BAS design issues
We know it's tough out there for BAS designers who want to keep up with the latest in sensors and controls. You know that sensors and controls are “where the rubber meets the road” in HVAC. But the available hardware and software mutate faster than an H1N1 flu virus on steroids. Furthermore, you have to juggle the needs, wants, knowledge, and capabilities of the building owner, the controls contractor, probably a commissioning agent if it's a large project—and of course your own associates.
What's a designer to do? Well, one thing you could do is check out the views of several experts to gain a broader perspective on the current status of the controls industry.
Don't give up the (owner)ship
A majority of our interviewed experts agreed that, tempting as it may be, a designer cannot afford to assign total responsibility for controls to a subcontractor. Bruce Westphal, vice president of R&D for KMC Controls , put it this way: “One reason you might assign responsibility to the controls subcontractor is if you had a really good experience with a particular subcontractor and trusted that person. But my feeling is that the designer shouldn't leave responsibility for controls to the subcontractor because the designer needs to maintain quality control of the installation. Furthermore, if it doesn't work, the designer will be part of the lawsuit!”
Steve Tom, director of technical information at Automated Logic Corp . (ALC), added that the designer should pay attention to controls design simply because the designer is the one who knows what the owner's requirements are—how closely temperature and humidity need to be maintained—and who knows what trade-offs can be accepted. “For example,” said Tom, “it has to be the designer who makes the call on whether to use a standard industry control or to go with a high-end product. The controls contractor won't know how to make that call. It also depends on the type of contract you have.
“My background is military with flat-bid specs,” said Tom. “You weren't allowed to talk to the contractor; so the contractor was trying to come up with the lowest bid. And that makes it even more critical for the designer to have a great deal of information in the spec about controls. Contractors can't afford to provide anything better than what you require, because if they do, they probably won't get the project. As a result, you're going to get absolutely what you specify—and nothing more; so you need to be very clear on what you specify.”
Mark Hydeman, an ASHRAE fellow and a principal in the Alameda, Calif.-based design firm Taylor Engineering , tied the decision to energy efficiency. “Owners, design engineers, and contractors have different definitions for what constitutes a successful job.” said Hydeman. “The classic example is that you can maintain tight conditions using constant volume reheat and get very good levels of comfort, but that's not necessarily a good thing from the standpoint of energy conservation. With the competition to win jobs and the motivation to severely underbid, subcontractors just don't have the time to provide the necessary level of diligence.”
Ken Gillespie, a building energy systems technologist for Pacific Gas & Electric in California said there needs to be shared responsibility in every project because each person is likely to have specific knowledge needed to make important decisions. “The programmer needs to know how to tune a loop because the designer is not going to know how to do it. But the sequence needs to be developed so that it's tunable in the first place—so that it's actually functional,” said Gillespie.
Terry Hoffmann, director of BAS marketing for Johnson Controls Inc ., wants a place at the table for the equipment manufacturers. “When there's a performance-based contract, or there's a need to integrate a lot of different systems, we think the manufacturers are best involved,” said Hoffmann.
Blame game, anyone?
If it's true that designers need to be hands-on when it comes to defining controls in the more complex HVAC systems, it may seem that the problem is a simple matter of educating mechanical engineers on controls and sensors. But as H. L. Mencken once famously said, “For every human problem, there is a solution that is simple, neat, and wrong.”
Gillespie said a big part of the problem is that many designers don't want to become knowledgeable about controls technology. He often encounters an attitude of, “I've got my professional engineer license; I don't really need anything else.” Furthermore, he thinks there's a tendency for designers to leave controls to subcontractors because inattention to detail in writing the spec generally results in higher-cost systems.
Gillespie said, “Amory Lovins [founder of the Rocky Mountain Institute , an energy think tank] wrote a paper in 1991 about the disincentive for quality in the building industry. That exists because consultants are paid a percentage of total cost; so, to make more money, everyone wants a bigger project. If you come up with a great idea for saving energy and construction cost, there's no incentive to apply it because it will reduce your profit.”
