About the LNCSAbout the LNCS
When you receive your Liquid Nitrogen Cooling System (LNCS),
there will be some assembly required, as well as installation, filling,
and maintenance.
Select from the following topics for more information:
The LNCS is designed to maintain a high level of liquid nitrogen in the cooling heads heat exchanger. A sensor assembly monitors the exhaust gas stream leaving the cooling head for the presence of liquid nitrogen. When liquid is detected, the pressure in the dewar is reduced until liquid is not longer detected in the exhaust stream. Pressure is then increased until liquid is once again detected and the control cycle continues.
Pressure is controlled by two pairs of valves to achieve acceptable valve-cycle timing. Large orifice values for lower liquid level and small orifice valves for high liquid levels. There are two sources of pressurized nitrogen gas which are selectable. You can obtain the best performance from the LNCS by using a house nitrogen source at 70 kPa gauge (10 psig) to control dewar pressure. Alternatively, there is an internal pressure build tube in the base of the dewar that can be used when house nitrogen is not available.
The dewar can be pressurized either internally or externally. Internal pressurization limits the capabilities (primarily temperature responsiveness) of the cooler as the liquid level decreases. It also consumes more liquid nitrogen, therefore reducing the available remaining usage time of the dewar. External pressurization is preferred for optimal performance and is accomplished with a nitrogen gas source at the users facility regulated to between 55 and 70 kPa (8 and 10 psig). A three-way valve on the LNCS controls the operating mode.
There are five plumbing fittings that you can access for normal operation:
The first two are for connection of the cooling head and feed/exhaust hose. They are located at the top of the liquid delivery tower. One fitting (0.25-inch tube) is for the LN2 supply to the heat exchanger (in the cooling head) and the other fitting (3/8-inch tube) is for the exhaust gas from the heat exchanger.
There are two fittings for connection to a bulk LN2 source for filling purposes, one which is controlled by a solenoid valve and the other for manual filling.
The last fitting is the attachment point for the external dewar pressurization line, an 55 to 70 kPa gauge (8 to 10 psig) nitrogen source, which is used to control the operating pressure of the dewar to deliver nitrogen to the heat exchanger. Use of this port is optional as self-pressurization is available.
The following DSC/LNCS cooling performance specifications are based on operation with a full and pressurized liquid nitrogen tank.
Temperature Range: –180°C to 550°C
Linear Cooling Rates: Cooling rates available will vary depending on the temperature range. As a general guideline, cooling rates of 50°C/minute can be achieved over the range 400 to –40°C and cooling rates of 5°C/minute can be achieved over the range 400 to –170°C.
Ballistic Cooling: Ambient to –90°C 15 to 18 minutes
CAUTION: We recommend that you do not use the LNCS when running isothermal experiments above 400°C. Damage to the unit can occur if used at high temperatures for extended periods.
CAUTION: The operator of this instrument is advised that if the equipment is used in a manner not specified in the documentation, the protection provided by the equipment may be impaired.
CAUTION: Due to the size and weight of the cooling accessory, the LNCS should always be lifted by two people to prevent injury.
CAUTION: The cooling head assembly contains coated fiberfrax material. Excessive handling of this material could cause fiberfrax particles to be emitted into the air. See the MSDS sheet for safety measures to be observed when fiberfrax is used.
You must unplug the instrument before doing any maintenance or repair work; voltages as high as 120/240 Vac are present in this system.
WARNING: High voltages are present in this instrument. Maintenance and repair of internal parts must be performed only by TA Instruments or other qualified service personnel.
The LNCS uses the cryogenic (low-temperature) agent, liquid nitrogen, for cooling. Because of its low temperature [-195°C (-319°F)], liquid nitrogen will burn the skin. When you work with liquid nitrogen, use the following precautions:
Liquid nitrogen boils rapidly when exposed to room temperature. Be certain that areas where liquid nitrogen is used are well ventilated to prevent displacement of oxygen in the air.
Wear goggles or a face shield, gloves large enough to be removed easily, and a rubber apron. For extra protection, wear high-topped, sturdy shoes, and leave your pant legs outside the tops.
Transfer the liquid slowly to prevent thermal shock to the equipment. Use containers that have satisfactory low-temperature properties. Ensure that closed containers have vents to relieve pressure.
