Using Humidity and Temperature Sensors to Improve Sustainability in Greenhouses and HVAC Systems

Controlling humidity and temperature is essential in diverse environments like greenhouses, datacenters, offices, and industrial facilities. In each case, there are negative consequences from excessive or insufficient humidity and temperature, and the efficiency and sustainability of operations can be enhanced by properly controlling humidity and temperature.

Of course, you can’t control what you don’t measure. This article begins with a review of the three ways to measure humidity and looks at the usefulness of each type of measurement; it looks at the relationship between humidity and temperature and considers factors related to the optimal humidity and temperature levels for datacenters, commercial buildings, and greenhouses.

It looks at common communications protocols for humidity, and temperature sensing, then presents humidity and temperature sensors along with a controller from Schneider Electric, including the company’s Green Premium label program designed to optimize sustainability. Those sensors and control can be used together with heating, ventilation, and air conditioning (HVAC) systems to control humidity and temperature at optimal levels for specific environments.

Humidity, it’s not all relative

The basic definition of humidity is the concentration of water vapor in the air. There are three common ways to measure humidity, absolute, specific, and relative:

• Absolute humidity is measured as the mass of water vapor per mass of dry air (usually in grams per kilogram) or the mass of water vapor per volume of moist air (in grams per cubic meter).
• Specific humidity is the ratio of water vapor mass to the total mass of the moist air parcel.
• Relative humidity (RH) is the most common measurement used for environmental controls and indicates the absolute humidity level relative to the maximum humidity possible at the same temperature.

RH is used for environmental controls. It depends on the temperature and pressure of the air. The amount of water vapor needed to achieve saturation (100% RH) increases as the temperature rises — the same amount of water vapor in cooler air results in higher relative humidity. For example, a cubic meter of air at sea level and +8°C can hold about 8 g of water. That same air mass at +30°C can hold about 28 g of water vapor.

The dew point is an important parameter when considering humidity controls. It’s the temperature the air needs to be cooled to (at a given pressure) to achieve 100% RH. If the air is cooled further, water vapor is forced out as liquid.

What’s the right amount of humidity?

The right amount of humidity depends on the needs of the activity. In datacenters, optimizing the environment for the most efficient cooling of the equipment and reliable equipment operation is important. Human comfort is the key in offices and commercial buildings, and in greenhouses, humidity needs to be optimal to enable maximum production. Determining the right level of humidity involves the use of physiology and psychometrics. Psychometrics, sometimes called hygrometry, is the engineering specialty dealing with the physical and thermodynamic properties of gas-vapor mixtures like humid air.

People, plants, and datacenters

People are simple. According to the Occupational Safety and Health Administration (OSHA), the temperature in offices should be +68°F to +76°F (+20°C to +24.4°C) with an RH of 20% to 60%. In residences, the temperature should be set to the comfort of the occupants, with a recommended RH range of 30% to 50%.

Plants are more complicated. The ideal greenhouse temperature is between +64°F and +75°F (+18°C and +23.9°C), depending on what is being grown. The optimal RH for plants is 65% to 75% at night and 80% during the day. Humidity levels outside those recommendations can interfere with plants’ biological processes, resulting in slower growth and/or lower-quality crops (Figure 2).

Figure 1: For optimal plant growth and quality, the RH in greenhouses needs to be different during the daytime and night. (Image source: Schneider Electric)

Enthalpy is a key psychometric consideration in datacenter cooling. It measures the total heat content in a sample of air and is used to determine the amount of fresh outside air that can be added to recirculated air to minimize energy consumption and cooling costs. Enthalpy considers both the dry air and humidity within the air sample and is implemented using psychometric curves that identify when outdoor air can be added to the system to improve cooling efficiency (Figure 2).

Figure 2: Psychometric curves identify how much outdoor air can be added to a datacenter HVAC system to improve cooling efficiency. (Image source: Schneider Electric)

While cooling is the most important consideration for humidity control in datacenters, it’s not the only consideration. If the dew point is reached, it can result in condensation, reducing equipment reliability. Low humidity can result in static electricity and damage sensitive components. Excessive fluctuations in humidity can cause circuit boards to expand and contract and damage circuitry or components. Additionally, some equipment, like tape drives and media, can be sensitive to changes in humidity as low as 5% an hour.

