TK04 Thermal Conductivity: User Guide

Content

Introduction

Thermal conductivity is the coefficient of proportionality relating conductive heat flow to a thermal gradient. The Teka Berlin TK04 system determines thermal conductivity based on a transient heat flow method. A line source is heated with constant power while recording source temperature. Thermal conductivity is calculated from the resulting heating curve.
The TK04 uses two types of probes: the full-space (VLQ) needle probe for soft sediments and the half-space (HLQ) probe for hard rock samples. Measuring a single point in a section takes ~54 min per sample, allowing for 3 replicates to be taken. A self-test including a drift study is conducted at the beginning of each cycle. To measure thermal conductivity the heater circuit is closed and the temperature rise in the probe is recorded. Thermal conductivity is calculated from the rate of temperature rise while the heater current is flowing. The thermal conductivity of each sample is the average of three repeated measurements for the full-space method and three to six repeated measurements for the half-space method.
Precision of the method is better than 2%, based on extended evaluation of the method; accuracy is about 5% because of random variations of thermal conductivity in natural materials.

Theory of method

Thermal conductivity is measured by transient heating of an isotropic material with a known heating power generated from a source of known geometry and measuring the temperature change with time. The needle probe contains a heater wire and calibrated thermistor. It is assumed to be a perfect conductor because it is much more conductive than unconsolidated sediments. With this assumption, the temperature of the probe has a linear relationship with the natural logarithm of the time after initiation of heating:

T(t) = (q/4pk) ln(t) + C,

where:

T = temperature,

q = heat input per unit length per unit time (W),

k = thermal conductivity (W/[m·K]),

t = time after initiation of the heat, and

C = a constant.

A simple way of calculating the thermal conductivity coefficient k is picking temperatures T₁ and T₂ at times t₁ and t₂, respectively, from the temperature vs. times measurement curve:

ka(t) = q/4p[ln(t₂) – ln(t₁)]/(T₂ – T₁).

ka(t) is the apparent thermal conductivity because the true conductivity (k) is approached only by a sufficiently large heating duration. The method assumes that the measurement curve is linear and ignores the imperfections of the experiment expressed in the constant C.

Thermal conductivity is an intrinsic material property for which the values depend on chemical composition, porosity, density, structure, and fabric of the material. Thermal conductivity profiles are used along with in situ temperature measurements to determine heat flow, which is an indicator of age of ocean crust and fluid circulation processes.

Available Probe Types

All probes consist of a source (i.e., a metal needle with an embedded heating wire and a temperature sensor), a handle or body (depending on the probe type), and a connection cable.
Full-space probes (VLQ) are needle probes equipped with a handle at one end of the source. They are completely inserted into the sample.
Half-space probes (HLQ), or "Pucks", are placed on top of the sample. The source is embedded into the bottom side of a puck-like probe body and has on-site contact with the material/sample.

Analytical process

The approximate amount of time needed per sample is as follows:

Measuring the Standard

1. Prior to initial testing of received cores at a site, the TK04 system should be tested and calibrated to ensure that there are no potential mechanical or software issues. Additional tests using the Standards should be run as part of the troubleshooting process if you experience issues during actual testing (See Troubleshooting).

2. The Macor Standard for the Standard VLQ consists of its black holding shell, while the Macor Standard for both the Standard HLQ and Mini HLQ is a white disc. Calibration tests for any of the available probe types should provide results of TC=1.626-1.637±2%. The Macor standard drift calculations are based on a Macor standard (1.637 ± 0.033 W/mK) because its properties are closest to basalt cores (See Appendix: TK04 Recommended Heating Power for information).

   Standard VLQ- MACOR Standard	Standard MACOR Disc
   Probe Test TC Value Expected Results: TC= 1.626±2% W/mK	Probe Test TC Value Expected Results: TC= 1.637±2% W/mK

3. To conduct a probe test, scan the STND MACOR disc TCON (H) label kept above the testing apparatus and ensure that the appropriate heating time and drift control (DCL) settings are input under the Configuration settings (See Configuring the Measurement Program).

