Thermocouple Probe
From GeoMod
Temperature probe Construction: Purpose: The probe needs to be able to acquire an accurate and consistent temperature at various locations in a river environment. These locations include but are not limited to river water surface, river bed surface, mid river depths, and sub-riverbed positions down to 30 cm. The probe needs to be water proof and corrosion resistant since the chemical makeup and red-ox potential of the water is not known. A data logging function will also be needed to record temperature data over extended periods of time ranging from 30 seconds to several days. Portability of the probe will be an important consideration since the majority of probe usage will be in remote areas in the field.
Construction: Phase 1: The initial design of the probe utilizes a ½ inch diameter stick of rigid electrical conduit cut to a length of 6 feet. A ½ inch threaded compression coupling was attached to one end of the conduit. A 6 inch diameter round stainless electrical box cover was then attached to the ½ inch compression fitting through the center punch out hole in the box cover. Two 3/8 inch holes where drilled in the box cover ¼ inch from the central punch out hole on opposite sides. The function of the box cover is to create a location along the pole that restricts penetration. In other words, the pole will be inserted into the riverbed until the bed comes into contact with the plate, which will restrict insertion. This creates the base to which the sensing and recoding equipment is attached. A thermocouple probe is then inserted into each of the 3/8 inch holes drilled into the box cover. The thermocouple probes are type K probes with a ¼ inch stainless steal sheathing. Both probes are also grounded to the sheathing and sealed with a rubber handle with 4 foot extension cable. The grounded thermocouple gives a temperature response time of less than 2 seconds for type K thermocouples. One probe is 24 inches in length while the other probe is 36 inches in length. The probes have a +/- 0.5 degree Celsius bias and are accurate to 0.1 degrees Celsius. The probes are held in place with two pipe clamps a piece which are tightened around the handles of the thermocouples. This allows the thermocouples to be adjusted up and down the pole, adjusting the depth of penetration of each thermocouple. Clamping to the handle prevents the pole from direct grounding contact with the thermocouples. Two wire type K thermocouples are then attached to the conduit by means of pipe clamp at varying locations. The pipe clamps can then be moved up or down the pole to obtain measurements at different depths in the river water. Each thermocouple is insulated with a waterproof polyvinyl sheathing with an exposed junction. The exposed junction allows rapid temperature response of less then 1 second. An Omega model HH309 four channel data logger is then attached to the top of the conduit. The HH309 can record temperature in Celsius or Fahrenheit on four separate channels. It has a resolution of 0.1 degrees Celsius. It can store up to 16,000 records per channel and has an NIST traceable Certificate of Calibration. It can log temperatures form as frequently as 1 measurement a second up to one measurement every 2 hours. Each of the four thermocouple leads is then numbered and plugged into a port on the HH309. A measuring tape is then taped to the conduit with the base, 0 cm, end of the tape at the box cover. The other end is taped to the top end of the conduit. This creates a marked reference along the length of the conduit for obtaining depth of water data.
Phase 2: The two thermocouples with the stainless steel sheathing are removed. This is due to issues encountered when the thermocouples are inserted into the sediment. Readings became very sporadic and range in reading differed by several hundred degrees, an impossibility for natural conditions. This is likely due to electrical interference induced by the iron in the sediment interacting with the stainless steel sheathing to which the thermocouples are grounded. Two additional wire type thermocouples are added to the probe in lue of the stainless steel sheathed thermocouples. These are attached to a 24 inch length of ½ inch electrical conduit with a ½ inch threaded compression coupling on one end. The exposed junctions of the wire thermocouples are covered with two layers of gauze cloth. The cloth is then held in place with Duct tape. The thermocouple junctions are located at 5 centimeters and 20 centimeters form the compression coupling. A ½ inch threaded coupling is then used to attach this new conduit piece to the existing probe assembly. The two new wire type K thermocouples are then plugged into the HH309 data logger.
Phase 3: The 24 inch length of ½ inch conduit is removed from the probe assembly. It was deemed unnecessary after comparing various depth data with bed surface data. A sufficient correlation was made to validate using bed temperature data which can be acquired much faster and much more easily than sub bed temperatures. The two wire type K thermocouples were removed from the 24 inch length of conduit. One was attached to the underside of the box cover to measure the temperature of the bed surface. The other was attached to the top of the box cover to measure water temperature immediately above the river bed. These thermocouples where then plugged into the HH309 Data logger.
