These two are sold as a set, one is a 740-N4-2(300 SLPM.AIR)-3-V4 and the other is a 740-N3-2(10 SCFM.AIR)-V1 for parts. Please see the photos for a better idea of description, the N3 is missing the sensor and the wires are cut, the N4 is complete as shown. I dont know what these do or what they are used for. The purchase is for units only, please dont expect something not shown or stated in the description. If you have ANY questions please ask and ill get back to you within 24 hours and post the answer to the listing. Feel free to make me an offer and they can be yours today.