Gillespie thinks building owners often are to blame. “I call it the least-cost disease.” he said. “Designers and contractors bid without expertise, and it's hard for an owner to know enough to eliminate them from the bid list, so owners look mostly at cost. That's where commissioning came in, as an attempt to bring value back into the building process. One approach to eliminate the poor-design and poor-construction firms is the RFQ (request for qualifications) method where the owner has a preselected group of contractors known to have the expertise. But it's very difficult for many owners—including some government entities—to do that. The feds had to struggle for a long time to win the right to do it.”
So are we doomed to failure by the essential nature of our capitalistic system? Not everyone thinks so. In fact, to hear Bob Schultz tell it, the profit motives of owners will be our salvation. Schultz, chief applications engineer at TAC Americas , sees a new dawning of hope in America. “For the first time,” he said, “with LEED and green building, we're starting to see owners who think beyond simple systems. Our industry is really great at ripping out pneumatics and putting in direct digital controls, so owners get a simple control system with minimal data collection and minimal alarming.
“That's where we've been for the last 15 to 20 years.” said Schultz. “The people we hire are very technician-oriented wire pullers and board installers. They're not sophisticated when it comes to strategy and manipulation of data. But in the last 12 to 24 months, all of a sudden I have this sense that things are changing. I had a customer in the other day. He didn't want to talk about basic control systems. He wanted to talk about knowing where the energy is going and how he could translate it into the number of trees that would need to be planted to replace it. He wanted to have competitions among the students living in his dorms on how they could reduce their energy use. He wanted to be more involved in looking at the data and seeing what's going on!”
Specs and sensors
According to Hydeman, the primary elements to specifying a control system are the points list and the control sequences. He said, “If you just have those two items, you can put out the bid and have a system installed.
“Behind that, you have to know which sensors really matter,” said Hydeman. “I call them the 'money sensors.' They're things like the condenser water supply temperature sensor in a cooling tower. A couple of degrees off on that can cost you anywhere from thousands to hundreds of thousands of dollars in energy costs. And if it's a data center, it can cost you that much in terms of reliability. Some other examples of money sensors are the high-limit switches on economizers, CO2 sensors at the zone level, and humidity sensors. These are all sensors that—if they drift over time—experience hysteresis (instability) or exhibit long-term aging, they can cause energy efficiency, health, or safety problems and compromise the system. That's why they're the money sensors, and that's why designers need to know about them.”
Gillespie thinks green system designers need to pay careful attention to flow meters. “If you're doing a project that aims to save energy, you have to install a certain amount of instrumentation,” he said. “A lot of engineers are afraid of insertion turbines, and they should be. Insertion turbines are OK in clean lines. I've had one in service for years—a dual-turbine model that has nylon turbines. But if you have dirty lines, lines that are exposed to the outside air, they're going to be a problem. There are lots of particulates and junk in a cooling tower, so they get plugged. That's where you go to a full-bore mag meter or an insertion mag meter. It used to be that they just weren't cost-effective, but now they are. They've become the popular choices, but you're talking about a sizable investment in an instrument.”
Gillespie also said any designer who wants to save energy for an owner needs to pay attention to the IT aspects of the spec. “You may want to sample every 5 min on a regular basis, but to do trending and diagnostics, you probably want to sample once a minute or even more frequently. With all that, you need substantial controller memory to archive a few days' worth of data and data storage to store and retrieve a year's worth of data. So, in designing your system, you need to pay attention to the architecture that exists around the controllers, which is an IT issue.”
In addition, the interviewed experts agreed that a successful spec must include details about the accuracy, allowable drift, and recommended calibration frequency for critical sensors. Table 1 has Gillespie's recommendations for HVAC sensor accuracy requirements. And for unusual conditions, such as clean rooms, saltwater environments, or high humidity, the sensor enclosure type and accuracy also should be specified. Even in normal conditions, if a sensor is to be mounted inside a drop ceiling functioning as a return air plenum, the sensor must be specified as all-metal—and no plastic—to meet fire codes.
