The portable chair frame that supports a sling of the preferred embodiments includes a brace, a first pair of collapsible bars, a second pair of collapsible bars, and two joints that each provide a point of contact with a ground surface. The method can include obtaining a target pattern comprising a plurality of target pattern features corresponding to a plurality of features to be imaged on a substrate. To that end, a retransmit message requesting retransmission of a set of message segments to a given receiver is received from the given receiver. The cover defines two fixing apertures in respective opposite sides. In addition, the invention is directed to a continuous multi-stage process comprising reacting at least one reactant in a liquid phase in an up-flow column reactor, and forming a gas or vapor phase by-product wherein the gas or vapor phase by-product is continuously removed at the top and at least one intermediate stage. Each of the optical sensors includes a waveguide for conducting light, and a reagent region positioned between the waveguide and a fluid in the borehole to absorb a portion of the light from the waveguide, the portion being dependent upon a concentration of a chemical species in the fluid.