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Food justice research is undoubtedly concerned with equity

Further, their discursive distance from soilless forms of urban agriculture reflected the lack of emphasis that regional supporting organizations and planning initiatives put on these types of growing methods, as they continue to privilege soil-based ways of farming the city. This research hints at important connections between the way growing sites and organizations in San Diego County represent themselves, including their growing methods, primary topic of interest, and institutional affiliation. Our analysis suggests that soilless sites, which are largely for-profit, tend to focus their website content on the innovative methods they use to grow food in urban environments. In contrast, soil-based organizations tend to represent themselves as centered on community and food access. These broad patterns provide important insights into urban agriculture trends in the county and partly support common assumptions held about the goals and motivations of urban agriculture. However, closer examination tells a more nuanced story. Our results show that no single characteristic, whether the use of technology, institutional affiliation, or primary topic, predicted the way our growing sites and organizations represented themselves in narratives on their websites. There were some trends, but the relationship between growing method and the narrative presented is tenuous at best. Overall, two broad conclusions and future research paths can be drawn from the results of this research. First, a politics of technology that creates fixed connections between certain growing methods and values and uses this connection to assume the motivations of urban agriculture participants is misleading and lacks analytical rigor. If we pay attention to the various ways in which urban agriculture organizations represent themselves,drainage gutter it is clear that this connection between growing methods and values is tenuous. For instance, soilless urban agriculture is often associated with entrepreneurialism and therefore cast aside as profit-driven.

While the majority of our soilless sites in our population were for-profit, the link between growing method, for-profit status, and narrative topic was weak. Capital is an underlying reality of all of our sites, especially in the context of neoliberal governance in which even nonprofits are increasingly reliant on private sources of funding , including philanthropy and revenue-generating social enterprises. Entrepreneurialism, therefore, transcends the use of advanced technology and is more meaningfully connected to broader processes like neoliberalism . Future research should continue to unravel these simplistic constructions that constrain research findings and ignore potential tools for improving urban food landscapes. Second, it is important to acknowledge that the genuine motivations and agendas of actors may not match their public narratives and website content. It is therefore critical for researchers to examine the practices that underlie the narratives and self-reported motivations that we have explored and categorized in this chapter. This analysis will require researchers to embed themselves in local urban agriculture networks to observe urban agriculture in practice. Ethnography offers useful tools for this detailed analysis including in-depth interviews and participant observation that allows researchers to examine the relationship between discursive representations and practices of urban agriculture. This methodology will capture the nuanced, everyday interactions that may be hidden by the narratives presented on websites or even in survey data. Avoiding a politics of technology that interprets the connection between technology and capital to mean a singular profit-motive is imperative for gaining a better understanding of the urban agriculture movement. Soilless urban agriculture sites and organizations engage a plethora of environmental and social concerns. Simply equating technologically-advanced urban agriculture with entrepreneurialism, ignoring additional narratives, and forgoing additional critical inquiry creates blind spots in sustainable and equitable food movements.

Based on the narratives examined here, the two forms of agriculture often share values like improving food access, fostering sustainability, and empowering marginalized groups through education and training. We expect the lines to continue to blur in the future as soil based urban growing becomes more entrepreneurial and soilless growing becomes more prolific and accessible. Preliminary interviews already suggest that this is the case in San Diego County. For instance, UrbanLife Farms is planning construction of a new rooftop, hydroponic farm and will integrate it into their broader mission of education and providing job-training for youth in marginalized communities. Project New Village has also expressed an interest in pursuing these growing methods to further their mission of building community wealth and social capital in Southeastern San Diego. This research sought to ‘untangle’ the connections between growing method and narratives. This is an important step in trying to understand some of the common biases against soilless urban agriculture, many of which are rooted in ideological beliefs that are produced and reproduced through popular narratives. However, we recognize that the narratives advertised by urban agriculture sites and organizations on their websites do not accurately reflect the many values that are embedded in these sites or their practices and advocated by their members. This content analysis can only tell us how urban agriculture sites and organization represent themselves in public forums. Still, this analysis begins the task of unraveling a priori assumptions and examining the narratives that accompany urban agriculture practices. These narratives are important actants in urban agriculture actor-networks and are used by actors to strengthen support and attract funding. Deconstructing these narratives is an important step to unveiling co-optation and hollow branding strategies .

