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Temperature and Relative Humidity Sensor

A temperature and relative humidity (T&RH) sensor is a simple, durable and low-cost type of electronic hygrometer used in building science. Commonly available models use thin-film polymer technology. The dielectric properties of the polymer film changes with the change with ambient air relative humidity, and the capacitance of the sensor changes correspondingly.

The T&RH sensor directly measures temperature and relative humidity and calculates other psychrometric properties. Unlike dewpoint hygrometer which measures dewpoint, its relative humidity readings are significantly affected by ambient dry bulb temperature. Therefore, it is important to maintain accurate temperature readings on a field test setting. Certain things such as condensation on the sensor probe, direct sunlight, body heat and humidity, non-representative sources of heat or stagnant and/or non representative air samples need to be avoided.

Image of a temperature and relative humidity sensor and three related measurement sticks. Each stick has a sensor measurement end on it and wiring on the back that connects to the hand-held sensor computer.

T&RH sensor specification on operating temperature ranges vary depending on application types such as ducted or wall-mounted, indoor or outdoor application types. The widest range is between -40 to 60°C and 0 to 100% RH. T&RH sensor accuracy in general varies from ±1% to ±3% RH and ±0.2°C to ±0.6°C as common depending on the operating temperature range. However, for every 1% RH at 20°C the dewpoint temperature varies by less than 0.2°C. A device with an accuracy of ±2% RH and ±0.2°C is able to measure dewpoint temperature to better than ±0.5°C.

 

 

Image of a Mini-Split system console with multiple temperature and relative humidity sensor wires attached to the exterior grid wiring. Wires are taped to the bottom of the device to keep them in place and a hand is holding one wire end at a connection point along the front.

T&RH Sensor Used On Mini-Split System
Field Test [1]

It is recommended for researchers to purchase T&RH sensors and extension wires with right application and lowest uncertainty, unless pre-test uncertainty analysis indicates otherwise. These sensors come pre-calibrated with manufacturer listed total uncertainty limit. But overtime, the readings will drift and lab and in field calibration are required. Good records of T&RH calibration, spot-checking and maintenance need to be kept. It is common in a field test setting to do some salt based spot checking on high and low RH setting such as 11% and 75% RH.

Indoor Aspirated Shield

Sometimes making temperature measurements can be a challenge. Thermocouple measurements can be biased due to undesired heat transfer effects (i.e. a nearby hot surface). Further, a thermocouple only measures at one point in space. To get around these limitations, you can use an aspirated shield. This assembly is useful for protecting sensors while measuring outdoor temperature or estimating a room’s average air temperature.

Outdoor Aspirated Shield

Outdoor temperature and humidity are most often measured with a combined temperature and relative humidity sensor that is housed in an aspirated shield so that it does not artificially heat up in the sunlight. Outdoor aspirated shields have a different geometry than an indoor aspirated shield for temperature measurements, but have the same purpose.

 

Reference:
1. Christensen D. et. al. Field Monitoring Protocol: Mini-Split Heat Pumps, NREL report No. TP-5500-49881; DOE/GO-102011-3227.