The Eberline Model RO-2 is a portable air ion chamber instrument designed for detecting beta (β), gamma (γ), and x-ray radiation. It offers four linear ranges for measuring dose rates of x-ray and γ radiation. The ion chamber is vented to atmospheric pressure and engineered for a flat energy response in the x-ray region. The RO-2 is calibrated to γ radiation (¹³⁷Cs). A single rotary switch manages instrument functions, including turning it off, checking battery status, setting the zero, and selecting the operating range.

Function Description

The RO-2 operates based on an ion chamber with battery potential between its inside wall and a center electrode. When air within the chamber ionizes due to radiation, a minute current flows through the chamber. This current causes the operational amplifier's minus input to become slightly positive, resulting in a negative swing of the amplifier output. This output is connected to feedback elements via a divider circuit. The feedback elements, also connected to the amplifier input and ion chamber, conduct away the current generated in the chamber. The meter, tied to the amplifier's output, indicates proportionally to the amplifier output voltage. Range changes are achieved by selecting different feedback elements and points on the feedback divider.

The ion chamber, located inside the case below the meter, comprises the lower two inches of a three-inch diameter chamber assembly. The remaining volume houses electronic components, including the amplifier A1. The chamber wall is made of one-sixteenth inch phenolic, and the face is a one-mil aluminized Mylar® layer, with another one-mil Mylar® layer glued to the case, totaling two mils thick. The active air volume is 208 cm³.

The chamber's interior has a conductive graphite aquadag coating maintained at a positive battery voltage, as is the inside of the Mylar® face. The chamber's exterior is coated with aquadag and grounded for electrostatic shielding. The center electrode, coated with conductive aquadag, is supported on the guarded feedthrough's center conductor at the chamber's top. The guarded feedthrough's guard ring is positioned on insulators between the center conductor and the positive outer ring, preventing leakage from the chamber voltage to the center electrode. The guard ring and center electrode are maintained at the same potential (ground potential) to prevent leakage from the guard to the electrode.

The chamber is sealed, except for a small hole in the top wall that vents to the electronic section behind the chamber. This section, in turn, vents to the main instrument case through a plastic hose connected to a drying box filled with silica gel desiccant. This setup ensures that any air drawn into the chamber (due to atmospheric pressure changes, temperature changes, or transport) passes over the desiccant, maintaining dry air to prevent leakage.

The positive voltage for the inside chamber wall is supplied by a separate battery (BT3), which exclusively powers the chamber. Its current drain is minimal, ensuring an indefinitely long shelf life, and it is not checked by the battery checking circuits.

The operational amplifier A1, a single TO-5 size transistor package, is located in the top of the chamber assembly. It features dual MOS FET inputs with a very high input impedance (10¹⁵ Ω). The noninverting input (pin 3) is grounded, while the other input (pin 2) connects to the ion chamber center electrode and feedback elements. The amplifier's output (pin 6) feeds both the meter and feedback circuits.

When ion chamber current flows towards amplifier input pin 2, the input becomes slightly more positive, causing the output to become more negative. This negative output draws current from one of the feedback elements to precisely match the ion chamber's contribution. A higher ion chamber current necessitates a higher voltage across the feedback element to draw off the current, making the amplifier voltage output proportional to the radiation rate in the chamber.

The meter is driven directly from amplifier output pin 6, through dropping resistor R8 and one of four calibrating resistors (R9 through R12). The meter reaches full scale on all ranges at approximately the same amplifier voltage output. These calibrating resistors account for manufacturing tolerances and enable calibration at various elevations and temperatures. Function switch S1, section C, selects the appropriate calibration resistor. Capacitor C4 sets the time constant for the meter circuit.

The RO-2's operating range depends on the feedback circuitry between the amplifier output and input (pin 2), which connects to the ion chamber. Feedback resistor R1 is always in the circuit. For the 500 and 5000 mR/h ranges, S3 closes, bringing R2 into the circuit. Since R2 has 100 times less resistance than R1, R1's effect becomes negligible. In the ZERO, BAT 1, BAT 2, or OFF positions of the range switch, S1 closes and shorts out the feedback resistors, preventing feedback current from generating voltage across them. Any remaining voltage on the amplifier output, visible on the meter, indicates a zero offset error. This offset can be removed using ZERO control R3, which rebalances the amplifier with both inputs of A1 at zero potential.