Communicating the humidity and temperature

Humidity and temperature sensors communicate with the HVAC system controller using several common technologies. Depending on the system architecture, the most common options include the following:

• Humidity and temperature can be communicated using a 4 to 20 milliampere (mA) analog current loop, a well-established and robust sensor communications standard. The signaling current is unaffected by voltage drops in the communications loop if the supplied voltage is greater than the sum of the voltage drops around the loop at the maximum 20 mA signaling current.
• Temperature can be measured and communicated using negative temperature coefficient (NTC) thermistors whose resistance decreases as their temperature increases. The resistance can provide accurate analog temperature measurements with proper calibration at a set temperature, usually +25°C.
• Humidity and temperature can be communicated digitally using Modbus RS485 half-duplex remote telemetry unit (RTU) communications. This protocol communicates serial data over a two-wire link like the connections used for a current loop, making it easy to implement.

Sensing the environment

Schneider Electric offers facilities designers a selection of humidity and temperature sensors with various communications protocols to suit specific system architectures. These sensors are designed for use in demanding environments. They have an ingress protection (IP) rating of IP65 and are dust-tight, allowing no ingress of dust, and are protected from water jets from any direction. They are rated for operation from -40°F to +140°F (-40°C to +60°C) and up to 100% RH (Figure 3).

Figure 3: The TM1SHTCC4 humidity and temperature sensor has an IP65 ingress protection rating. (Image source: Schneider Electric)

These sensors include:

• The TM1SHC4 humidity sensor with a 4 to 20 mA current loop output and an accuracy of ±3% RH.
• The TM1SHTCN4 humidity and temperature with a 4 to 20 mA current loop for the humidity sensor with an accuracy of ±3% RH and an NTC temperature sensor (10 kiloOhms at +25°C) that can measure from -40°F to +140°F (-40°C to +60°C) with an accuracy of ±1°C.
• The TM1SHTCC4 humidity and temperature sensor with a 4 to 20 mA current loop for the humidity sensor with an accuracy of ±3% RH and a 4 to 20 mA current loop for the temperature sensor with a measurement range of -40°F to +140°F (-40°C to +60°C) and an accuracy of ±1°C.
• The TM1SHTM4 humidity and temperature sensor with an RS485 Modbus RTU output and a humidity measurement of ±3% RH and a temperature measurement range of -40°F to +140°F (-40°C to +60°C) and an accuracy of ±1°C.

Controlling the environment

These humidity and temperature sensors are designed to be used with Schneider Electric’s Modicon M171 and M172 logic controllers and I/O modules like the TM171EP27R designed for analog inputs, including current loops and NTC thermistors. The TM171EP27R enables monitoring and control of large areas and features a total of 27 inputs and outputs, including:

• Discrete outputs for signals
  • 3 relay outputs single pole single throw (SPST) with same common
  • 2 relay outputs SPST with same common
  • 2 relay outputs single pole double throw (SPDT) with independent common
• Discrete output for high currents
  • 8 amperes (A) relay SPDT
  • 5 A relay SPST
• Analog inputs
  • 2 analog input NTC
  • 4 configurable
• Analog outputs
  • 3 voltage or current of 0 to 10 V or 4 to 20 mA
  • 2 open collectors or current 1 kHz, 12 V, 35 mA, or 4 to 20 mA

The TM171EP27R features IP20 degree of protection, an operating temperature range of -20°C to 55°C and is designed for use in a range of environmental control applications, including (Figure 4):

• Air handling unit
• Air/water-cooled chiller
• Compressor rack
• Heat pump
• Heat recovery unit
• Precision air conditioner
• Rooftop unit
• Ventilation unit

Figure 4: The TM171EP27R has 27 inputs and outputs and an IP20 protection rating. (Image source: Schneider Electric)

Green Premium for improved sustainability

Modicon HVAC controllers like the TM171EP27R discussed above meet Schneider Electric’s Green Premium label requirements. Green Premium solutions use a combination of the Internet of Things (IoT) and digital tools to use energy and other natural resources efficiently. They enable facilities engineers to reduce the environmental impact and minimize the CO₂ footprint. Key features include:

Resource performance – Green Premium labeled devices improve resource efficiency throughout their life cycle. Resource efficiency includes the efficient use of energy and the minimization of CO₂ emissions, water, air, and other natural resources. To achieve the optimal outcome, Schneider Electric uses Life Cycle Assessments and provides users with comprehensive and verified environmental footprint information about each product.

Circular performance – Schneider Electric helps customers optimize the total cost of ownership of their assets. The company provides IoT-enabled services and upgrade, repair, retrofit, and remanufacture opportunities. Green Premium offers digitally available ‘Circularity profiles,’ giving users information on the product’s circularity, end-of-life instructions, and available take-back services.

Summary

Controlling humidity and temperature is essential in various agricultural, commercial, datacenters, and other environments. The optimal RH is based on different factors depending on the facility being considered. Humidity and temperature must be monitored in real-time to ensure optimal conditions and maximize operational sustainability.