ALC's Tom helped create a tool based on ASHRAE Guideline 13 called Control Spec Builder ( www.ctrlspecbuilder.com ), which won Consulting-Specifying Engineer's Product of the Year Award for controls in 2005. It goes beyond ASHRAE 13 and gets into the effects of specifying the control hardware and software. It can prepare sequences of operation for typical control systems and will print the points list and schematics.
“Because so many mechanical engineers don't know how to write controls specifications, they turn to their local controls vendor,” said Tom. “Naturally, the vendor writes the spec around their own products. So we decided to make Control Spec Builder generic rather than just putting out another proprietary spec. I won't pretend it's a cure-all, but it's a good starting point.”
CSI MasterFormat Division 25 for Integrated Automation, available from the Construction Specifications Institute at www.csinet.org , is another available spec-writing tool. It allows you to create a comprehensive specification for HVAC, lighting, security, and fire suppression.
But Tom isn't sure that's always the best way to go. “I see the need for those systems to share information, but that's easily handled by standard protocols like BACnet,” said Tom. “That's different from trying to have one system that does all those things bundled together. The problem I see is that it's not the way the industry generally works. On most projects, you'll have an electrical contractor, a separate contractor for the mechanical system, and possibly a third contractor for the control system. It often works better to have separate systems that share information but are not the same system.”
Several of the interviewed experts discussed when and where wireless sensors should be used—and if designers should worry about security issues with wireless systems. Though some warned of the potential for hacking into wireless systems, Hydeman, who has designed several such systems for data centers, extolled their virtues. “In a wired system, delivery of control points costs about $1,000 to $1,500 each, installed,” he said. “But the wireless sensors are coming in around $30 to $100 per point. So it allows you to do things you could never do with wired sensors, like have a sensor on each rack, which is great for energy efficiency and also for tuning the system.”
However, he said such a density of wireless sensors creates a maintenance problem with the need to change batteries, so going wireless is probably less practical in office buildings.
On the other hand, Mark Ziolkowski, director of HVAC components at Johnson Controls, says his company offers wireless room sensors and that they have optimized the data transmission rate so that batteries can have up to a 7-year life. But he allows that changing batteries becomes a tough sell when sensors are located in ductwork or in the space above a drop ceiling that is functioning as a plenum. He sees a potential solution to the maintenance issue in energy harvesting.
“We're investigating energy harvesting techniques, but there are some challenges, said Ziolkowski. “For instance, some sensors, like humidity sensors, require a significant amount of power, but energy harvesting techniques can't supply that type of power yet. Another challenge is when systems sit idle for extended lengths of time, harvesting energy isn't feasible; meanwhile, the need to constantly monitor temperature continues.”
Ziolkowski said another difficulty with wireless sensors is the lack of an industry wide protocol, though ZigBee is showing signs of eventually winning that distinction in the commercial building space.
What about pneumatics?
The interviewees were asked if they see any place for pneumatic controls for new construction. Most said pneumatic controls are a thing of the past. But Hydeman thinks there's still a place for them. “It used to be that you could do much faster actuation with pneumatic controls, so lab valves were pneumatic.” he said. “But now they have fast-acting electronic actuators that are pretty cost competitive. You can also get a lot more pressure out of a pneumatic valve. But the real beauty of pneumatics is that they're the true plug-and-play device. There is just one protocol (air pressure), and everybody's equipment can be hooked together. Also, people who know pneumatics can maintain the systems from one end to the other.”
A Microsoft controls protocol?
One thing nearly all interviewees commented on is that the protocol wars are a major impediment to progress in HVAC controls. We heard numerous stories of owners who were shocked to learn that vendor A's add-on system cannot talk with vendor B's legacy system. And not even systems implemented with open standard protocols are plug-and-play.
Schultz was one of several interviewees who pointed to the recent acquisition of Richards Zeta by Cisco Systems as a major event.
