Other cellular mechanisms taken into consideration include the protoplast, solutes within the protoplast (solute potential), transpiration rates of the plant and the tension of cell walls. Measurement is limited depending on the method used, some of which are explored and explained below. Not all methods can be used for all organisms, due to size and other properties. For example, a diatom doesn't have the same properties as a plant, which would place constrictions on what could be used to infer turgor pressure. Units used to measure turgor pressure are independent from the measures used to infer its values. Common units include bars, MPa, or newtons per square meter. 1 bar is equal to .1 MPa. Turgor pressure can be deduced when total water potential, Ψw, and osmotic potential, Ψs, are known in a water potential equation. These equations are used to measure the total water potential of a plant by using variables such as matric potential, osmotic potential, pressure potential, gravitational effects and turgor pressure. After taking the difference between Ψs and Ψw, the value for turgor pressure is given. When using this method, gravity and matric potential are considered to be negligible, since their values are generally either negative or close to zero. The pressure bomb technique was developed by Scholander et al., reviewed by Tyree and Hammel in their 1972 publication, in order to test water movement through plants. The instrument is used to measure turgor pressure by placing a leaf (with stem attached) into a closed chamber where pressurized gas is added in increments. Measurements are taken when xylem sap appears out of the cut surface and at the point which it doesn't accumulate or retreat back into the cut surface. Atomic force microscopes use a type of scanning probe microscopy (SPM). Small probes are introduced to the area of interest, and a spring within the probe measures values via displacement. This method can be used to measure turgor pressure of organisms. When using this method, supplemental information such as continuum mechanic equations, single force depth curves and cell geometries can be used to quantify turgor pressures within a given area (usually a cell). This machine was originally used to measure individual algal cells, but can now be used on larger-celled specimens. It is usually used on higher plant tissues but wasn't used to measure turgor pressure until Hüsken and Zimmerman improved on the method.