Switches S2 and S3 are glass-encapsulated magnetic reed switches with very high open-circuit impedance. They are activated by a permanent magnet on a swinging arm outside the chamber assembly. S2, the shorting switch, is on the right side of the chamber assembly. The magnet is over S2 when the range switch is at OFF, BAT 1, BAT 2, or ZERO. S3, which places R2 in the circuit, is on the left side of the chamber assembly. The magnet is over S3 when the range switch is at the 5000 or 500 mR/h positions. For the 50 or 5 mR/h positions, the magnet is over the chamber's center, leaving neither S2 nor S3 pulled in, and R1 as the active feedback resistor.

The second section of S1 (S1-B) selects either full amplifier output voltage or one-tenth of the voltage to supply the feedback elements. Full amplifier voltage is used for the 5000, 50, BAT 1, and BAT 2 positions. One-tenth output voltage is used for the 500, 5, and ZERO positions.

Usage Features

To use the RO-2, first turn the function switch to BAT 1 and then to BAT 2. The meter should read above the BATT cut-off line in both positions. Next, turn the function switch to ZERO. Verify that the meter reads zero; if not, adjust it with the ZERO knob. Note that for instruments with the "DAWN" operational amplifier, the zero control may work in reverse; simply turn it in the appropriate direction.

Set the function switch to the desired range of operation, corresponding to the full-scale reading of that range. When measuring beta, low energy gamma, or x-ray emissions, open the sliding beta shield on the bottom of the case and face the instrument's bottom toward the radiation source. To open or close the shield, depress the friction release button on the left side of the case and manually move the slide, or allow it to fall due to gravity. When the shield is open, protect the thin face from puncture damage.

The zero setting can be checked in any radiation field by simply selecting the ZERO position. When selecting the most sensitive range (5 mR/h), temporary meter deflection due to switching transient noise can be avoided by first selecting 50 mR/h, allowing the needle to settle, and then switching to 5 mR/h. The effective center of the ion chamber is marked by dimples on the front and sides of the instrument case.

Since the ion chamber is vented to atmospheric pressure, it is sensitive to changes in air pressure and temperature. Correction factors are provided in tables to adjust meter readings if use conditions differ from calibration conditions. If both pressure and temperature vary, multiply the meter reading by both factors. Avoid exposing the instrument to fields significantly above full scale for the selected range, as this can cause amplifier gate stress and potential damage.

Maintenance Features

Preventive Maintenance: Replace batteries when the meter indicates at or below the BATT OK line during a battery check. Batteries are accessible by separating the cover assembly from the case and unstrapping the desiccant box. BAT 1 checks the right-hand battery, and BAT 2 checks the left-hand battery (viewed from the top). The center battery (BT3), which powers only the ion chamber, has an indefinite shelf life and is not checked by the internal battery circuits. It is good practice to move the center battery to a BT1 or BT2 position when a new battery is needed, and install the new battery in the center position. This ensures full power from fresh batteries and powers the chamber with a fresh battery. Always turn the RO-2 off before changing batteries. Remove batteries if the instrument will be inactive for a long period.

Dry or replace desiccant crystals when they turn clear or pink. To remove the desiccant box, separate the cover assembly and case, then open the nylon retaining straps and remove the tape from around the box lid. When new or dried crystals are in place, retape the lid, leaving a small space for air to enter and leave the box. Desiccant crystals can be repeatedly dried by heating to 250 °F for twelve hours or 400 °F for one hour. Do not heat the plastic box. To prolong desiccant life, place the box in a small plastic bag with a rubber band around the plastic hose. It is crucial to keep the chamber assembly dry to prevent leakage currents due to moisture. If the desiccant is saturated and the unit becomes erratic, renew the crystals and cycle the instrument between room temperature (or lower) and +140 °F three or four times to flush the chamber air across the desiccant.

Calibration: For maximum accuracy, calibrate the RO-2 at approximately the expected temperature and air pressure for its use. If conditions differ, an offset can be used during calibration to ensure proper readings. To calibrate, remove the control locks and position the RO-2 in a known gamma field. The effective center of the chamber is marked by case indentations. Adjust the calibration control (corresponding to the range switch position) for the proper meter indication. For best accuracy, choose a field strength that causes the meter to read around the four mark. Repeat for all four ranges. The entire ion chamber must be in the gamma field during calibration. Be aware that the required gamma field intensities are potentially hazardous, so observe proper precautions.

