1. SENSOR INSTALLATION

a. Temperature Sensor(s)

1. The temperature sensors are resistance temperature detector (RTD) devices. These sensors are composed of calibrated-resistance platinum wire coils mounted on supporting substrate. The elements are factory calibrated to offer 100 ohms resistance at 70°F over a range of 0 to 100°F. The outputs are linear with an accuracy of 1 percent over the range of the sensor.
   
   
   
   
2. The sensors are to be installed in the room air handler zone return air path. The location should be such as to sample the average return air temperature, allowing no mixing with the incoming makeup air or other supply air flow.
   
   
   
   
3. The sensors are to be installed through pre-drilled holes in the sheet metal ducting. The sensor elements are to face the incoming airflow. The sensor support will be attached to the sensor wiring junction box. The space between the rear face of the junction box and the sheet metal duct will be gasketed with flexible caulking strips (Permagum or equal) to minimize outside air aspiration.

b. Relative Humidity Sensor(s)

1. Relative humidity sensors sense ambient absolute humidity (grains of moisture) as well as ambient drybulb temperature (via an adjacent RTD element). The absolute humidity affects the resistance of a semiconducting polymer substrate.
   
   
   
   The sensor is powered by an incoming 4 to 20 milliamp DC current loop provided by the connected input/output device. It includes an amplifying and processing circuit that compares the sensor resistance change and corresponding sensed temperature. The two input values are utilized to compute a corresponding relative humidity value from 0 to 100 percent. The device then creates a linear change in resistance ranging from 4 milliamps at 0 percent RH to 20 milliamps at 100 percent RH.
   
   
   
   
2. The sensors are to be installed in the room air handler zone return air path. The location should be such as to sample the average return air temperature, allowing no mixing with the incoming makeup air or other supply air flow.
   
   
   
   
3. The sensors are to be installed through pre-drilled holes in the sheet metal ducting. The sensor elements are to face the incoming airflow. The sensor support will be attached to the sensor wiring junction box. The space between the rear face of the junction box and the sheet metal duct will be gasketed with flexible caulking strips (Permagum or equal) to minimize outside air aspiration.

c. Pressure Transducers

1. Pressure transducers are strain-gauge devices that utilize the motion of a diaphragm to impinge on a thin-film resistance-change device via a mechanical movement. The spring tension of the movement is set to correspond to a fixed range of pressure. Room pressure transducers are scaled from 0 to 0.2-inch water gauge differential pressure measured across the flexible diaphragm barrier. This measurement is converted to a linear, 4 to 20 milliamp output across the desired scale.
   
   
   
   
2. The pressure transducers are installed in a remote location (typically inside a control cabinet) and are connected to the sampled zone via 1/4-inch polyethylene tubing. The tubing is connected using standard pneumatic fittings, including brass barbed fittings and compression fittings. Note that compression fittings must have internal ferrules to prevent crushing when the outer ferrules are forced onto the tubing with the compression nuts.
   
   
   
   
3. The pressure transmitting tubing must enter the sampled zone through the ceiling membrane. A bulkhead-type fitting attached to a rigid plate is typically used to create a fixed mounting for the tubing connection.
   
   
   
   
4. The tubing routing must be free of kinks and any obstructions. The 1/4-inch diameter tube will transmit accurate values to 30 feet distant from the sensed zone. Distances beyond 30 feet require the installation of 3/8-inch diameter tubing to compensate for pressure losses due to tube wall friction.
   
   
   
   
5. The transducers have two available ports for tubing connection, the high and low sides. Each is connected to the corresponding side of the flexible diaphragm. If the control system design is to measure differential room pressure to ambient then the high side is connected to the sensed zone; the low side is left open to the interior of the control cabinet. Note that the control cabinet must be vented to the ambient to accurately sense the zone pressure.

2. SENSOR WIRING

1. The sensors are to be wired utilizing shielded instrumentation cable. The cable will have an outer PVC jacket with twin #22 gauge insulated conductors. The twin conductors will be wrapped in a spiral wrapping of aluminum foil. A braided, tinned copper wire conductor is intertwined with the two conductors to ensure positive contact with the foil shields. The shielding functions as an antenna to collect surrounding electromagnetic radiation. Without the shielding it is possible to induce current in the sensor wires, possibly degrading the control signals.
   
   
   
   
2. The sensor cables are to be run in a suitable path from the sensors to the control points of connection. The cables should be routed with no sharp bends or kinks. The cable should be inspected to make sure that there is no damage to the outer PVC jacket. Note that cabling should be routed away from any sources of electrical fields, including motors, transformers or power supplies.
   
