Reducing Lag

A thermowell is required to protect the thermal element of a thermometer or temperature transmitter thermobulb from corrosion and erosion, to give it adequate support, and to permit its removal without interrupting the process. The use of the thermowell will unavoidably introduce a temperature time lag to the changes in the process temperature and response to the temperature relayed to the instrument. This is caused by the transmission of heat through the thickness of the metal well and the inevitable dead air space between the well and bulb. 

The sleeve forces the bulb to one side of the well insuring a metal-to-metal contact throughout the length of the sensitive thermobulb. Sometimes, two sleeves are used on both sides of the bulb. The corrugations in the sleeve itself provide a metallic path between the well and the bulb on the opposite side. The advantage of using a sleeve is that it can be applied to thermowells installed in a vertical, horizontal, or in an upside-down position. 

The standard type thermowells manufactured and supplied are provided with the following bulb diameter-to-well bore relationship. The dimensions listed in catalogs are usually given as a 1/4-inch bulb fits into a 0.260-inch bore, a 3/8-inch into a 0.385-inch, a 1/2-inch into a 0.510-inch, a 9/16-inch into a 0.572-inch, a 5/8-inch into a 0.635-inch, an 11/16-inch into a 0.707-inch, a 3/4-inch into a 0.760-inch and a 7/8-inch into a 0.885-inch. 

A mixture of oil and graphite is better than pure graphite for use at higher operating temperatures. Other substances used in vertical thermowells are glycerine, napthalene, oils and various types of greases and proprietary heat transfer fluids. 

Thermowells in a horizontal position can be filled with graphite, carbon, metsdlic dust such as copper, aluminum or clean iron filings or a corrugated aluminum sheath to reduce the insulating properties of the air gap. Solder is sometimes used between the thermobulb and the thermowell (tin, 420°F, lead, 600°F). 

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Contamination functional barrier depending on the degree of contamination of the functional barrier, only reduced lag time available. 

Silicon nitride rotors have proved superior to steel rotors in this application. The advantage of silicon nitride lies in its low density which allows lower torque, more rapid acceleration, and reduced lag (the silicon... 

Advantages. This system reduces lags and allows finer control. 

Trimethylene Di-iodide. Use 76 g. of trimethylene glycol, 27 - 52 g. of pmified red phosphorus and 254 g. of iodine. Lag the arm C (Fig. Ill, 40, ) with asbestos cloth. Stop the heating immediately all the iodine has been transferred to the fiask. Add water to the reaction mixture, decolourise with a httle sodium bisulphite, filter, separate the crude iodide, wash it twice with water, dry with anhydrous potassium carbonate and distU under reduced pressure. B.p. 88-89°/6 mm. Yield 218 g. (a colourless liquid). 

Staged reactions, where only part of the initial reactants are added, either to consecutive reactors or with a time lag to the same reactor, maybe used to reduce dipentaerythritol content. This technique increases the effective formaldehyde-to-acetaldehyde mole ratio, maintaining the original stoichiometric one. It also permits easier thermal control of the reaction (66,67). Both batch and continuous reaction systems are used. The former have greater flexibiHty whereas the product of the latter has improved consistency (55,68). 

When fouling occurs, even mechanical cleaning does not remove all traces of the bio film. Previously fouled and cleaned surfaces are more rapidly colonized than new surfaces. Residual biofilm materials promote colonization and reduce the lag time before significant fouling reappears. 

Fig. 2. Assay reaction curves for (a) substrates, (b) enzymes, and (c) enzymes exhibiting a lag phase or reduced reaction rate where t is measurement time.

Time-Delay Compensation Time delays are a common occurrence in the process industries because of the presence of recycle loops, fluid-flow distance lags, and dead time in composition measurements resulting from use of chromatographic analysis. The presence of a time delay in a process severely hmits the performance of a conventional PID control system, reducing the stability margin of the closed-loop control system. Consequently, the controller gain must be reduced below that which could be used for a process without delay. Thus, the response of the closed-loop system will be sluggish compared to that of the system with no time delay. 

Positioner Application Positioners are widelv used on pneumatic valve actuators, VIore often than not, thev provide improved process-loop control because thev reduce valve-related nonlinearitv, Dvnarnicallv, positioners maintain their abilitv to improve control-valve performance for sinusoidal input frequencies up to about one half of the positioner bandwidth. At input frequencies greater than this, the attenuation in the positioner amplifier netvv ork gets large, and valve nonlinearitv begins to affect final control-element performance more significantlv. Because of this, the most successful use of the positioner occurs when the positioner-response bandwidth is greater than twice that of the most dominant time lag in the process loop. 

The death rate coefficient is usually relatively small unless inhibitoiy substances accumulate, so Eq. (24-10) shows an exponential rise until S becomes depleted to reduce [L. This explains the usual growth curve (Fig. 24-21) with its lag phase, logarithmic phase, resting phase, and declining phase as the effect of takes over. 

In order to avoid error, dtie to time lag in the temperature meastirement of large motors and vttriation in the cooling air or gas temperature all reasonable precautions must be taken to reduce these variations and errors as much as possible. 

If hydrogenation of the reaction mixture is begun in the presence of platinum oxide, a long induction period or lag occurs before the catalyst is reduced. 

Photog.) restrainer (VaTniah) reducer, verzogem, v.t. retard delay, postpone. Verzogerung,/. retardation delay, postponement lag. 

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