CryoStat Calibration

The cryostat temperature reading does not necessarily reflect the temperature at the sample. The discrepancy is large enough to cause significant differences between data taken with a cryogenic bath and data taken with the cryostat set to the temperature of the bath.

One solution is to modify the voltage-to-temperature curve stored in the temperature controller so that the reading accurately reflects the temperature of the sample. This option requires additional interaction with the temperature controller to download, upload, and select the voltage-to-temperature curves. However, this is straightforward with the Lakeshore controller and their Curve Handler software.

Required Items

  • An ASAP 2020 Plus analyzer equipped with an installed Micromeritics cryostat unit.
  • A selection of adsorptives to cover the temperature range to be calibrated — such as nitrogen, argon, methane, ethane, and propane. Choose adsorptives whose saturation pressure is less than 800 mmHg at the measurement temperatures.
  • The Lakeshore Curve Handler software (free download): http://www.lakeshore.com/products/Pages/curvehandler.aspx.
  • An empty sample tube for the Micromeritics gas adsorption analyzer being used.
  • A spreadsheet application — such as Microsoft Excel. Knowledge of how to load a .TXT file containing tabular data into a spreadsheet application is beneficial.

Calibration Procedure

  1. In the analysis application, go to the Unit menu. Ensure there is a checkmark to the left of Disable Elevator and no checkmark to the left of Enable CryoStat. The cryostat will be adjusted manually.
  2. Install a blank sample tube into the gas adsorption analyzer.
  3. Raise the cryostat and lock it in place following the general cryostat procedure.
  4. Set the cryostat to the required temperature. On the LakeShore temperature controller, press SETPOINT, enter the temperature (in kelvin), then press ENTER.

  1. Wait until the cryostat reaches the entered temperature. The time varies depending on how big the temperature jump is. For example, going from 200 K to 215 K may take 15 to 20 minutes; going from 298 K to 77 K might take 90 minutes. Once the cryostat has reached the operating temperature, record the set temperature, the measured temperature, and the cryostat signal probe voltage.
  1. Create a sample file to measure saturation pressure.

In Analysis Conditions:

  1. Select the adsorptive to be used.
  2. Select the Absolute pressure dosing option.
  3. Enter the pressure settings shown in the Pressure Settings for ASAP 2020 Plus table below.
  4. Click Dosing and enter the dosing settings in the Dosing Options for ASAP 2020 Plus table shown below. Click OK to return to the Analysis Conditions window.
  1. Click Equilibration. In the table, enter an absolute pressure of 1,000.00000 and equilibration interval of 5. Click OK.

 

Pressure Settings for ASAP 2020 Plus
  Starting Pressure (mmHg) Pressure Increment (mmHg) Ending Pressure (mmHg)
Line 1 0.000000   100.000000
Line 2 100.000000   980.000000
Dosing Options Settings for ASAP 2020 Plus
Field Enter...
Absolute pressure Tolerance 5.000 mmHg
Relative pressure tolerance 5.0 %
Low Pressure incremental dose mode Select this option
Dose amount 20.0000 cm3/g STP
Equilibration Delay Minimum. 0.00 h
Maximum. 999.00 h
Maximum number of decants 6
  1. Once the analyzer is set up and the temperature for the first data point has reached equilibrium, start the analysis defined by the sample file. The analyzer will dose the sample tube with 20 cm3/g STP of adsorptive . Once the tube pressure reaches the adsorptive saturation pressure, the generated isotherm will climb straight up. Take several points at this saturation pressure, then pause the analysis and set the cryostat to the next temperature.

Once the cryostat has reached the set temperature and has equilibrated a few minutes, record the cryostat temperature and the probe signal, then resume the analysis. Perform this cycle for the number of temperature points required for the adsorptive being used. Typical isotherms are shown below. Note that the temperature labels are for clarity and are not a part of the normal report.

Additional adsorptives will allow the calibration to be extended. A set of local fits may be needed to cover a wide temperature change.

  Depending on the selected adsorptive, a warning message may be given indicating that 950 mmHg is not the saturation pressure for the temperature being used. This is a normal message and will not interfere with the experiment. Click OK and continue with the analysis.
 

These analyses will not end and will have to be terminated manually by stopping the analysis window. To terminate the analysis, click Cancel , then wait a few seconds and click Cancel again (do not double-click the Cancel button).

The analysis will end abruptly with adsorptive in the sample tube. The sample tube must be manually evacuated before the cryostat is lowered.

Manually close all valves. Open values 2, 7, and 9 in that order. Once the pressure in the sample tube is less than 300 mmHg, open valve 1.