Future research should continue to examine the narratives that growing sites and organizations use to promote themselves and the agendas of their diverse actors involved in growing sites and organizations. Indeed, a whole network of people with different backgrounds, personal experiences, decision-making power, and motivations create and reinforce narratives around urban agriculture, not just the directors who likely inspire the content emphasized in mission statements and websites. Further, researchers should engage more detailed methods like ethnography to examine the practices and hidden power dynamics that underlie these narratives. Although many scholars are already embedding themselves in their local urban agriculture networks, participating and observing, to better understand motivations and power relations , few have critically explored the role of technology and considered the breadth of networks shaping urban agriculture. These networks extend beyond garden gates and warehouse walls into composting facilities, federal buildings, local media offices, ethnic markets, Whole Foods supermarkets, farm to table restaurants,plastic gutter and consumers’ kitchens. Future work should examine these networks in full, accounting for the multitude of actors, narratives, and practices driving the discursive and material realities of urban agriculture in the Global North. Tensions surrounding the use of advanced technology in urban agriculture are often rooted in competing understandings of social justice grounded in assumptions regarding the role of land, labor, and capital . These different conceptualizations of justice are particularly evident in debates around the benefits of soil-based and soilless urban agriculture. Such debates have recently pitted food scholars and advocates against each other at a variety of professional meetings including the recent Food Tank™ Summit in San Diego, California. In these contexts, where organizers typically seek to present a ‘balanced’ perspective by including multiple interest groups on panels, discussions of the future of urban agriculture often act as carriers for different yet simplified narratives of food justice, in which the urban food movement is envisioned at a metaphorical fork in the road with the choice of either a high-tech, entrepreneurial or a nature-based, grassroots future. Social justice, specifically food justice, plays an important role in these dichotomous and divisive arguments. Arguably, all forms of urban agriculture, regardless of their relationship to the soil, have the potential to promote or prevent social justice.

Therefore, it is necessary to examine how urban agriculture initiatives, with various degrees of technological intensity, define and do justice. This research seeks to evaluate the justice narratives and practices that shape three urban agriculture spaces with social missions in San Diego County. Urban agriculture thrives in this county and is increasingly diverse including soil-based and soilless growers – both of which are represented in our study sites. I compare these three spaces by focusing on land, labor, and capital and their relationship to distribution, participation, and recognition – three key aspects of justice. Specifically, I assess the outcomes and opportunities generated at each site that produce benefits for marginalized groups such as increased food access, improved self-sufficiency, job training, community engagement, participation in local food system planning and decision-making, and ownership of resources. At the same time, I examine the sociospatial contexts– geography, regional economies, demographics, and institutional environments – that contribute to sites’ ability to produce benefits for marginalized communities. Justice is a central concept in urban agriculture with ‘social justice’ often cited as a goal of urban food projects in the United States. In general, food justice is concerned with addressing exploitation, racism, and oppression within the food system. It is expressed to varying degrees under monikers such as food security, food justice, and food sovereignty – all of which rely on particular understandings of justice . Food security is undoubtedly the least radical of the three. It is a reformist strategy that focuses on market-based interventions – like increasing access to food retailers – and regulatory reform to ensure that individuals have access to food . Programs such as SNAP , food banking, and initiatives to increase access to supermarkets all fall under the purview of food security. The food movement, which seeks more transformational approaches to food systems, is often concerned with strategies like food justice and food sovereignty that address inequities beyond access to food and tend to focus on communities rather than individuals . Food justice is broadly defined as the idea that every person has the right to access affordable, healthful, and culturally appropriate food produced in an ethical and environmentally sound way . It is a progressive strategy that focuses on removing the disparities, especially those based on race, class, and gender, that underlie food system inequities . As such, food justice looks beyond food itself and begins to address the multiple ways in which cultural, social, economic and political inequality shapes our food system, including the production, distribution, and consumption of food. The localization of food production, which allows for greater connections and accountability, has been a common approach to reduce these disparities. Food sovereignty, arguably the most radical of the three , is defined as “the right of peoples and governments to choose the way food is produced and consumed in order to respect our livelihoods, as well as the policies that support this choice” . Here, the distribution of power, particularly power in planning and managing food systems, is key . This perspective, which has been embraced in the Global South, typically implies a rejection of capitalism and neoliberalism that are viewed as causing inequality and preventing communities from being in control of their own food ways. Often, this perspective translates into building alternative and self-sufficient food systems, including supporting community oriented projects and indigenous practices. Geographer David Harvey argues that “different socio-ecological circumstances imply quite different approaches to the question of what is just or not” . In the United States, the dominant perspective is distributive justice – the idea that outcomes such as jobs, health, and income must be fairly distributed among citizens . This approach to justice underlies concepts like food security, as well as food justice , although the two differ in their approach to fairness – the prior typically stressing equality and the latter emphasizing equity . Equality is a prolific theme in food access research where the argument is made that all people should have equal access or the right to food. However, focusing on equality of outcomes has been widely critiqued for its failure to account for the broader social contexts that produce injustice such as patterns of suburbanization , racial and economic segregation , white privilege , and individual mobility . Equity-based distributive justice is still concerned with outcomes; however, it provides more insights into the social context of injustice and considers the “historical antecedents of inequality” including “slavery, exploitation, and dispossession of the land, labor, and products of women, the poor, and people of color” . Opportunities such as access to resources like land and capital also become important in equity-based distribution.