Disassembly: Separate the cover assembly from the case by unfastening the front and rear latches and lifting the instrument. The chamber face is very thin and easily damaged; protect it with cardboard if the case is off for an extended period. The amplifier A1 is highly susceptible to static charges. When the cover is removed, ensure that any item touching the RO-2 assembly is first grounded to the chassis. Grasping the chassis with one hand and touching the item with the other eliminates static charge. Solder irons must be clipped to the chassis. For added protection, remove batteries and clip chamber terminals 1, 3, 4, 7, and 8 (counting clockwise from the chamber face toward the viewer) together. Remove clips before reinstalling batteries.

To remove the desiccant box, pull open the nylon straps and slip the plastic tube off the pipe at the chamber assembly side. To separate the chassis plate from the cover, remove the four screws at the corners of the chassis plate and lay the plate aside. This provides access to the range switch, calibration potentiometers, zero control, meter, and magnet swing arm. To remove the meter, remove the instrument handle, meter wires, and the three screws in the meter bezel. To access detector electronics, loosen the two screws holding the chamber mounting brackets to the chassis plate. Rotate the chamber assembly slightly to free it. Cleanliness inside the chamber assembly is critical. The inside of the ion chamber can be reached by removing the clamp ring and then the Mylar® face. The center electrode can be removed by unscrewing it from the center pin; if the pin rotates, hold it with pliers in the electronic section. Removing the four screws in the corners of the slide holder on the bottom of the case will remove the slide and holder.

Reassembly: Generally, reassembly reverses the disassembly procedure. The chamber face and the thin window under the slide are 1-mil-thick aluminized Mylar® film. The window on the can is glued to the aluminum surface with electrically conductive adhesive, and the window on the chamber is clamped against a conductive rubber gasket to maintain proper electrostatic fields. If only one side of the Mylar is aluminized, ensure the aluminized side faces the chamber interior. To replace the chamber face, lay the Mylar® over the rubber gasket and press the clamp ring evenly to avoid wrinkles. In very moist conditions, flushing the chamber with dry nitrogen before closing it with the face may be advisable; otherwise, temperature cycling may be necessary later.

The magnet swing arm assembly should be adjusted to be against the right-hand stop when the range switch is at OFF through ZERO, against the left-hand stop when the switch is at 5000 and 500, and centered when the switch is at 50 and 5. The magnet should just clear the meter insulators as it swings past. Adjust the swing arm operation by moving its mounting bracket, moving the switch cam assembly, and bending the arm as needed. Ensure the magnet does not contact the meter case under any movement conditions.

Troubleshooting: The schematic diagram (Figure 6-1), system block diagram (Figure 3-1), and Section III "Theory of Operation" are primary aids for troubleshooting. Since the instrument has no active electronic components except for amplifier A1, the most likely sources of trouble are the amplifier, a broken wire, poor batteries, or leakage in the high impedance areas of the chamber assembly.

If the meter remains pegged and will not zero when ZERO is selected, check for proper battery voltages. Try pressing the magnet down against the chassis plate to ensure S2 is pulled in. Using the schematic (Figure 6-1) as a guide, check for proper voltages at the chamber assembly terminals. Terminal 1 should be at plus battery voltage, terminal 2 slightly less negative than negative battery voltage, and terminal 3 should be ground. If these voltages are normal but terminals 5 and 8 have the same polarity, the amplifier is likely defective and should be changed. Note how the old amplifier was installed to install the new one correctly.

The amplifier can be destroyed by improper handling due to static charges. Take every precaution to eliminate static charges around the unit. Do not remove the shorting device shipped with the new amplifier until other shorting clips are in place and the amplifier is soldered in. Transfer the clip to the terminal or other soldered lead only after installation is complete. Ensure the soldering iron is grounded to both the instrument chassis ground and the shorting clip around the amplifier when soldering.

High impedance components, including pin 2 of A1, feedback resistors R1 and R2, and reed switches S2 and S3, must be clean, free of fingerprints, and carefully installed. Their leads are air-insulated, not supported on insulators. All wiring in the chamber assembly should be dressed away from these components. If leakage currents are a problem, ensure the silica gel in the desiccant box is dark blue. Temperature cycling, as described in preventive maintenance, may correct the problem. A high-purity, mild Freon® solvent spray may help clean high impedance components, but do not use alcohol spray as it dissolves the aquadag coating.

Eberline RO-2 - Page 14

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