Demand mode

Demand mode The mode of air supply in which inhalation creates a negative pressure inside the face-piece, causing the regulator to release air into the face-piece. Respirators that operate in this mode are not recommended and have been largely replaced by respirators operated in the pressure-demand mode, in which the face-piece is maintained under a slight positive pressure at all times. 

One of the most attractive precision dispensing methods for low-viscosity adhesives is the use of microjet printing technology.10 This technology is based on piezoelectric demand mode ink-jet printing, which can produce droplets of polymeric resins 25 to 125 flin in diameter, at rates up to 1000 drops per second. 

Having in mind the inhibitory effect of hydrogen peroxide on peroxidases, the procedure of peroxide addition influences the preservation of the enzymatic activity into the system. The peroxide can be added in a portionwise, continuous, or feed-on-demand mode, by using a peroxide-stat [8, 86], as was mentioned before in Sect. 10.1.2. When adding peroxide in a continuous mode, a low concentration of this reactant and thus less enzyme deactivation are ensured. 

Qbtaining pathogens and toxins from their natural environment is another sensible, though skill-demanding mode. In 1993, the Japanese cult Aum Shinrikyo sent a group of 16 cult doctors and nurses to Zaire, on a supposed medical mission. The... 

Self-contained breathing apparatus with a full facepiece operated in pressure-demand or other positive-pressure mode or (2) Supplied-air respirator with a full facepiece operated in pressure-demand or other positive-pressure mode and equipped with an auxiliary escape-type self-contained breathing apparatus operated in pressure-demand mode. 

Escape (1) Self-contained breathing apparatus in demand or pressure-demand mode or (2) Full-face mask respirator with chin-style or front- or back-mounted type industrial size canister specifically approved for protection against formaldehyde. 

Tight-fitting powered air-purifying respirator witii high-efficiency filters (3) Full-facepiece supplied-air respirator operated in demand mode (4) Half-mask or full-facepiece supplied-air respirator operated in a continuous-flow mode or (5) Full-facepiece self-contained breatiiing apparatus operated in demand mode. 

Eirefighting Full-facepiece self-contained breadiing apparatus in positive-pressure demand mode. 

Combination Type C supplied-air respirator [see below], demand type, with full facepiece, and auxiliary self-contained air supply or (2) Open-circuit self-contained breathing apparatus with full facepiece, in demand mode or (3) Type C supplied-air respirator [see below], demand type, with full facepiece. 

In the 1950s, Hansell observed the production of drops by electro-mechanically induced pressure waves [8]. In this type of system, a voltage pulse applied to a piezoelectric material that is directly or indirectly coupled to the fluid, at ambient pressure, induces a volumetric change in the fluid. This volumetric change creates pressure/velocity transients within the fluid that are directed so as to produce a drop from an orifice [9-11]. Since the voltage is applied only when the drop is needed, these types of systems are called drop-on-demand or demand mode. ... 

In many commercially available demand-mode ink-jet systems today, a thin-film resistor is substituted for the piezoelectric transducer. When high current is passed through this resistor, the ink in contact with it is vaporized, forming a bubble over the resistor [17]. This vapor bubble serves the same function as the piezoelectric transducer. This t5rpe of printer is referred to as a thermal ink-jet printer. 

The high-temperature demand mode ink-jet process used in printing UV-curing polymer microlenses can be used to create highly controlled spacers in flat panel displays. Figure 11-26 shows an example of printed spacer bumps that would meet the physical and thermal (in excess of 200°C) durability requirements for flat panel displays. Bumps as small as 25 pm diameter and 10 pm high can be created, and bumps this size or larger would span the requirements for most spacers in displays. 

The 3D Systems Thermojet Printer [38] also uses a demand mode piezoelectric array printhead dispensing a thermoplastic material to build solid models. Printing resolutions (actually addressability) of 300, 400, and 600 dot per inch (drop spacings of 85 pm, 63 pm, and 42 pm) are claimed. The maximum model size is 25 cm x 19 cm x 20 cm. 

When a safety function is aiiocated to a safety instrumented function, it wiii be necessary to consider whether the appiication is in demand or in continuous mode. The majority of appiications in the process sector operate in demand mode where demands are infrequent, in such cases, Tabie 3 in iEC 61511-1 ANSi/iSA-84.00.01-2004 Part 1 (iEC 61511-1 Mod) is the appropriate measure to use. There are some appiications where demands are frequent (for exampie, greater than one per year) and it is more appropriate to consider the application as continuous mode because the probability of dangerous failure will be primarily determined by the failure rate of the SIS. In such cases. Table 4 in IEC 61511-1 ANSI/ISA-84.00.01-2004 Part 1 (IEC 61511-1 Mod) is the appropriate measure to apply. Continuous mode applications where failure would result in an immediate hazard are rare. Burner or turbine speed control may be continuous mode applications if protection systems are insufficient for all failure modes of the control system. 

In the choice of a proof test interval, considerations should be given to the demand rate for Demand Mode systems, the failure rate of each component being tested, and the overall system performance requirements. 

Safety Integrity Level Probability of Failure on Demand (PFDavg.) Low Demand Mode Risk Reduction Factor (RRF)... 

Results of the evaluation typically include a number of safety integrity and availability measurements. Most important, the average probability of failure on demand (PFDavg) and the safe failure fraction (SFF) is calculated for low demand mode. Probability of failure per hour is calculated for high demand mode. From charts, the SIL level that the... 

When the process design is successfully oriented toward safety, risk analysis teams will usually estimate residual hazardous situations that occur only once in many years (clearly low demand mode). The average time period between hazardous events is often estimated to be over 10 years. Thus, we have a situation where the SIS is activated only once every ten years or more. During normal operation it is static. A safety isolation valve may sit motionless for years Contrast this with the operation of a BPCS. The control signals are normally d)mamic with some signals moving considerably at all times. 

The safety and availability of a set of equipment used for a safety instrumented function may benefit from testing. However, that depends on redundancy and how often the demand occurs. Three modes of operation have been defined in lEC 61508 for equipment providing a safety instrumented function continuous demand mode, high demand mode and low demand mode. This book will use the lEC 61508 definitions to designate those three different situations. 

In continuous mode, the demand is effectively always present. Dangerous conditions always exist and a dangerous failure of the safety instrumented function will immediately result in an incident. There are no safety benefits that can be claimed for manual proof testing or even automatic on-line diagnostics in a single channel system (Tool). By the time the diagnostics detect the fault and initiate action, it is too late. Therefore, in continuous demand mode probability evaluation cannot take credit for any diagnostics except in redundant systems. 

In high demand mode, the probability evaluation is done by comparing the calculated probability of failure per hour (PFH) against the PFH table shown in Figure 7-4. 

In low demand mode safety instrumented functions, the person performing the SIF verification calculations must ... 

The calculations may be done with simplified equations, fault trees, Markov models or other techniques depending on the complexity of the model and the demand mode of operation. 

Continuous / High Demand Mode Verification Calculation... 

Remember that in continuous demand mode credit can be given for automatic diagnostics or for proof test procedures only for redundant systems. In addition, many of the calculation assumptions made for low demand mode do not apply. The "probability of dangerous failure per hour" must be calculated based on all dangerous failures. 

Figure 7-4. Continuous Demand Mode Dangerous Probability Limits per SIL...

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