calibrate the Model 8640.
• Chapter 3 describes how to maintain the Model 8640.
• Chapter 4 describes how to troubleshoot the Model 8640.
• Appendix A lists the Model 8640 specifications.
• Appendix B describes the Model 8640 theory of operation.
• Appendix C describes how the Model 8640 is wired.
• Appendix D describes how to measure face velocity.

References to the Model 8640 front panel are highlighted in this guide. For example: The ALARM light is red. A black four-pointed star [Note] is used throughout this guide to call attention to important notes or comments.

Chapter 1: Operation

The Model 8640 Fume Hood Face Velocity Controller is easy to use, with simple and straightforward operation. All necessary information about face velocity is displayed on the controller's front panel. The system is also easily calibrated via adjustment screws accessed through holes in the front panel.

From an operations standpoint, the Model 8640 functions on two levels:

1. The controller's front panel provides indication of the face velocity via an analog meter, indication of the selected set point with a six-position selector switch, and indication of low face velocity with an ALARM light.
2. The six-position selector switch on the controller's front panel also permits the operator to select the face velocity control set point.

Specific details about the Model 8640 front panel display and controls are described below.

Front Panel Display

Analog Meter

An analog meter indicates the face velocity. The meter has a range of 0 to 200 ft/min (0 to 1.0 m/s).

Set Point Indication

The six-position face velocity set point selector switch indicates the face velocity set point selected.

ALARM Light

The red ALARM light indicates low face velocity. The ALARM light turns on when the face velocity falls approximately 25 ft/min (0.125 m/s) or more below the selected set point. The alarm is activated after approximately a 10-second delay. When the face velocity increases to within 25 ft/min (0.125 m/s) of the selected set point, the ALARM light turns off after a delay of several seconds. The ALARM light also turns on whenever the set point selector switch is set to the EMERGENCY position.

Front Panel Controls

A six-position selector switch is provided for operator selection of the following face velocity control set points:

*English units are in feet per minute (ft/min), and metric units are in meters per second (m/s). The values for metric controllers are listed in parenthesis.

Figure 1: Model 8640 Front Panel Display and Controls: Depicts the front panel of the controller, showing the analog meter, set point selector switch, calibration screws (ZERO and CAL), and the ALARM indicator light.

Chapter 2: Calibration

The Model 8640 Fume Hood Face Velocity Control System must be calibrated to accurately indicate and maintain face velocity. The calibration is a simple, two-step procedure:

1. Set the velocity calibration at zero flow.
2. Set the velocity calibration span.

The following items are also needed to calibrate the Model 8640:

• Masking Tape
• Jeweller's straight-slot screwdriver
• Portable Air Velocity Meter (e.g., TSI VELOCICALC or VELOCICHECK)

This chapter contains all the information needed to calibrate the Model 8640.

Calibration Procedure

Set the velocity calibration zero

Place a piece of tape over the velocity sensor opening to seal off the air flow through the sensor. The tape must block all air flow from passing through the sensor.

[Note]: The velocity sensor should be mounted on the inner sidewall of the hood four inches into the fume hood and four inches below the sash when the sash is full open.

Wait two minutes for the sensor to stabilize. Set the velocity calibration zero by turning the ZERO adjustment screw clockwise or counter-clockwise until the indicator needle on the analog meter is aligned with the 0 ft/min (0 m/s) line. (Turn the ZERO adjustment screw clockwise to move the needle to the right and counter-clockwise to move the needle to the left.)

Remove the tape from the sensor.

Set the velocity calibration span

Set the set point selector switch to the velocity that the hood will normally control at (100 ft/min is typical). Set the baffles to the normal operating position.

Wait 30 seconds for the sensor to stabilize.

Open the sash to the normal operating height or 60 percent of the full open position.

Use a portable air velocity meter to measure the average face velocity passing into the hood through the sash opening. Refer to Appendix D for the proper method of measuring average face velocity.

Compare the face velocity indicated on the Model 8640 to the face velocity measured by the portable air velocity meter.

Set the velocity calibration span by turning the CAL adjustment screw clockwise or counter-clockwise until the face velocity measured by the portable air velocity meter is the same as the face velocity indicated by the needle on the analog meter. Turn the CAL adjustment screw clockwise to increase the face velocity or counter-clockwise to decrease the face velocity.