Unions found it hard to organize workers brought to farms by intermediaries

In the first scenario, when propanil is no longer applied in the buffer zones and no other herbicide is used to replace it, we found that total revenues in Butte and Colusa counties would decline by $1.68 million, and that net revenues would decline by $1.58 million, assuming that the price of rice does not increase in response to the 0.4% decrease in production . In the second scenario, with lambda-cyhalothrin ground-applied before planting instead of being aerially applied, and assuming 15% and 23% yield losses as explained above, we found that total revenues in Butte and Colusa would decline by $5.75 million, while net revenues would decline by $4.66 million. Again, this assumes that the price of rice does not change in response to the reduction in quantity of rice produced. The combined revenue losses of the draft regulations, due to changes in application of both propanil and lambda-cyhalothrin, would be a $7.43 million loss in total revenues and a $6.25 million loss in net revenues for Butte and Colusa counties. Our analysis indicates that the draft regulations will likely have a substantial negative impact on California rice growers in Butte and Colusa counties, with a decrease in total revenues of $7.4 million and a decrease in net revenues of $6.2 million, if rice prices do not shift because of the decreases in production. The results change substantially if price is allowed to increase in response to a reduction in quantity of rice in California. However,10 liter drainage collection pot this is not a very realistic scenario given that rice prices are greatly influenced by world market prices, California only accounts for about one-fifth of U.S. rice production, and the United States is active in the international rice market.

The magnitude of the predicted revenue losses can be accounted for by the fact that there are no ideal substitutes for propanil and lamda-cyhalothrin, the large expected yield losses due to weed and rice water weevil damage in untreated buffer areas, and a sizable amount of rice acreage is affected by the draft regulations. The price of the alternative treatment in comparison to the current treatment is unlikely to be a major factor because farmers will most likely leave buffers untreated with herbicide and switch to ground applications of lamda-cyhalothrin, which involves a negligible increase in per-acre costs. Additionally, because lamda-cyhalothrin also controls tadpole shrimp, another pest of seedling rice, early pest management may become more expensive. Due to the very high share of fields affected, additional management costs due to the regulations, which are not estimated here, could be substantial. Even if the additional management costs under the draft regulations would be only $100 per field, this would lead to additional revenue losses of $470,000.The federal National Labor Relations Act excludes farm workers. California is the only major farm state with a state law that grants union rights to farm workers, establishes election procedures under which workers decide whether they want to be represented by unions, and remedies unfair labor practices committed by employers and unions. The Agricultural Labor Relations Act was enacted in 1975 after a decade of strife, as the fledgling United Farm Workers union challenged farm employers and the Teamsters for the right to represent farm workers. Experience during the late 1960s, with farm employers sometimes selecting the Teamsters to represent their workers without elections, led to provisions in the ALRA. These allowed the Agricultural Labor Relations Board to recognize a union as the bargaining representative of farm workers only after workers vote in secret-ballot elections. After the ALRA went into effect in Fall 1975, there were over 100 elections a month on the state’s farms, and it appeared that many of the state’s farm workers wanted to be represented by unions. Between 1975 and 1977, Figure 1 shows there were almost 700 elections on California farms, and unions were certified to represent workers on two-thirds of the farms involved .

