Multi-Battery Radiacmeters

TM 11-6665-232-40

circuit, consisting of meter M1, resistor R3,

ground return for the ion chamber; pin H pro-

variable resistor R2, and calibration variable

vides ground return for the grid circuit; pin J

resistor R4. The negative terminals of parallel-

grounds a shielding coating on the outside of the

connected batteries BT3 and BT-4 are con-

detector assembly.

nected to the positive terminal of meter M1, and

d. During operation, the ionization chamber

the negative terminal of BT6 (which is series-

conductive coating is held positive with respect

connected to BT5) is connected through resis-

to the collecting electrode. Ionization current

tive network R5, R6, and R7 to the negative

develops a positive going voltage on pin 6 of V1

terminal of M1. This arrangement results in volt-

as a result of its flow through grid resistor R1.

ages of opposite polarity being applied to M1.

The total effect is an increase in plate current

During zero adjustment of the meter, function

flow through V1, a result of an increase in ioni-

switch S2 (c below) is set to ZERO, and variable

zations in the ionization chamber.

resistor R2 is adjusted until the current from

BT5 and BT6 through V1 is equal and opposite

2-4. Multi-Battery Radiacmeters

that from BT3 and BT-4. At this time M1 indi-

cated 0. When switch S1 is set to OFF, it shorts

The multi-battery radiacmeter uses six bat-

the meter and dampens the pointer movement

teries to supply the operating power for the

when the instrument is not in use. Resistor R3

radiacmeter. As shown on schematic diagram,

establishes the resistive range of the R2R3 cir-

figure FO-2, batteries BT-1 and BT-2 provide

cuit from 6.8 kilohms to 21.8 kilohms.

bias for the ionization chamber; BT-3 and BT-4

(3) Plate and screen grid voltage (approxi-

supply filament power for cathode follower V1;

mately + 10.4 volts) is obtained from series-

BT-5 and BT-6 supply plate voltage for V1.

connected batteries BT-5 and BT-6. The

Switches S1 and S2 are shown in the normal

positive terminal of BT-5 is connected to the

operating position.

plate (pin 1) and the screen grid. (pin 2) of V1

a. Ionizational Chamber Bias. The difference in

through connectors P1-C and J1-C. The nega-

potential between the walls of the ionization

tive resistive network R5, R6, and R7; the paral-

chamber and the collector electrode is provided

lel circuit of meter M1 and calibration variable

by batteries BT-1 and BT-2.

(1) The positive terminal of BT-1 is con-

resistor R4; and connectors P1-C and J1-C. The

negative terminal of BT6 is connected to the

nected to the ionization chamber through ter-

cathode (pin 4) of V1 through resistive network

minals 3 and 5 of S2A and connectors P1-F and

R5, R6, and R7; the parallel circuit of meter M1

J1-F.

(2) The negative terminal of BT-2 is con-

and calibration variable resistor R4; and con-

nectors P1-D and J1-D.

nected to the grid of V1 through terminals 9 and

c. Function Switch S2 (fig. FO-2). Function

11 of S2B, connectors P1-H and J1-H, and resis-

switch S2 is a two-pole, three position switch,

tor R1.

b. Meter M1 Circuit.

spring-locked to the center (OPERATE) posi-

tion.

(1) The voltage developed across the high-

(1) When S2 is set to ZERO (during zero

megohm resistor (located with tube V1 in the

adjustment of the meter (b above) ), the ioniza-

upper portion of detector assembly) by the cur-

tion chamber is shorted (through R1) to the con-

rent from the ionization chamber is applied to

trol grid of V1 through connectors P1-F and

the grid of cathode-follower V1. An increase in

J1-F, and contacts 5 and 2 of S2A. As a result, no

potential causes tube V1 to conduct more heav-

potential difference exists between the ioniza-

ily and thus causes an increase in cathode cur-

tion chamber and the control grid of V1. At the

rent. Meter M1, in the cathode circuit of tube V1,

same time, the negative terminal of BT-2 is

detects this increase in cathode current which

connected to the control grid of V1 through con-

results in a meter deflection. Gamma radiation

tacts 8 and 11 of S2B, connectors PiH and

dose rates are then read direct from the scale of

J1-H, and R1; and the positive terminal of BT-2

meter M1. Calibration variable resistor R4 (in

is connected to the cathode circuit of V1 through

parallel with meter M1) is used to calibrate the

resistors R6, R7, and R5. As a result, normal

radiacmeter.

bias conditions (control grid more negative than

(2) Zero control R2/S1 consists of switch S1,

cathode) exist for V1.

a single-pole, two-position switch, and variable

resistor R2 mounted on a common shaft. When

(2) When S2 is set to CHECK (during the

check of radiacmeter circuits), the ionization

S1 is set to ON, approximately 1.34 volts from

chamber is shorted (through R1) to the control

parallel-connected batteries BT-3 and BT-4 is

grid of V1 through connectors J1-F and P1F,

applied to the filament of V1 and to the meter

2-3