气流池使用的插槽大于插槽限制

有三个传感器任务并使用相同的池，池'limit_sensor'设置为1，但池限制不起作用，三个池一起运行

sensor_wait = SqlSensor(
    task_id='sensor_wait',
    dag=dag,
    conn_id='dest_data',
    sql="select count(*) from test",
    poke_interval=10,
    timeout=60,
    pool='limit_sensor',
    priority_weight=100
)

same_pool1 = SqlSensor(
    task_id='same_pool1',
    dag=dag,
    conn_id='dest_data',
    sql="select count(*) from test",
    poke_interval=10,
    timeout=60,
    pool='limit_sensor',
    priority_weight=10
)

same_pool2 = SqlSensor(
    task_id='same_pool2',
    dag=dag,
    conn_id='dest_data',
    sql="select count(*) from test",
    poke_interval=10,
    timeout=60,
    pool='limit_sensor',
    priority_weight=10
)

有回填日志，气流1.10.0

[2018-10-12 11:20:35,036] {jobs.py:2198} INFO - [backfill progress] | finished run 0 of 1 | tasks waiting: 0 | succeeded: 0 | running: 3 | failed: 0 | skipped: 0 | deadlocked: 0 | not ready: 0

在网络用户界面中Admin->Pools你可以看到：

Pool    Slots    Used Slots    Queued Slots
limit_sensor    1    3    0

我应该如何做才能使池限制发挥作用？谢谢。