Allow several seconds for the control system to respond to changes and stabilize when making CAL adjustments. Each full turn of the CAL adjustment screw will increase or decrease the face velocity by approximately five ft/min.

The face velocity indicated on the analog display is not the same as the actual average face velocity unless the controller has been calibrated.

For example: If the set point selector switch is set to 100 ft/min, the needle on the analog display will indicate 100 ft/min +/-10 percent, while the actual average face velocity may be only 70 ft/min. In this case, one would turn the CAL adjustment screw clockwise to increase the actual average face velocity to 100 ft/min.

Verify that the Model 8640 controller maintains a constant face velocity by changing the sash position and then comparing the measured face velocity (portable air velocity meter) to the indicated face velocity (8640).

Figure 2: Model 8640 Calibration Adjustment Screw Locations: Shows the front panel with calibration screws highlighted and indicates the direction to turn them to adjust the analog meter needle.

Chapter 3: Maintenance and Repair

The Model 8640 Fume Hood Face Velocity Controller is almost maintenance-free. Periodic inspection of system components, calibration, and occasional velocity sensor cleaning are all that are needed to ensure the Model 8640 is operating at optimum performance.

System Component Inspection

Periodically inspect the various system components for proper performance and signs of excessive wear.

Calibration

The Model 8640 should be calibrated annually. Refer to Chapter Two for calibration instructions.

Velocity Sensor Cleaning

Significant build-up of contaminants, including dust and dirt, on the velocity sensor can degrade its performance and cause it to go out of calibration. It is recommended that the velocity sensor be periodically inspected for accumulation of contaminants. The frequency of these inspections depends on the quality of the room air being drawn across the sensor. Simply put, if the air in the room is dirty, the sensor will require more frequent inspection and cleaning.

CAUTION: Turn off power to the Model 8640 prior to cleaning the velocity sensor.

CAUTION: Do not use compressed air to clean the velocity sensor.

Visually inspect the velocity sensor. The air flow orifice should be free of obstructions. The small, cylindrical, ceramic sensor protruding from the orifice wall should be white and free of accumulated debris.

Accumulations of dust or dirt can be removed with a soft-bristled brush (such as an artist's brush), or by blowing the orifice clean with canned compressed air. Water, alcohol, acetone, or trichlorethane may be used as a solvent to remove other contaminants if required.

WARNING: Do not attempt to scrape contaminants from the velocity sensor. The velocity sensor is quite durable; however, scraping may cause mechanical damage and possibly break the sensor. Mechanical damage due to scraping voids the velocity sensor warranty.

WARNING: Use extreme care when cleaning the velocity sensor. The ceramic sensor may break if excessive pressure is applied or if the cleaning apparatus abruptly impacts the sensor.

CAUTION: If a solvent is used to clean the velocity sensor, ensure that it is completely dry before performing calibration.

Replacement Parts

All components of the Model 8640 are field replaceable. Contact TSI Industrial Test Instruments Group at (651) 490-2888 or your nearest TSI Field Representative for replacement part pricing and delivery.

Chapter 4: Troubleshooting

The fume hood exhaust system must be operating properly before troubleshooting the Model 8640. A properly operating exhaust system will typically provide enough static pressure to draw at least 100 ft/min (0.050 m/s) through the face with the sash fully open and the control damper fully open.

If the velocity sensor or actuator is suspected to be defective, follow the tests given at the end of this chapter.

To test velocity sensor:

Disconnect the velocity sensor from the sensor cable. Depress the latching piece on the connector to pull it out. Use an ohm meter to measure the resistance between the sensor pins indicated in Figure 3. The resistance between pins 3 and 4 should be between 10 and 12.5 ohms. The resistance between pins 2 and 4 should be between 120 and 140 ohms.

Figure 3: Model 8640 Velocity Sensor Pinouts: Shows a diagram of the velocity sensor connector with pin numbers and a description of resistance tests for troubleshooting.

To test the electric actuator (p/n = 800118):

Move the fume hood sash to a 50-percent open position. Turn the velocity controller to the minimum position. The damper should be closed somewhat. Measure face velocity through the sash. Turn the velocity controller to emergency. Measure velocity through sash. It should go up. If velocity goes down, check jumper wire installation.

