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Temperature Sensors

Apogee temperature sensors use high precision thermistors to measure air and soil temperatures.  We offer a few different products which vary in design and purpose, including the Radiation Frost Detector, and two Thermistor Temperature Sensors.

SF-110 Radiation Frost Detector from Apogee InstrumentsRadiation Frost Detector

The SF-110 is a unique temperature sensor designed to predict the temperature of fruit buds and leaves rather than the air temperature.  Because this difference can be 4 to 8 C (8 to 15 F), a standard air temperature sensor in a multi-plate radiation shield can fail to alert growers even when fragile flower buds are freezing.  Growers are aware of this inaccuracy, and often overcompensate by activating frost protection fans and heaters when they are not necessary.  In large orchards this cost can be $200 for every minute of operation.  Integrating the SF-110 radiation frost detector into your crop protection system saves money by eliminating the guesswork in timing the activation of orchard frost protection.

The SF-110 is a combination of two temperature sensors (precision thermistors) in a single housing. One sensor is designed to mimic a plant leaf and the other a flower bud. The SF-110 provides close approximations to leaf and bud temperatures and can be used for prediction of frost on leaves and buds.

On clear, calm nights, leaf and bud temperature can drop below freezing even if air temperature remains slightly above 0 °C (see Figure 1 below). This is called a radiation frost and is due to the lack of air mixing (wind) near the surface, and a negative net longwave radiation balance at the surface (more longwave radiation is being emitted from the surface than what the surface is absorbing from the clear sky). Under cloudy and/or windy conditions, radiation frost events do not occur.

The temperature measurement range of the SF-110 is -40 to +70 °C with an accuracy of ± 0.1 °C from 0 to +70 °C. However, the sensor is intended for applications in cropped fields and orchards when temperatures will be near freezing, and where air temperature measurements are not a good predictor of frost formation (Figure 1). Shop>

SF-110 Radiation Frost Detector Graph

Figure 1: Leaf and bud temperature approximations measured with an Apogee SF-110 compared to air temperature (top panel) and wind speed (bottom panel) on the evening of October 26, 2011. Leaf and bud temperatures were both below air temperature after 6 PM (hour 18) and reached the freezing point well before the air.

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Thermistor Temperature Sensor from Apogee InstrumentsThermistor Temperature Sensor

The thermistor temperature sensor is a small thermistor enclosed in a weatherproof housing. It is suitable for measuring air and soil temperatures with a datalogger.

The temperature measurement range is -50 to 70 C with an accuracy of 0.1 C above 0 C. Shop>








Fine-Wire Thermistor

The fine-wire thermistor is ideally suited for taking measurements of surfaces or small samples where even a narrow field of view infrared radiometer may not be suitable due to the integration over its conical field of view. The small size also provides a more rapid response to changes in temperature than larger sensors.

In spite of the small size, the sensor is remarkably rugged and can be repeatedly pushed into plant tissues with a tweezers.  The wires are flexible so it is less sensitive to breakage than similarly-sized fine wire thermocouples.

The temperature measurement range is -50 to 70 C with an accuracy of 0.2 C above 0 C. Shop>

 ST-200 Fine-Wire ThermistorST-200 Fine-Wire Thermistorst-200-fine-wire-thermistor-6.jpg






ST-110 Thermistor

This precision thermistor temperature sensor is optimized for air temperature measurement. Reflective white heat-shrink reduces solar loading on the cable behind the thermistor.  A constantan wire transition is used to minimize thermal conduction to the sensor tip.  The sensor is designed for long-term outdoor installations in radiation shields in all climates.

The temperature measurement range is -50 to 70 C with an accuracy of 0.1 C above 0 C. Shop>

 ST-110 thermistor




For non-contact temperature measurements see our infrared radiometers.


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