Image forming apparatus having removable developing units

An image forming apparatus comprising:

• a first image forming portion having;
• a first image bearing member;
• a first developing unit detachably attached to the image forming apparatus and developing an electrostatic latent image formed on the first image bearing member by using a first developer including a first toner and a carrier,
• the first developing unit having a first developing container with a first opening and containing the first developer, a first developer bearing member disposed at the first opening and developing a latent image born on the first image bearing member, a first developer supplying portion supplying the first developer to the first developing container, and a first developer discharging portion configured to discharge a part of the first developer according to supplying of the first developer by the first developer supplying portion,
• a second image forming portion having;
• a second image bearing member;
• a second developing unit detachably attached to the image forming apparatus and developing an electrostatic latent image formed on the second image bearing member by using a second developer including a second toner and a carrier,
• the second developing unit having a second developing container with a second opening and containing the second developer, a second developer bearing member disposed at the second opening and developing a latent image born on the second image bearing member, a second developer supplying portion supplying the second developer to the second developing container, and a second developer discharging portion configured to discharge a part of the second developer according to supplying of the second developer by the second developer supplying portion;
• a first conveying pipe detachable from the first developing unit and attached to the first developing unit when the first developing unit is disposed in the image forming apparatus and for conveying the first developer discharged from the first developer discharging portion to a collection portion;
• a second conveying pipe detachable from the second developing unit and attached to the second developing unit when the second developing unit is disposed in the image forming apparatus and for conveying the second developer discharged from the second developer discharging portion to the collection portion;
• a first detection unit which detects a toner content of the first developer in the first conveying pipe;
• and
• a second detection unit which detects a toner content of the second developer in the second conveying pipe,
• wherein the first developing unit can be removed from the image forming apparatus in a state that the first conveying pipe and the first detection unit remain in the image forming apparatus, and
• wherein the second developing unit can be removed from the image forming apparatus in a state that the second conveying pipe and the second detection unit remain in the image forming apparatus.