If the actuator has no movement, remove screws to separate the controller from the fume hood. Turn the setpoint selector switch so that it is between two positions (any two positions). The voltage across pins 5 and 6 should slowly change from approximately 10 volts to approximately -0.4 volts. Turn the velocity setpoint switch to emergency. The voltage across pins 5 and 6 should be approximately 10 volts. If these voltages are correct, check actuator/damper for proper mounting.

To test the electric to pneumatic interface (e/p) (p/n = 800119):

Move the fume hood sash to a 50-percent open position. Turn the velocity controller to the minimum position. The damper should be closed somewhat. Measure face velocity through the sash. Turn the velocity controller to emergency. Measure velocity through sash. It should go up. If velocity goes down, check jumper wire installation.

If the actuator has no movement, remove screws to separate the controller from the fume hood. Turn the setpoint selector switch so that it is between two positions (any two positions). The voltage across pins 5 and 6 should slowly change from approximately 10 volts to approximately -0.4 volts. Turn the velocity setpoint switch to emergency. The voltage across pins 5 and 6 should be approximately 10 volts. If these voltages are correct, check actuator/damper for proper mounting.

Appendix A: Specifications

System Performance Specifications

• Response Time: 5 seconds nominal for 50 ft/min step change
• Temperature Compensated Range: 55 to 95° F

Model 8640 SUREFLOW Components

Face Velocity Controller

• Face Velocity Display Range: 0 to 200 ft/min

Outputs

• Voltage Output: 24 VAC, 4 watts max
• Alarm Contacts: SPST (N.C.) 0.5 amps at 10 watts max, 24 VAC max. Contacts close in alarm conditions and loss of power.

Operating Conditions

• Operating Temperature: 40 to 120° F
• Input Power: 24 VAC, 5 watts max

Dimensions

• Dimensions: 5" x 3.125" x 1.5"
• Weight: 0.7 lb

Velocity Sensor

• Dimensions (DxH): 2.75 in. x 1.25 in.
• Weight: 0.2 lb
• Power Dissipation: 0.09 watts at 0 ft/min., 0.14 watts at 100 ft/min.
• Cable Specifications: 4-conductor, 22 AWG, 4-pin polarized at both ends with a standard length of 5 ft, a maximum length of 10 ft.

Transformer

• Primary: 120 VAC
• Secondary: 24 VAC
• Rating: 20 VA
• Dimensions (HWD): 2.0 in. x 2.25 in. x 1.5 in. (mounts to 4 in. x 4 in. duplex box)
• Weight: 1.3 lb.
• UL/CSA Approved: Yes
• Cable Specifications: 2-conductor, 18 AWG with standard length of 25 ft, maximum length of 100 ft.

Damper/Actuator

• Standard Materials: 16-gauge galvanized, Type 304 SS or Type 316 SS
• Optional Coating: Plasite No. 7122 finish
• Types of Actuators: Electric or pneumatic
• Input Power*: Electric: 24 vac, 7.5 watts max; Pneumatic: 24 VAC, 3 watts max
• Time for 90° Rotation: 5 sec.

*Actuator power only. No controller.

Appendix B: Theory of Operation

Introduction to the Model 8640 Fume Hood Face Velocity Controller

The Model 8640 Face Velocity Controller maintains a constant hood face velocity regardless of sash position. This improves operator safety by ensuring there is always adequate face velocity into the hood. Controlling the face velocity also reduces the quantity of conditioned air exhausted.

The Model 8640 controls a fume hood by measuring the face velocity and modulating the exhaust volume in order to maintain a constant face velocity. The face velocity is measured by a sensor mounted in the hood's inner sidewall.

How it Works

The exhaust system in an operating fume hood produces a low static pressure which causes air to be drawn through the hood.

The Model 8640 measures the velocity of this airflow with a thermal anemometer sensor mounted in the sidewall of the hood. Since they are driven by the same differential pressure, the velocity across the face of the hood (face velocity) is the same as that across the velocity sensor. The controller compares the measured velocity to the selected set point and varies the control output to maintain that set point. The controller modulates either an electrically-actuated damper, a pneumatically-actuated damper, or the exhaust fan speed via an AC-adjustable frequency drive.

Velocity Sensor

The Model 8640 Fume Hood Face Velocity Controller uses a thermal anemometer sensor to measure the face velocity. The sidewall-mounted velocity sensor assembly actually consists of two sensors: an air velocity sensor and a temperature compensation sensor. The velocity sensor is heated to 35° Celsius above the ambient air temperature by the controller electronics. The temperature compensation sensor corrects for changes in the ambient air temperature and forces the velocity sensor to remain at a constant temperature above the ambient air temperature (constant overheat).