Unions on most large vegetable farms and many of the largest fruit farms were expected to transform the farm labor market by raising wages and obtaining benefits such as health insurance and pensions for the seasonal workers they represented. After pushing entry-level wages in lettuce contracts to twice the minimum wage, Business Week on March 5, 1979 predicted that the United Farm Workers would help seasonal farm workers “to win wage parity with industrial workers.” The UFW became a major force in state politics, and sued the University of California to stop the use of taxpayer funds to support labor-saving mechanization research. Union organizing slowed to an average of 30 elections a year in the 1980s, and the share of elections that resulted in a union being certified to represent workers fell to 55%. Unions or workers can request secret-ballot elections, and during the 1990s requests fell to an average of 10 a year, with unions winning half. In the first decade of the 21st century, the average number of elections fell to seven a year, and many involved workers trying to decertify the union representing them. In some years, the UFW requested no elections to win certification to represent more workers, and was decertified at farms including L.E. Cooke, Vista Vineyard, and Henry Hibino. Over 15 organizations have been certified by the ALRB to represent workers on California farms, but today, three major unions represent most of the farm workers covered by contracts. The best-known union, the UFW, reported 4,300 active members to the U.S. Department of Labor at the end of 2010, and 2,500 active participants in its Juan de la Cruz pension fund; that is, workers on whose behalf employers made pension contributions sometime during the year. Teamsters Local 890 represents several thousand workers employed in the Salinas area, while United Food and Warehouse Workers Local 5 represents workers in the Salinas areas and at several wineries and dairies around the state. The UFW does not have local unions.There are four major explanations for why farm worker unions have been unable to represent more California farm workers and transform the farm labor market. The first involves flawed union leadership, especially of the UFW. Journalist Miriam Pawel praised UFW leader Cesar Chavez as a charismatic leader, able to articulate the hopes and dreams of farm workers, but concluded that Chavez was unwilling to turn the UFW into a business union that negotiated and administered contracts.

Chavez seemed more interested in using the UFW to achieve broader social change than in organizing more farm workers who might challenge his leadership. The second explanation involves state politics. Democratic governors made key appointments to the ALRB between 1975 and 1982, Republicans between 1983 and 1998, Democrats between 1999 and 2004, Republicans between 2005 and 2011, and Democrats since. Sociologists Linda and Theo Majka concluded that the ability of farm worker unions to organize and represent farm workers in the 1970s and early 1980s depended on which political party made appointments to the ALRB. Since then,plastic gutter arguments about political interference with the ALRB have diminished. The third explanation deals with changes in the structure of farm employment. Farm worker unions were most successful in the 1960s and 1970s with farms that belonged to conglomerates with brand names that made them vulnerable to boycotts, including Seven-Up, Shell Oil, and United Brands . During the 1980s, many conglomerates sold their California farming operations to growers who were more likely to hire farm workers via intermediaries such as custom harvesters and farm labor contractors.The fourth explanation is rising unauthorized migration that added to the supply of labor, making it hard for unions to win wage increases. Figure 2 shows that the number of deportable aliens located, mostly foreigners apprehended just inside the MexicoU.S. border, was rising when unions had their maximum impacts on wages. This occurred between the mid-1960s and the late 1980s, after the Bracero program ended and before unauthorized migration increased in the 1980s with recession and peso devaluations in Mexico. By the mid-1980s, when apprehensions rose to almost 1.8 million a year, unions found it hard to organize workers fearful of being discovered by Border Patrol agents. It was also difficult to win wage and benefit increases after they were certified to represent workers because newcomers from Mexico were flooding the labor market.