The purity of liquid nitrogen decreases when exposed to air. If the liquid in a container has been open to the atmosphere for a prolonged period, analyze the remaining liquid before using it for any purpose where high oxygen content could be dangerous.
WARNING: Potential Asphyxiant. Liquid nitrogen can cause rapid suffocation without warning. Store and use in an area with adequate ventilation. Do not vent the Liquid Nitrogen Cooling System (LNCS) container in confined spaces. Do not enter confined spaces where nitrogen gas may be present unless the area is well ventilated.
The warning above applies to the use of liquid nitrogen. Oxygen depletion sensors are sometimes utilized where liquid nitrogen is in use.
The cell surfaces can be hot enough to burn the skin during a sample run. If you are conducting a subambient test on the DSC, cold could also cause injury. After running any type of experiment, you must allow the DSC cell to return to room temperature before you touch the inner cell surfaces.
CAUTION: Some surfaces of the LNCS and DSC system may get extremely cold during the use of the LNCS for cooling experiments. This presents a danger to exposed skin coming in contact with and adhering to the cold surfaces. We recommend that you do not remove the DSC lids when the instrument is at subambient temperatures to prevent moisture buildup in the system. However, if you do remove the lids or handle any cold surfaces, use forceps or gloves to prevent injury.
WARNING: Some of the DSC and LNCS surfaces get cold during use of the LNCS. The cold surfaces can cause condensation and, in some cases, frost to build up. This condensation may drip to the floor. Provisions to keep the floor dry should be made. A slipping hazard may result if the condensation is not cleaned up.
WARNING: Do not exceed 100°C with the LNCS cooling head installed and the LNCS power off. Serious damage to the cooling head could occur.
CAUTION: We recommend that you do not use the LNCS when running isothermal experiments above 400°C. The life of the DSC cell heating element can be shortened if the LNCS is used at high temperatures for extended periods.
There are several types of valves present on the LNCS, which are briefly explained in this section.
Four pressure
control valves, shown in the figure to the right, are used to control
the operating pressure of the dewar.
There are two pressure relief
valves shown in the figure below. One
is used to prevent pressure buildup in the transfer hose if liquid is
accidentally trapped after an autofill sequence by closing the bulk tank
valve before liquid has had time to vaporize. The
other pressure relief valve prevents excessive pressure buildup in the
dewar.
The large silver-colored autofill valve, shown in the figure above, is used when filling the dewar tank automatically.
The gold-colored tank pressure relief valve, shown in the figure above, prevents buildup of pressure inside the dewar if a pressure control failure occurs.
The black handled valve is used to select the pressure source to control dewar pressure. The valve handle points toward the stainless steel tube that is connected to the dewar for internal pressurization.
Turning the black valve counterclockwise opens the pressure build line in the dewar. This is used when a house nitrogen gas source is not available to supply pressurized gas to the pressure control valves.
When you want to connect to a bulk nitrogen gas supply,
turn this valve clockwise as shown here. Make
sure the supply is regulated to 10 psi.
To find out how to fill the LNCS dewar, click here 
Filling the LNCS.
Once the LNCS has been properly installed, follow the steps below to set up the instrument parameters and condition the LNCS-DSC system for optimum performance.
It is best to start the LNCS as directed below before you run an experiment. This allows the LNCS to stabilize and will prevent samples from being exposed to cryogenic temperatures prior to starting a run.
Verify the correct cooler type (e.g., LNCS) and verify the desired standby temperature on the Tools/Instrument Preferences/Cooler Page of the DSC instrument control software.
Verify that a source of dry nitrogen is connected to the base purge and cooling gas (LNCS) purge. Select the gas to be used with the Gas 1 port on the back of the DSC instrument (see NOTE below).
NOTE: Dry nitrogen is used for the base purge and LNCS purge but, depending on the temperature range of the experiment, helium may be required for Gas 1 (cell purge). If the lowest temperature experienced during a run does not fall below –100°C, nitrogen may be used.
Dry the LNCS system before turning on the LNCS by following Step 1 of the conditioning procedure. See Conditioning the LNCS.
Verify that the post-test conditions (accessed through the Procedure Page by clicking the Post Test button) are set as desired. A temperature window above ambient should be used to prevent the cell from cooling down between experiments (e.g., typical values are 35 to 50°C). Once these conditions are verified, select Go to Standby Temp from the Control menu to invoke the standby temperature set on the Instrument Preferences/DSC Page.
