April 2005
  
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Higher Performance RH Transmitters

 

Growing concern about indoor environmental
quality issues, including mold, mildew and
moisture problems also reinforce the need for
accurate humidity sensing devices.

  Louis Viglione, Eng
Viconics Electronics Inc.

Advantages of Specifying Higher Performance RH Transmitters for HVAC Systems

The accuracy of humidity transmitters applied to building control systems can significantly impact energy use in HVAC systems. Growing concern about indoor environmental quality issues, including mold, mildew and moisture problems also reinforce the need for accurate humidity sensing devices. Achieving maximum benefit from increased accuracy should be one of the key criteria in the selection of HVAC relative humidity transmitters. Electronic and micro-controller-based transmitters are available from many manufacturers with 5%, 3%, 2% (and a few offer more expensive 1% accuracy products). Generally, tighter accuracy commands higher prices due to increased component, manufacturing and calibration costs. How really important is accuracy? What is the real impact of higher accuracy devices on installation, health and operational costs? Before we can answer these questions, we should first look at the types of sensors that are available and the factors that influence actual sensing and control accuracy in actual applications.

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Humidity transmitters consist of a sensor as well as a transducer that converts the sensor reading into an output signal. For HVAC transmitter applications, there are basically two common categories of sensing elements used to convert relative humidity measurements into electrical signals: capacitive sensors and bulk-polymer resistive or impedance based sensors. These electrical signals are then linearized, temperature compensated and calibrated before being converted to 0-10V or 4-20mA signals by the associated electronics that feed into BAS systems or humidification control equipment directly.

Each type of sensing element has advantages and weaknesses. Capacitive sensors tend to be nearly linear devices that exhibit low temperature dependence, however, the much lower sensitivities of these devices place a strain on the sensing electronics. Bulk polymer sensors on the other hand, feature sensitivities tens of thousands of times more than their capacitive counterparts however these are non-linear devices that feature higher temperature dependence. In the end, what is important is the overall RH transmitter accuracy and not just that of the sensing element.

In operation, the actual accuracy of a typical, 3% RH transmitter can be significantly worse than the manufacturer’s general 3% accuracy specification. By closely examining some of the manufacturers’ specifications we can find linearity, temperature compensation, hysterisis and drift can all add to the losses in accuracy from the general accuracy specification. General accuracy is typically specified at 25°C (77°F), and typically ranges from 2 to 5%. Linearity is the deviation between the transmitter’s actual signal at various RH points across the operating range (typically 10 to 90% RH), and linearity errors can range from 1 to 3 %. Generally speaking the higher the linearity the more accurate the transmitter and a few manufacturers offer multi-point calibration and linearization. Errors associated with temperature compensation typically range from 1 to 3 % and vary from device-to-device or the actual electronics interface. Hysterisis errors are due to physical phenomena with the sensing elements that cause the error in measurement when the same humidity condition is approached from the lower end then from the higher end.  Hysterisis errors can range from as much as 1 to 2%. Drift is caused by a shift in sensor characteristics with time due to aging, and can range from 1 to 2% per year.

If we combine all these errors, in actual operation over typical temperatures and operating ranges, a 3% accuracy transmitter typically yields 4 to 6% overall accuracy. Studies have also found that worst-case accuracy can often exceed 8%. A diligent review of manufacturer specification is essential during the selection process, since not all 3% device will perform equally.

How does this inaccuracy impact on operational costs? The following analysis, looks at the operating cost for humidification of an average 50,000 sq. ft. commercial building located in northern mid-western USA, shows the “effective cost of accuracy”.

Building size: Approximately 50, 000 Sq Ft. facility housing 200-250 employees
Design conditions: 72 F / 40% RH was maintained for a period of 2000 Hours.
ASHRAE minimum fresh air standards (15cfm/sq. ft.)
Mean outdoor temperature during humidity season 0 °F (32°C)
Humidity load approximately 100 lbs./Hr of steam was introduced.
Mean humidity: 15% RH (25%RH controlled to attain setpoint)

Cost calculation:

34 KW (Electric steam humidifier)
2000 Hrs running time
10 cents/KWH (national average USD)

Annual cost:

$6800

Cost / % accuracy:

$272 per 1% humidity error added annually (based on mean of 15% and 25% controlled

Based on the above example a humidity transmitter off by as much as 8% effectively cost the building owner up to $2,176 annually. With a small increase in up front investment and a more careful selection of humidity sensing equipment the owner would have had a larger opportunity for cost savings with increased user comfort. A similar argument and analysis can also be made for dehumidification applications in warmer climates.

Aside from the operational costs, there are other very important, documented factors to consider. Indirect costs associated with health and comfort, productivity, damage to the building envelope, mold, equipment and static, are all difficult to quantify, and vary from building to building, and can often be many times the direct operational cost!

To achieve accuracy, manufacturers must address these issues by offering cost-effective, latest generation RH transmitters with improved performance that feature microcontroller-based devices with multipoint calibration, improved temperature compensation and low-drift sensors. Selecting and specifying the best humidity transmitter, with the tightest possible accuracy within budget constraints, can have a huge impact in reducing long term direct operating costs, indirect costs (mentioned previously) as well as improving overall energy efficiency. An added environmental advantage is the reduction in greenhouse gas emissions. By careful selection, investing a mere $10-20 more for a better quality RH transmitter can go a long way. The almost instantaneous return on investment makes very good business sense that would be certain to convince most building owners.

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