“Wow! That's a wake-up call,” said Schultz. “Our industry has been moving from the mechanical room up into the IT arena in the last 20 years. But Cisco has started at that IT level, and it looks like they're moving down into our turf right now. What happens if Microsoft decides to throw a few billion dollars at this? I can see Microsoft saying, 'We need one programming language for field-level controllers to be used by everyone, and, oh by the way, we, Microsoft, are going to sell that, and it's going to be in every DDC controller for the next 50 years.' That could turn the industry upside down.”
The road to a green future
For the moment, however, it appears that the fundamentals of good HVAC design are the same as they've been for some time. Gillespie describes Jay Santos and Schultz's five-day controls course. “Jay teaches that you need to develop an overall controls strategy for your facility or campus. You need a strategy that comes to grips with the kinds of products you want, the skill set of your operators, how much training you're going to provide them, who's going to maintain the system, and what level of performance monitoring you're going to require. These are all issues that the owner needs to address. The days of 'set it and forget it' are quickly drawing to a close. You need to have a clue about what your equipment is doing, and you aren't going to have a clue about it without spending some money. I think 'green' should be about the owner wanting to know.”
We agree. But the most obvious reason an owner will want to know what's going on in his or her HVAC system is to control the bottom line. That's why the road will lead to a green future only if designers become knowledgeable enough to convince owners of the value of paying attention to controls. The designer who can deliver understandable and actionable data is the designer who will get the job today and in the future.
Table 1: Through-system measurement accuracy requirements
<table BORDER = '0' CELLSPACING = '1' CELLPADDING = '3' ID = 'id1536057-96-table'><tr ID = 'id1537020-163-tr'><td ID = 'id1537022-163-td' CLASS = 'tfoot' COLSPAN = '2'>*Editor's note: Using an average heat content of the gas to convert to kBtu introduces a ~2% error.
Source: “A Specifications Guide for Performance Monitoring Systems,” by Kenneth Gillespie, et al. Published March 3, 2007, by the California Energy Commission's Public Interest Energy Research (PIER) and developed in collaboration with the U.S. Dept. of Energy. Downloaded June 1, 2009, from cbs.lbl.gov.
</td></tr><tbody ID = 'id1536066-96-tbody'><tr ID = 'id1536068-96-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536074-96-td' CLASS = 'table'>Measurement point or metric</td><td ID = 'id1536080-97-td' CLASS = 'table'>Accuracy required</td></tr><tr ID = 'id1536231-99-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536237-99-td' CLASS = 'table'>Outside air temperature (F)</td><td ID = 'id1536243-100-td' CLASS = 'table'>0.2 F</td></tr><tr ID = 'id1536250-102-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536256-102-td' CLASS = 'table'>Outside air wet bulb temperature (F)</td><td ID = 'id1536261-103-td' CLASS = 'table'>0.2 F</td></tr><tr ID = 'id1536268-105-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536275-105-td' CLASS = 'table'>Zone temperature (F)</td><td ID = 'id1536280-106-td' CLASS = 'table'>0.5 F</td></tr><tr ID = 'id1536287-108-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536294-108-td' CLASS = 'table'>HVAC electric only energy use (kWh)</td><td ID = 'id1536299-109-td' CLASS = 'table'>1.5% of reading</td></tr><tr ID = 'id1536306-111-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536312-111-td' CLASS = 'table'>Water temperature (F)</td><td ID = 'id1536318-112-td' CLASS = 'table'>0.1 F, if > 5 F delta-T</td></tr><tr ID = 'id1536324-114-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536331-114-td' CLASS = 'table'>Water delta temperature (F)</td><td ID = 'id1536336-115-td' CLASS = 'table'>2% of reading</td></tr><tr ID = 'id1536343-117-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536350-117-td' CLASS = 'table'>Water flow (gpm)</td><td ID = 'id1536355-118-td' CLASS = 'table'>2% of reading, > 20-1 turndown</td></tr><tr ID = 'id1536362-120-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536368-120-td' CLASS = 'table'>Natural gas flow (scfm)</td><td ID = 