Developing device having inductance sensor

1. A developing device comprising:

a developer bearing member which bears developer including toner and carrier to develop an electrostatic latent image formed on an image bearing member;

a developing container which accommodates the developer;

a conveying screw which conveys the developer accommodated in the developing container; and

an inductance sensor which includes a detecting portion which detects magnetic permeability of the developer accommodated in the developing container,

wherein in a state the inductance sensor is not attached to the developing container and a predetermined magnetic material is disposed at a position in contact with the detection portion, an output value when the detecting portion detects magnetic permeability of the predetermined magnetic material is A, and

in a state the inductance sensor is not attached to the developing container and the predetermined magnetic material is disposed 1 mm away from the detection portion in a vertical direction passing through the detection portion, an output value when the detecting portion detects magnetic permeability of the predetermined magnetic material is B, with B/A≥0.1 being satisfied,

wherein the conveying screw includes:

a first conveying portion which has a first rotary shaft portion and a first blade portion which is spirally formed on an outer circumferential surface of the first rotary shaft portion and which conveys the developer in a conveying direction of the conveying screw; and

a second conveying portion which has a second rotary shaft portion and a second blade portion which is spirally formed on an outer circumferential surface of the second rotary shaft portion and which conveys the developer in the conveying direction of the conveying screw,

wherein the first conveying portion is disposed opposite the detection portion with respect to the conveying direction of the conveying screw,

wherein the second conveying portion is disposed within one pitch downstream of the first blade portion from a downstream end of the detection portion with respect to the conveying direction of the conveying screw, and

wherein a shaft diameter of the second rotary shaft portion is greater than a shaft diameter of the first rotary shaft portion.

2. The developing device according to claim 1 ,

wherein assuming that the shaft diameter of the second rotary shaft portion is R 1 , the shaft diameter of the first rotary shaft portion is R 2 , and an outer diameter of the first blade is R 3 , the inequality R 1 >(R 2 +R 3 )/2 is met.

3. The developing device according to claim 1 ,

wherein a length of the second conveying portion in the conveying direction of the conveying screw is 0.5 to 2 times a length of one pitch of the first blade portion.

4. The developing device according to claim 1 ,

wherein the conveying screw further includes a rib formed as to protrude outwardly from an outer circumference of the first rotary shaft portion, and

wherein the rib is disposed with respect to the conveying direction of the conveying screw and the rib is disposed opposite the detection portion.

5. The developing device according to claim 4 ,

wherein the rib is provided with a magnet.

6. The developing device according to claim 1 ,

wherein the inductance sensor further includes an output portion which outputs a pulse signal in accordance with the magnetic permeability detected by the detection portion.

7. The developing device according to claim 1 ,

wherein the inductance sensor further includes a circuit board, and

wherein the detection portion is an area on the circuit board, on which an area a pattern of a coil is formed.