The velocity sensor and temperature sensor form two legs of a Wheatstone bridge as shown in Figure 4. The bridge circuit forces the voltages at points A and B to be equal. Air flowing past the velocity sensor cools the sensor, reducing its electrical resistance, which causes the voltage at point A to decrease. An operational amplifier instantly responds to this change by increasing the power at the top of the bridge until the voltage at point A increases and is equal to the voltage at point B. As more air flows past the sensor, more power is required to maintain a balanced bridge. Thus, the voltage required at the top of the bridge to maintain a constant overheat is directly related to the velocity of the air flowing past the sensor. This is the principal of operation of all constant-temperature thermal anemometers.

Figure 4: Model 8640 Bridge Circuit: A schematic diagram illustrating the Wheatstone bridge configuration used by the velocity sensor, showing the relationship between sensors, an operational amplifier, and the bridge output.

Velocity Controller

The velocity controller reads the voltage at the top of the bridge circuit and compares this value to the selected set point value. If the face velocity is outside of the control deadband, the velocity controller outputs a control signal to the final control element (damper or AC-adjustable frequency drive) to bring the face velocity back to set point.

Appendix C: Wiring Diagrams

This appendix provides wiring diagrams for different configurations of the Model 8640 system.

Figure 5: Model 8640 Electric Actuator Wiring Diagram

This diagram illustrates the electrical connections between the velocity sensor, velocity controller, transformer, and an electric actuator. It details the sensor cable, controller output cable, and transformer cable connections.

Figure 6: Model 8640 Pneumatic Actuator Wiring Diagram

This diagram shows the wiring for a system using a pneumatic actuator. It includes connections for the velocity sensor, velocity controller, transformer, and an electric/pneumatic interface connected to the pneumatic actuator.

Figure 7: Model 8640 Variable Frequency Drive Wiring Diagram

This diagram outlines the wiring for connecting the Model 8640 to a variable frequency drive. It shows the connections from the velocity sensor, velocity controller, and transformer to the variable frequency drive input.

Appendix D: Face Velocity Measurements

Prior to calibrating a fume hood face velocity controller, the average face velocity of the fume hood needs to be established. ASHRAE standard 110-1985, Method of Testing Performance of Laboratory Fume Hoods, section 6.3 Face Velocity Measurements, describes how to establish an average face velocity. The following procedure complies with the requirements of ASHRAE 110-1985.

TSI fume hood face velocity controllers should be calibrated with the fume hood sash open 18 inches. The average face velocity should also be established at this position, so that the two correlate.

A portable air velocity meter is required to make the face velocity measurements. TSI recommends the Model 8350 VELOCICALC air velocity meter due to its ability to measure low velocities accurately and for its averaging capabilities, which greatly simplify the calculation of average face velocity.

Figure 8 shows a typical 5-foot fume hood that needs to have the average face velocity established. The first step is to move the sash so there is 18 inches between the bottom of the sash and the top of the air foil.

Figure 8: Face Velocity Measurement Locations

This diagram illustrates a typical 5-foot fume hood with 12 measurement points marked for determining average face velocity. It indicates the sash position (18 inches from the top of the air foil) and provides dimensions for the measurement grid. If your fume hood is larger or smaller, measurements should be added or deleted in 12-inch increments. The 3, 9, or 15-inch positions remain unchanged.

It is important that air velocity measurements are made in the same plane in which the sash travels. An air velocity meter positioned away from this plane will not measure the true face velocity.

Start the air velocity measurements in the upper left corner and traverse the width of the hood from left to right. Record each air velocity measurement until all 3 rows of measurements are complete. If you are using the TSI VELOCICALC, store each reading into the instrument's memory. The average face velocity is then automatically computed by pressing the "average" key. If your air velocity meter does not store and average, the equation below can be used to calculate the average face velocity:

Average face velocity (ft/min) =
(Reading 1 + Reading 2 + ... + Reading (n)) / Total number of readings (n)

Example: A five-foot fume hood requires 12 readings.

The face velocities (ft/min) were as follows:

Average face velocity = (98+99+99+100+95+92+95+96+98+97+96+99) / 12 = 1164 / 12 = 97 ft/min.

The average face velocity is now the value used when calibrating a fume hood face velocity controller.