'id1536374-121-td' CLASS = 'table'>2% of reading, > 10-1 turndown, with pressure and temperature compensation*</td></tr><tr ID = 'id1536381-123-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536388-123-td' CLASS = 'table'>Air flow (cfm)</td><td ID = 'id1536393-124-td' CLASS = 'table'>5% of reading down to 150 ft/min, > 10-1 turndown</td></tr><tr ID = 'id1536400-126-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536406-126-td' CLASS = 'table'>Power (kW)</td><td ID = 'id1536412-127-td' CLASS = 'table'>1.5% of reading</td></tr><tr ID = 'id1536419-129-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536425-129-td' CLASS = 'table'>Chiller cooling output (tons)</td><td ID = 'id1536431-130-td' CLASS = 'table'>3% of reading</td></tr><tr ID = 'id1536438-132-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536444-132-td' CLASS = 'table'>Chiller cooling energy (ton-hrs)</td><td ID = 'id1536449-133-td' CLASS = 'table'>3% of reading</td></tr><tr ID = 'id1536860-135-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536866-135-td' CLASS = 'table'>Boiler heating output (kBtu/hr)</td><td ID = 'id1536871-136-td' CLASS = 'table'>3% of reading</td></tr><tr ID = 'id1536877-138-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536883-138-td' CLASS = 'table'>Boiler heating energy (kBtu)</td><td ID = 'id1536888-139-td' CLASS = 'table'>3% of reading</td></tr><tr ID = 'id1536895-141-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536901-141-td' CLASS = 'table'>Electric energy use (kWh)</td><td ID = 'id1536906-142-td' CLASS = 'table'>1.5% of reading</td></tr><tr ID = 'id1536912-144-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536918-144-td' CLASS = 'table'>Total HVAC energy use (kWh) (includes air side, water side and natural gas)</td><td ID = 'id1536924-145-td' CLASS = 'table'>3% of reading</td></tr><tr ID = 'id1536931-147-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536937-147-td' CLASS = 'table'>Chiller performance (kW/ton)</td><td ID = 'id1536942-148-td' CLASS = 'table'>4% of reading</td></tr><tr ID = 'id1536948-150-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536954-150-td' CLASS = 'table'>ChW plant performance (kW/ton)</td><td ID = 'id1536960-151-td' CLASS = 'table'>4% of reading</td></tr><tr ID = 'id1536966-153-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536972-153-td' CLASS = 'table'>Total boiler performance (kBtuo/kBtui) (COP)</td><td ID = 'id1536977-154-td' CLASS = 'table'>4% of reading</td></tr><tr ID = 'id1536983-156-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1536990-156-td' CLASS = 'table'>Total air handler performance (kW/cfm)</td><td ID = 'id1536995-157-td' CLASS = 'table'>6% of reading</td></tr><tr ID = 'id1537001-159-tr' VALIGN = 'middle' STYLE = 'background-color: #eeeeee'><td ID = 'id1537007-159-td' CLASS = 'table'>Net usable building floor area</td><td ID = 'id1537012-160-td' CLASS = 'table'>2%</td></tr></tbody></table>
<table ID = 'id1512996-0-table' CELLSPACING = '0' CELLPADDING = '2' WIDTH = '100%' BORDER = '0'><tbody ID = 'id1513006-0-tbody'><tr ID = 'id1513008-0-tr'><td ID = 'id1513010-0-td' CLASS = 'table' STYLE = 'background-color: #EEEEEE'> Author Information </td></tr><tr ID = 'id1512829-3-tr'><td ID = 'id1512831-3-td' CLASS = 'table'> Kronick is a Minneapolis-based freelance writer specializing in engineering and architecture. He is also a writing trainer who has presented more than 1,000 business writing and technical writing seminars on four continents. Ivanovich has been the editor-in-chief of Consulting-Specifying Engineer since 2007 and has a master's degree in building systems engineering from the University of Colorado at Boulder. </td></tr></tbody></table>
Learn more—take classes
Whether it's an owner's profit motive, a sense of professionalism, or an altruistic wish to address environmental issues that eventually motivates a designer to become more knowledgeable about HVAC control systems, there are plenty of educational opportunities out there. PG&E's Ken Gillespie is one of several of those interviewed who would like to send all designers to a five-day controls course taught by Jay Santos, PE, principal, Facility Dynamics Engineering, and Bob Schultz. The course is presented by the University of Wisconsin at Madison. Gillespie also recommends other sources. For example, he's on the advisory board of a program at Laney College in Oakland, Calif.
