Direct-reading Methods

Direct-reading instruments are very important tools for detecting and measuring worker exposure to gases, dusts, mists, vapors, aerosols, and fine particulates that could be in the air around them and that pose a threat to inhalation hazards. These instruments display real-time or near real-time measurements, and their use is specifically required by some OSHA standards. Some examples that have specific OSHA standards for direct reading instruments are: general industry, shipyard employment, and the construction industry. There are many different kinds of direct reading instruments available, each of them are different and designed for their specific purpose. Just because the instruments are direct-reading does not mean they cannot be inaccurate sometimes. The user must follow the procedure correctly and properly operate the direct reading instrument to get accurate readings on air contaminants.  These instruments can take measurements without manipulation of the sample by the user or sending the sample to an offsite laboratory. Devices that are direct-reading instruments are simple, taking a sample in a single point in time of exposure. These devices contain a sampling system, signal processing electronics, a display system, and a detector.

The Wheatstone bridge was originally developed by Charles Wheatstone to measure unknown resistance values and as a means of calibrating measuring instruments, voltmeters, and ammeters, etc, by the use of a long resistive slide wire. Although most people today use digital multimeters that provide the simplest way to measure a resistance, but the Wheatstone Bridge can still be used to measure very low values of resistances down in the mili-ohms range. A Wheatstone bridge circuit is an example of a direct reading instrument. They are used widely for the measurement of resistance, capacitance, and inductance. A basic Wheatstone bridge circuit contains four resistances, a constant voltage input, and a voltage gage. If the bridge is initially balanced, the initial voltage reading Vg should be zero. When the resistance changes are small (<5%), the second order term n is approximately zero and can be ignored. In practice we often use the same resistance value for all four resistors. By thoughtfully selecting the target and reference resistances, the Wheatstone bridge circuit can amplify small changes in resistance and/or compensate for changes in temperature.

Another sensor used to measure combustibility also relies on detecting a change in electrical conductivity; in this sensor, the change is produced when the combustible gas is adsorbed onto the solid surface of a metal-oxide semiconductor (MOS.) In 1959 M. M. (John) Atalla and Dawon Kahng at Bell Labs achieved the first successful insulated-gate field-effect transistor (FET), which had been long anticipated by Lilienfeld, Heil, Shockley and others by overcoming the "surface states" that blocked electric fields from penetrating into the semiconductor material. Investigating thermally grown silicon-dioxide layers, they found these states could be markedly reduced at the interface between the silicon and its oxide in a sandwich comprising layers of metal (M - gate), oxide (O - insulation), and silicon (S - semiconductor) - thus the name MOSFET, popularly known as MOS. The MOS transistor conducting region is either p-type (making it a "p-channel" device) or n-type ("n-channel" device) material. The latter are faster than p-channel but are more difficult to make. MOS devices hit the commercial market in 1964.

http://www.computerhistory.org/semiconductor/timeline/1960-MOS.html

http://www.efunda.com/designstandards/sensors/methods/wheatstone_bridge.cfm

http://www.sciencedirect.com/science/article/pii/S1871553209000991

https://www.osha.gov/SLTC/directreadinginstruments/index.html