Farm worker unions acknowledge their difficulty organizing and representing farm workers, and hope for federal and state legislative changes to restore union power. Their primary federal goal is enactment of the Agricultural Jobs, Opportunity, Benefits and Security Act , a compromise negotiated with farm employers that would legalize currently unauthorized farm workers and make employer-friendly changes to the H-2A guest worker program. Unions believe that legal workers grateful to them for legal status would be easier to organize. However, AgJOBS is unlikely to be negotiated soon, prompting the UFW to urge changes to the ALRA. The UFW won an amendment to the ALRA in 2002 that guarantees a union contract within eight months, and another in 2011 that allows the ALRB to intervene after employers unlawfully interfere before a union election. Unions certified to represent farm workers want to negotiate agreements that set wages and benefits and protect the union as an institution by requiring workers to join the union and pay dues. There is no master list of contracts signed between farm employers and unions, preventing analysis on which union certifications failed to result in contracts. However, it is clear that most of the over 800 farms on which unions were certified to represent workers never had a union contract. Furthermore, unions were unable to renew contracts with many of the farms that signed first contracts. Unions tackled the difficulty of turning election victories into contracts with mandatory mediation in 2002, an amendment to the ALRA that should have been unnecessary. The ALRA includes a unique remedy to encourage employers to bargain in good faith with their certified union. If employers fail to bargain in good faith, the ALRB can order the employer to make employees whole for lost wages and benefits during the time that the employer failed to bargain. Unions led by the UFW argued that the make-whole remedy was not effective because of long lags between when an election is held and ALRB certification of the results. Employers often contest the ALRB’s certification decision in the courts and, by the time the employer is ordered to begin good faith bargaining, there may have been significant worker turnover and shifts in union priorities. Meanwhile, separate procedures to determine the amount of make whole owed to workers can take years, frustrating workers who expected wage and benefit increases soon after voting for union representation. Unions argued that such employer behavior discouraged worker interest in the benefits of collective bargaining. The California Legislature agreed, approving an amendment to the ALRA that allowed mandatory mediation if employers and unions are unable to reach a first agreement via good-faith bargaining. Since 2003, employers and their certified unions bargain for at least 180 days to reach a first contract . If they fail, either party can request help from a mediator for an additional 30 days of bargaining. If this mediated effort fails, the mediator can set the terms of an agreement that the ALRB can impose on the parties. Mandatory mediation, which aims to ensure that unions get first contracts quickly, was denounced by growers as a perversion of collective bargaining, whose goal is to allow the parties closest to the workplace to negotiate wages, benefits, and working conditions. Fears that unions would frequently invoke mandatory mediation, to try to gain via mediation what they could not win at the bargaining table, prompted limits on how often it could be invoked; no union could request mediation more than 75 times between 2003 and 2007. This limit proved unnecessary. Mandatory mediation has been invoked seven times in nine years. In two cases, Hess Collection Winery and Boschma and Sons Dairy, a mediator imposed a collective bargaining agreement; in two others, Bayou Dairy and Frank Pinheiro Dairy, the employer went out of business. In Pictsweet, Valley View Farms, and D’Arrigo, the parties reached a collective bargaining agreement during the mediation process.

Angular sensors can also be used in some cases to measure linear velocity

In the marketplace, people generally care more about the sensed quantity and how well the sensor performs for their specific application, while academic researchers and sensor designers are also interested in how the sensor measures the quantity. This section is concerned with the latter. The means by which a sensor makes a measurement is called the transduction mechanism. Transduction is the conversion of one source of energy to another, and all sensors utilize some form of energy transformation to make and communicate their measurements. It should be noted that this is not an exhaustive list of transduction mechanisms. This list only covers a small fraction of the many universal laws describing the conversion of one energy form to another. Rather, this list focuses on transduction principles that describe converting one energy type to electrical energy. This is because all electrical sensors must take advantage of at least one of these mechanisms, and often more. What this list does not cover is transduction from any energy type to another type other than electrical. For example, the thermal expansion principle that governs the liquid-in-glass thermometer example at the beginning of this chapter is not described,plastic plant containers because that sensor operates on the principle of converting thermal energy to gravitational energy. This list also does not include modes of biological or nuclear signal transduction mechanisms for the sake of brevity.A potentiometric sensor measures the open-circuit potential across a two-electrode device, such as the one shown in Figure 1.3C. Similar to amperometric sensors, the reference electrode provides ‘electrochemical ground’.