“With a National Science Foundation Grant, they've built a lab there with equipment from several control manufacturers,” said Gillespie. “They're focusing on controls and systems, and they're getting engineers to take the classes. At a recent curriculum meeting, I was stressing the importance of instrumentation—how a sensor works and what's important to know about it. [I also teach] how to do energy-savings calculations, and how that information can be used in the control system.”
Gillespie also recommends courses offered by UC Berkeley Extension and PG&E's Pacific Energy Center, some of which are taught by Mark Hydeman and his associates in the design firm of Taylor Engineering. One of those courses is on fundamentals of HVAC controls. “The wonderful part of that course,” said Hydeman, “is that we have controls contractors, building owners, building operators, mechanical contractors, and consulting engineers all sitting in the same room. And the reason I've taught that for seven years is that I learn something every time I sit down with this group. It's a great exchange of ideas, which is what this industry is about.”
Besides classroom education, several of the people interviewed for this article stressed the value of ASHRAE's publication 13-2007, “Specifying Direct Digital Control Systems.” The document covers all aspects of defining DDC and includes a sample specification outline. Another often-recommended information source is the a website maintained by the Iowa Energy Center, which contains an “Introduction to DDC” (Mark Hydeman said “I have all the students in my classes read it.”) and an “I/O Tutorial.” Interviewees also recommended the following:
“Technology Assessment of Emerging Advanced Building Control Strategies” (Application Assessment Report # 0725: www.etcc-ca.com/images/stories/demand-based_building_controls1.pdf ), a report prepared for PG&E's Emerging Technologies Program; issued: April 12, 2009. This concise 29-page document lists strategies for greatly improving the energy efficiency of any BAS.
“Specifications Guide for Performance Monitoring Systems,” by Kenneth L. Gillespie Jr., et al. ( cbs.lbl.gov/performance-monitoring/specifications/pdf/PM%20Spec%20Guide%20Version%201_2007-03-23.pdf ), which contains complete sample specifications for basic, intermediate, and advanced performance monitoring systems.
National Building Controls Information Program, administered by the Iowa Energy Center ( www.energy.iastate.edu/Efficiency/Commercial/nbcip.htm ), which contains numerous studies, two of which provide eye-opening data on the lack of consistency in currently available humidity sensors.
Utah State University's Utah Water Research Laboratory ( uwrl.usu.edu/researchareas/index.html ), which contains studies on flow meters.
Colorado Engineering Experiment Station Inc. ( www.ceesi.com ), which contains studies on flow meters.
Lawrence Berkeley National Lab's page on ventilation studies ( eetd.lbl.gov/ie/viaq/v_pubs.html ).
Honeywell's Automation and Control Solutions division recently published free, downloadable software called Demand Control Ventilation Savings-Estimator ( customer.honeywell.com/economizertools ). The tool helps engineers show owners the energy-savings potential of economizers.
The Energy Business Intelligence website ( www.esource.com/public/products/cec_form_send.asp ) managed by the State of California's Public Interest Energy Resource (PIER) contains numerous research reports on controls as well as related HVAC topics.
The Control Design Guide, which is part of the Functional Testing Guide offered by Portland Energy Conservation Inc., at www.peci.org/ftguide . Chapter 3 looks at sensor accuracy and installation issues in detail.