NOTE: The DSC cell should be covered when not loading samples and should not be opened below ambient temperatures.
Proceed to Step 2 of the conditioning procedure found in the next section, Conditioning the LNCS to further stabilize the DSC-LNCS system after installation. This cyclic experiment allows the DSC-LNCS system to stabilize resulting in optimized baseline and calibration.
Recalibrate the DSC after conditioning the system. See Calibrating the DSC.
NOTE: When setting up experiments, be sure to verify the post-test conditions. A temperature window above ambient should be used to prevent the cell from cooling below ambient between experiments.
If you are using the Liquid Nitrogen Cooling System (LNCS), the run will start when the flange, which is part of the cell structure, has reached a temperature below –160 °C and when the system has detected adequate liquid nitrogen is present. The cell temperature is then set to 20°C and the experimental method is started.
Each time the LNCS heat exchanger is installed on the DSC the following conditioning procedure should be run before calibration and experiments are performed. The first step of conditioning is used when the system is first installed and periodically thereafter to dry the system to remove moisture in the DSC cell and heat exchanger BEFORE turning on the LNCS. The second step is used to stabilize the DSC-LNCS system by cycling the system to optimize baseline performance.
Follow the instructions below:
Verify that the DSC cell is empty and cover the cell. If an AutoLid mechanism is present, verify that the lids are seated properly. (Refer to Aligning the AutoLid for instructions to align the lid, if needed.)
Access the Tools/Instrument Preferences/Cooler Page of the DSC instrument control software. Verify that the correct cooler type (LNCS) is selected, check Leave LNCS on, and verify the desired Standby Temperature.
Using the DSC instrument control software, access the Experimental View Summary Page. Select the "Standard" mode, then select the "Cell/Cooler Conditioning" test template from the list. This test is performed with the LNCS off.
Click on the Procedure Page.
Verify the default conditions of 120 minutes at 75°C and select Apply. These conditions are suitable for typical situations.
Access the Post Test Parameters window and enter a temperature range window of 35 to 50°C to return the cell to slightly above ambient. Once the LNCS is operating, it is very important that the cell is always kept at or slightly above ambient temperature before and after experiments.
Start the experiment.
Upon completion of this experiment, the base and cell purges must remain on continuously. If the purges do not remain on, the atmospheric moisture will contaminate the system and, depending on the time involved and relative humidity, the procedure may have to be repeated.
The following cyclic experiment is performed after the first step in order to allow the DSC-LNCS system to stabilize, resulting in optimized baselines and calibration.
Select Control/LNCS/Cool from the menu. This will enable the LNCS and begin cooling the cell. Once the LNCS has started, the flange temperature will cool rapidly to its operating temperature.
Verify the instrument preferences and post-test conditions as outlined in steps 2 and 6 in "Step 1: Drying the System" above.
Verify that the cell is emptied and cover the cell.
Observe the Signal Display pane. Verify that "Set Point Temperature" displayed is at the midpoint value of the Temperature Range specified on the Post Test Parameters window. This indicates that the post test temperature control is active. If the post test temperature control is not active (i.e., the "Set Point Temperature" reads 0.00°C), select Go to Standby Temp from the Control menu to invoke the standby temperature set on the Instrument Preferences/DSC Page.
Create and save the following "Custom" method:
1 Data Storage On
2 Equilibrate 50°C
3 Isotherm 60 minutes
4 Mark end of cycle
5 Equilibrate 300°C
6 Mark end of cycle
7 Isotherm 30 minutes
8 Mark end of cycle
9 Equilibrate –180°C
10 Mark end of cycle
11 Isotherm 10 minutes
12 Mark end of cycle
13 Ramp 20°C/min to 300°C
14 Mark end of cycle
15 Isotherm 10 minutes
16 Repeat segment 8 for 7 times
Start the experiment created in step 5. The flange temperature must be below 100°C when operating an LNCS. If the run is started when the flange is above 100°C, then an error message will be posted and the run will be terminated. During normal operation the flange temperature should be less than –145°C at the start of a run.
After conditioning the LNCS (by performing both the drying and stabilization steps), evaluate the last baseline run in the method above for any artifacts. Calibrate the DSC before running experiments using the LNCS.
Refer to the TA Instruments LNCS Getting Started Guide for information on maintenance. Click the following button to open the Adobe Acrobat Reader file for the manual.