   
   
   
3. The shielding wrapping must be grounded at one end of the cable only. This grounding connection allows stray electrical fields to be collected and drained off to earth. Grounding both ends of a cable will defeat the purpose of the shield and can allow extraneous electrical fields to induce current in the control conductors. The ground connection is typically made at the control cabinet end of the cable run.
   
   
   
   
4. Wire terminal connections should be secure. Braided wire must have an approved ring crimp-type connector attached before attachment to a connecting lug. Solid conductors must be bent into a ring shape and attached to the terminal post with washers above and below the wire loop. Straight terminal block connections should be made with sufficient torque on the attachment points to secure the wire in place.

3. SENSOR CALIBRATION VERIFICATION

a. RTD sensors are calibrated at the factory using NIST-traceable instruments. Sensors that do not display linear outputs are rejected. To ensure that an installed sensor is performing accurately, its output should be verified after installation and controls configuration. The following protocol assumes that the control system is configured to receive, process, and re-transmit the incoming RTD temperature sensor data.


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1. Secure an NIST-traceable thermometer and install it in the airstream adjacent to the RTD to be verified.
   
   
   
   
2. Allow both sensors to stabilize for a minimum of 2 minutes.
   
   
   
   
3. Verify and Log Both Readings
   
   Note: There may be a difference between the two values. An offset may be required to adjust the RTD sensed value to match the standard value. The control system will have an offset feature. Note the total negative or positive offset on the Controls Calibration and Verification Form. Offset adjustments are expected, as they compensate for any fixed resistance values attributable to the wiring or wiring connections.
   
   
   
   
4. After logging the initial values adjust the controller setpoint to a setting at least 1 degree F above the initial value. Allow both sensors to stabilize a second time. Log the two values.
   
   
   
   
5. Perform the above task over a minimum of 5 controller settings. Typically the adjustments should be made with two setpoints below and two setpoints above the initial value.
   
   
   
   
6. Review the logged data. The two values should correspond over the linear span of 4°F.
   
   
   
   
7. Note any differentials between readings of the sensed values. If the accuracy of the RTD sensor is published as ±1 percent of span then the maximum differential between the two values should be less than 0.5°F.
   
   
   
   
8. Log and tabulate all data on the Controls Calibration and Verification Form (below).

b. Relative humidity sensor accuracy is verified in a similar fashion to the RTD sensors above. A NIST-traceable thermohygrometer or psychrometer is required to accurately measure local relative humidity. Observe the initial readings after stabilization. Insert and log the necessary offset.

1. As with the above RTD protocol, the installed sensor accuracy must be verified over a 5-point range with incremental adjustments of a minimum 1 percent RH.
   
   
   
   
2. A minimum of 5 minutes stabilization time is required between changes in setpoint.
   
   
   
   
3. As in 3.a.7. above, if the accuracy of the RH sensor is published at ±2 percent of span, then the maximum differential between the two values should be less than 1 percent RH.
   
   
   
   
4. Note that while RH sensors can be field-calibrated, this practice is not recommended. An obviously malfunctioning unit should be returned to the manufacturer for recalibration.

c. Pressure transducer accuracy is verified in a similar fashion to the RH sensor. A tee fitting is installed in the connecting tubing and an NIST-traceable manometer is utilized to obtain comparison data. The verification protocol will include sampling and logging data over a 5-point span of pressure control settings. Minimum 0.02 inch water gauge adjustments are necessary to compare the installed and traceable instruments.

SAMPLE FORM



CONTROLS CALIBRATION AND VERIFICATION FORM



NOTE: USE A SEPARATE SHEET FOR EACH SENSOR OR TRANSMITTER

1. Project Name__________________________________________



Project Address__________________________________________



Project City/State/Country_______________________________



2. Date of Test__________________________________________



3. Technician____________________________________________



4. Sensor Type___________________________________________



5. Sensor Location_______________________________________



6. Sensor Manufacturer___________________________________



Date of Manufacture (If Known)___________________________



7. Verification Instrument_______________________________



8. Date of Last Calibration______________________________



9. Method of Connection__________________________________



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10. Initial Readings(Use Suitable Scale)



Installed Sensor_________________________________________



Verification Sensor______________________________________



Required Controlled Offset_______________________________

11. Readings


Installed Sensor    Verification Sensor



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12. Accuracy Verified by Technician Using the Above Procedure?



YES___________ NO____________



13. Document Checked and Verified by:



Name_________________________________________



Title________________________________________



Date_________________________________________

John Potter is national sales manager for American Cleanroom Systems. He is a 16-year veteran of the cleanroom industry and a member of the CleanRooms Editorial Advisory Board. Mr. Potter is also a technical consultant to the television and motion picture industry.