The cryostat can be lowered once the sample tube has been completely evacuated.

This saturation pressure data can either be read from the graph or an isotherm report can be made and the saturation pressure read from the report:

  1. Place the data in table format using a spreadsheet application such as Microsoft Excel.

Temperature and Saturation Pressure Values
  Set Temperature (K) CryoStat
Temperature (K)
Signal Probe Voltage Measured
Saturation
Pressure (mmHg)
Sample Tube
Temperature (K)
Nitrogen 65 64.999 1.04878 137.833 65.317
67.5
67.498 1.04474 206.334 67.778
70 69.999 1.04066 301.792 70.294
73 72.996 1.03572 454.217 73.239
73 72.996 1.03572 456.148 73.271
73 72.998 1.03571 452.139 73.239
75 75.002 1.03240 584.063 75.189
75 75.002 1.03245 587.793 75.240
77 76.993 1.02896 743.094 77.165
77 76.995 1.02897 742.343 77.156
78 78.002 1.02737 832.533 78.165
Ethylene 140 139.999 0.91437 111.175 142.470
145 144.999 0.90456 168.125 147.470
150 150.000 0.89477 245.617 152.350
Set Temp Cryostat temperature setting, in kelvin.
CryoStat Temperature Temperature reported by the temperature controller.
Signal Probe Voltage Signal probe voltage reading.
Measured Saturation
Pressure
Equilibrium pressure measured by the instrument when the isotherm is vertical.
Sample Tube
Temperature
Sample tube temperature calculated from the measure saturation pressure using NIST’s REFPROP program (bundled with the Micromeritics applications).

The spreadsheet data can be graphed in the REFPROP spreadsheet program. Plot the REPROP to determine temperature versus the Cryostat reading. Fit the data points:

Additional adsorptives will allow the calibration to be extended. The following graph contains data for nitrogen combined with data from ethane:

  1. The REPROP determined sample tube temperatures, combined with the signal probe voltages read from the LakeShore controller are then used to construct a cryostat calibration curve.

The LakeShore Curve Handler program (shown below) is used for this operation. (See the LakeShore Curve Handler manual for communications information.) Connect the cryostat to the computer via an Ethernet connection.

  1. Click Connect to establish communication with the LakeShore Temperature Controller. Once communication is established, all the calibration curves in the LakeShore Temperature Controller will be listed.

  1. The ColdEdge Cryostat/LakeShore Model 336 Temperature Controller uses the DT‑670 silicon diode as the temperature sensor according to ColdEdge Micromeritics Interface Owner’s Manual. Select the curve for the DT‑670 temperature probe. Click Save As and save the DT‑670 curve with a different name (i.e., DT‑670‑xx). Note that all calibration curves are stored as ASCII files in the Documents library with the file extension .curve. There will be a DT‑670.curve file and a DT-670‑xx.curve file.

  1. The LakeShore Curve Handler instruction manual provides several methods of entering user created calibration. The following process uses a file copy of a modified version of the DT‑670 curve previously saved.
  2. Produce a modified calibration curve by applying the fit from step 8 to the temperatures in DT‑760 curve.
  1. Start the LakeShore Curve Handler program and establish communication with the LakeShore Temperature Controller.

  1. Click Open and select the modified DT‑670‑xx.curve. Place this curve in the first empty bin. User curve bins start at Number 21.
  2. The new curve should now be able to be loaded into the LakeShore Curve Handler program. Click the User’s Curve tab in the lower left side of the window.

  1. Note in this example the modified curve DT-670-xx.curve is in bin 22 (left column) and the curve is displayed on the right. The values should be reviewed and any changes made. Click Save if changes have been made. Click Write to load the new curve into the LakeShore Temperature Controller. Exit the LakeShore program.
  2. To select the new curve, on the temperature controller, press the Input command (key 7), select the Input channel (usually A), use the arrows to navigate to the curve menu, press Enter, then select the curve. Use curve 22 in this example. Press Enter once more, then press Escape. The new, modified curve is now in place. More details can be found on page 52 of the LakeShore Model 336 Temperature Controller User’s Manual.

The table below shows the effect of the new curve:

Temperature and Saturation Pressure Values
Set Temp (K) Cryostat
Temperature (K)
Measured
Saturation Pressure (mmHg)
Uncalibrated Sample Tube
Temperature (K)
Calibrated
Sample Tube
Temperature
(K)
% Error
(Calculated
vs
Uncalibrated)
73 72.997 473.450 73.553 73.250 -0.414
75 74.996 615.020 75.603 75.215 -0.517
77.3 77.002 786.618 77.648 77.029 -0.804
 
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