The second electrode is the ion-selective electrode , which is sensitive to the analyte-of-interest. The ISE is connected to a voltage sensor alongside the RE. The voltage sensor must be very sensitive and have a high input impedance, allowing only a very small current to pass. There are four possible mechanisms by which ionophores can interact with ions: dissociated ion exchange, charged carrier exchange, neutral carrier exchange, and reactive carrier exchange. Dissociated ion-exchange ionophores operate by classical ion-exchange over a phase boundary, in which hydrophilic counter-ions are completely dissociated from the ionophore’s lipophilic sites, preserving electroneutrality while allowing sites for the ions in solution to bind to. Charged-carrier ionophores bond with opposite-charged ions to make a neutrally charged molecule, and the ions with which they bond are determined by thermodynamics and the Hofmeister principle. Neutral carrier ionophores are typically macrocyclic, where many organic molecules are chained together to form a large ring-like shape whose gap is close to the molecular radius of the primary ion. Finally, reactive carrier ionophores are mechanistically similar to neutral carrier ISEs, with the only difference being that reactive carriers are based on ion-ionophore covalent bond formation while neutral carriers are based on reversible ion-ionophore electrostatic interaction. Neutral carrier and reactive carrier ion exchange both are dependent on the mobility, partition coefficients, and equilibrium constants of the ions and carriers in the membrane phase. Some examples of the chemical structures of ionophores are shown in Figure 1.4. Positional sensors are some of the most common in the world, and there are likely several within reach of you as you read this. Smartphones and wearable health devices utilize various sensors to track how many steps you take in a day, the intensity of your workouts, and what route to take home from work. Displacement, velocity, and acceleration can sometimes all be found with a single device, as each quantity is the time-derivative of the prior.

In practice, however, it is common to use separate devices for any of these three measurements because the cost of these sensors is relatively cheap, and it is easy to build systematic errors if the timing mechanism is off. The measurements for displacement, velocity, and acceleration must be made with respect to some frame of reference. For example, consider a group of people playing a game of billiards in a moving train car. Observers on the train platform would assign different velocity vectors to the balls during play than observers on the train. Displacement and angle sensors commonly use potentiometers when the value is expected to be suitably small. A potentiometer transduces linear or angular displacement to a change in electrical resistance. For a displacement sensor, a conductive wire is wrapped around a non-conductive rod, and a sliding contact is attached to the object whose displacement is being measured. A known voltage is supplied across the wound wire, and as the object moves, the sliding contact will make contact with the wound wire, shorting that part of the circuit. Then, the output voltage across the wire is measured, which will be proportional to the amount of the wire shorted by the sliding contact, which is proportional to the object’s displacement. The same principles are applied to measure the angle for a potentiometer operating in angular displacement mode. There are other methods for measuring displacement, but these methods can also be used to measure velocity, as described in the following section. Velocity measurements utilize a variety of approaches ranging from radar, laser, and sonic sensor systems. These types of sensors use one of these modulating signals to send a sound or light wave in a direction and measure the time it takes to bounce off of a surface, return to the sensor, and activate a sensing element that is sensitive to that modulating signal. Using this, the device can calculate the distance between the sensor and the reflecting object by dividing lag time by the wave speed. Then, because these devices often operate at a high frequency, the measurement can be made again, and the change in distance divided by the change in the time between measurements yields a linear velocity.In a car, for example, the speedometer is a linear velocity sensor, but it makes its measurement using an angular velocity sensor on the drive shaft and calculates the linear velocity from the assumed tire size.

