This product is a replacement membrane cap for the Gravity Analog Dissolved Oxygen Sensor, which needs to be used in conjunction with the sensor and electrolyte. There is a transparent oxygen permeable membrane on the membrane cap which is easily damaged. Please take care to protect it. Do not touch it with sharp objects or nails. When using it, you need to drip in about two-thirds of the electrolyte before you put the membrane cap over the electrode.