Thermocouples and Radiation Shields Back to Freeze Page The objective of the radiation shields is to reduce the effects of

Thermocouples and Radiation Shields

Back to Freeze Page

The objective of the radiation shields is to reduce the effects of radiation heat exchange between the thermocouple and the surrounding environment. Radiation can have a significant effect on the temperatures of objects (tomato plant leaves, fruit, soil, etc.). At night objects can often be colder than the surrounding air due to heat loss by radiation to the nighttime clear sky. The same effect could be experienced by the thermocouple at night or a reverse effect could occur during the day because of solar radiation.


Thermocouple wire (p432) type T (Thermo Electric)	Radiation shield for Thermocouple (p467)	Radiation shield for Thermocouple (p468	Sensor mounting system for tomato rows (p469

The initial PVC radiation shield design used in this study has some disadvantages as pointed out by Dr. Khe Chau. First, the PVC is of relatively large mass thus imposing a lag on the temperature response of the sensor. Also as the PVC ages its reflectance will decrease, resulting in increased external radiation effects on the shield and sensor. In his fruit cooling experiments, Dr. Chau typically uses small bulbs formed by wrapping aluminum foil around the thermocouple. Small holes are added to the bulb to facilitate air circulation. The aluminum foil is of low mass and high reflectance.

Based on this advice from Dr. Chau, some modifications were made to the radiation shields used in this study. The existing PVC shields were wrapped in aluminum foil. While this addresses the reflectance issue, it does not resolve the mass issue. For the
purposes of the freeze protection study the mass problem may not be a critical issue given the slow temperature changes expected in field. Another reason for not completely changing over to aluminum bulb shields is the fact that with the application of water, ice will be forming on the shields. Bulbs with small holes are likely to become completely encased by ice thus removing any ability to sense the actual canopy air temperature. The PVC shield, open at the bottom, will allow some opportunity for the sensor to detect air
temperature. However, with ice encasing the outer PVC housing, the sensor may still reflect the effect of latent heat from the formation of ice (32F). To help reduce this effect, an umbrella of Tyvex and aluminum foil is attached to the top of the PVC Tee. Hopefully this will reduce the ice formation on the PVC Tee housing.


Sensor installation (p493) by Ed Rawlinson & Bjoern Koos	Sensor installation (p494) in Field B on Jan 18, 1999	Radiation shield (p495)	Radiation shield (p496)


Sensor support frame (p479)	Tyvex covers for radiation shield (p482) under foil	Sensor inserted under fruit skin (p480)	Sensor in sprinkler system (p483)

Prior to installation in the field all systems are tested to ensure correct wiring and thermal response in the range of interest (32F). As the first step all sensors from a given multiplexer are emmersed simultaneously in an agitated ice bath to test the equilibrium temperature response of the sensors. The second step of the test involves sequentially emmersing each thermocouple into the ice bath to check that the correct memory location responds. This confirms proper wiring and program data storage.


Lab calibration of thermocouple sensors (p472)	Wiring of sensors for Field A multiplexer (p473)	Ice bath calibration for sensors 32.3-4 F (p471)

F. The differences between PA10-Wobbler and PB10-Rotator (0.3 F) may be the result of two differences between the two systems. First because of the high costs of the CR10X dataloggers, the PA10-Wobbler system utilized a AM25T multiplexer (equipped with an internal reference temperature sensor) placed in a separate shelter housing the CR10X datalogger. The one CR10X drives both the PA10-Rotator and the PA20-Drip multiplexers. By comparison in the PB10-Wobbler system one shelter houses both the AM416 multiplexer and an older model CR10 datalogger equipped with a 10TCRT reference temperature sensor). Thus the significant differences between the systems include the datalogger/multiplexer types and housing plus the reference temperature sensor types. The PB20-Drip system has the same configuration as the PB10-Wobbler system except that lower-grade thermocouple wire (type J) was used in this field due to insufficient quantities of the the higher grade type TTX. This PC20-Drip ice bath equilibrium temperature (33.6 F) was approximately 1.0 F higher than that of the PB10-Wobbler temperature (32.6 F).

System	Mean Ice Bath Temp	TC Type	RefTemp Type
PA-Rotator (1010)	32.4	TTX	AM25T Internal
PA-Drip (1020)	32.4	TTX	AM25T Internal
PB-Wobbler (2010)	32.6	TTX	10TCRT on CR10
PC-Drip (2020)	33.6	JX	10TCRT on CR10

Given the questions surrounding the performance of the PC system, an additional thermocouple was added to the PB and PC multiplexers on January 18. These extra thermocouples will permit periodic ice bath tests to be performed in the field without disturbing the other canopy or fruit sensors. We will see if the difference between the PB and PC ice bath tests remains constant at 1.0 F.