Acceleration measurements are most commonly made with accelerometers. Accelerometers are most commonly MEMS devices that are extraordinarily cheap, have a low-power requirement, and utilize the capacitance transduction mechanism. The charged electrode of an interdigitated parallel-plate capacitor structure is vibrated at a high mechanical frequency. Then, when acceleration occurs, if it is perpendicular to the gap between the two capacitor plates, the force from the acceleration will cause the moving electrode of the parallel-plate capacitor to deflect towards the other plate, changing the space of the gap between the two, thereby changing the measured capacitance. The operating principle of most pressure sensors is based on the conversion of a pressure exertion on a pressure-sensitive element with a defined surface area. In response, the element is displaced or deformed. Thus, a pressure measurement may be reduced to a measurement of a displacement or a force that results from a displacement. Because of this, many pressure sensors are designed using either the capacitive or the piezoresistive transduction mechanisms. In each, a deformable membrane is suspended over an opening, such that the pressure on one side of the membrane is controlled while the pressure on the other side is the subject of the measurement. As the pressure on the measurement side changes, the membrane will deform proportionally to the difference in pressure. For a piezoresistive transducer, the membrane is designed to maximize stress at the edges, which modulates the resistance proportional to the deformation. For a capacitive transducer, the membrane is made of or modified with a conductive material, while a surface on the pressure-controlled side of the membrane is also conductive, and the pair act as a parallel-plate capacitor. Then, the membrane is designed to maximize deflection at the center of the membrane,blueberry container thereby changing the electrode gap and capacitance.Practically speaking, a sensing element does not function by itself. It is always a part of a larger ‘sensor circuit’: a circuit with other electronics, such as signal conditioning devices, micro-controllers, antennas, power electronics, displays, data storage, and more. Sensor circuits fit within the broader subject of systems engineering, which is a vast field in its own right. Figure 1.5 shows one possible sensor circuit configuration. Depending on the design of the circuit and which components are included in it, the signal that is output by the sensing element might be conditioned to the specifications of a connected micro-controller, saved onto a flash drive, shown on a display, and sent to a phone, saved on a remote server, or many other possibilities. Rather than discuss all possible sensor systems and circuit designs, we have selected the most common – and arguably most essential – components in any given sensor system and summarized them in this section.

In some form or another, all sensor circuits require power to operate. The components of a sensor circuit that generate, attenuate, or store energy to power the other circuit components are called power electronics. This may include batteries, energy harvesters, and various power conditioning devices. A sensor circuit can be made passive, where there is no energy storage within the circuit. The concept is similar to passive sensing elements described in section 1.2: passive sensor circuits use the naturally available energy to operate. This can be done if the quantity that is being measured can also be harnessed to power the device, such as light powering a photovoltaic sensing element. If there is no passive power generation, power electronics are vital for a sensing circuit’s function. This could be as simple as a coin-cell battery connected to the micro controller’s power I/O pins or as complex as a circuit with multiple energy harvesting and energy storage modalities. A sensor is not a sensor if it does not communicate its measured signal to another person or device. Communication electronics are what fulfill this function. Communication electronics can be wired or wireless. When communicating data to a person, wired communications electronics could be displays or speakers that communicate the data through images or audio. When communicating data to another computer, wired communication electronics come in the form of a ‘bus’, a catch-all term for all the hardware, wires, software, and communication protocols used between devices. At the time of this writing, wireless communications must be between the sensor circuit and another electronic device, though perhaps in future years, technology will develop a way for people to directly interface with wireless data transfer. In the meantime, wireless communications generally incorporate an antenna that attenuates an electrical signal into a directional RF frequency following one of many wireless communication protocols such as WiFi, Bluetooth, or RFID.In science and engineering, ‘error’ does not mean a mistake or blunder. Rather, it is a quantitative measurement of the inevitable uncertainty that comes with all measurements. This means errors are not mistakes; they cannot be eliminated merely by being careful. All sensors have some inherent error in their measurement. The best that one can hope for is to ensure that the errors are minimized where possible and to have a reasonable estimate of the magnitude of the error. One of the best ways to assess the reliability of a measurement is to perform it several times and consider the different values obtained. Experience has shown that no measurement – no matter how carefully it is made – will obtain the same values. Error analysis is the study and evaluation of uncertainty in a measurement. Uncertainties can be classified into two groups: random errors and systematic errors. Figure 1.8 highlights these two types of error using a dartboard example. Systematic errors always push the measured results in a single direction, while random errors are equally likely to push the results in any direction. Consider trying to time an event with a stopwatch: one source of error will be the reaction time of the user starting and stopping the watch. The user may delay more in starting the stopwatch, thereby underestimating the duration of the event, but they are equally likely to delay more in stopping the stopwatch, resulting in an overestimate of the event. This is an example of random uncertainty. Consider if the stopwatch consistently runs slow – in this case, all events will be underestimated. This is an example of systematic uncertainty. Systematic errors are hard to evaluate and sometimes even difficult to detect. However, the use of statistics gives a reliable estimate of random error. In the kingdom of electronics, silicon reigns.