   Example 1: A Standard HLQ properly positioned on the Standard MACOR Disc.

4. Once the proper settings are confirmed, you can test the probes on the standard as if it were a normal sample. For the Standard VLQ this consists of leaving the probe needle in the MACOR standard, while for the HLQ probes you will need to attach it to the Standard MACOR Disc with a rubber band (See Example 1).

The user should note that in many cases, the samples are not amenable to thermal conductivity analysis, whether because of fractures that lead to circulation, poor surface conditions, or other factors. If the Macor standard gives good results and a core section or piece does not, the sample may simply not work.

Preparing Samples for Analysis

Soft-Sediment Samples- Standard VLQ (Needle Probe)

1. Equilibrate core sections to room temperature for at least 4 hours in the core rack before bringing a target section to the thermal conductivity workstation.
2. Select measuring points in the core
   1. Intact core: middle of section; record offset in cm
   2. Cracked core: just above/below the middle (but in any case away from the crack); record offset in cm
3. Use the cordless drill to drill a ~2 mm hole into the core liner at the borderline between working and archive halves. If the sediment is unconsolidated, drill only through the core liner. If the sediment is semiconsolidated, drill a small hole in the sediment for the probe as well.
   Caution: it is very easy to bend the needle on the VLQ full space needles!
4. Optional - Apply thermal joint compound to the probe unless the sample is very soft and/or moist.
5. Carefully insert a clean full-space needle into the sediment. Avoid twisting the needle into the core. The needle must be completely inserted into the sample up to the handle. Do not attempt to force the needle into the sample if the resistance is too great; DO NOT BEND THE NEEDLE.

Figure 1. Thermal conductivity measurement on a soft-sediment section using full-space probe.

Soft-Sediment Samples- HLQ ("Puck" Probe)

1. Certain sediment samples are too hard for the VLQ Needle Probe to be safely inserted without risking damage to the probe. In these cases, the samples should be split and an appropriate section of core tested with the HLQ "Puck" Probe. If the curator has not designated the core as being a hard rock sample then the ambient temperature seawater bath can be ignored in favor of normal core equilibration on the core rack for the standard four hours.
2. Select measuring points in the core:
   1. Probe should be positioned so that the embedded needle sits flush with the split surface with no gaps. DI Water can be used to aid in creating a proper conductive connection between the probe needle and the core.
   2. Pieces must be at least 10 cm long. The sample diameter has to be at least equal to the probe diameter and the sample should be at least 2–3 cm thick. The needle on the HLQ half-space needle pucks must be in contact with the sample material on its whole length.
3. Use rubber bands to secure the puck to the specimen and run test with appropriate configuration input (See Configuring Measurement Program section).

Hard Rock Samples

1. Place hard rock samples in an ambient temperature seawater bath to equilibrate and saturate (4–12 hr). Keep the sample saturated until measurement. A bell jar and vacuum pump are available in the Physical Properties lab and can be used to aid in saturation.
2. If the surface of the split core is excessively rough, use a lap plate and grit from the Thin Section Lab to prepare a smooth surface on a split-core piece. Pieces must be at least 10 cm long. The sample diameter has to be at least equal to the probe diameter and the sample should be at least 2–3 cm thick. The needle on the HLQ half-space needle pucks must be in contact with the sample material on its whole length.
3. Equilibrate the sample and sensor needle in an insulated seawater bath for at least 15 min prior to measurement. Do not submerge the puck—let the water level rise to half the depth of the puck or less.
   Caution: If the water rises to the top of the HLQ puck, it will damage the electrical fittings!
4. Use rubber bands to secure the puck to the specimen.

Configuring Measurement Program

1. Load ThermCon software in offline mode. Ensure that the Text_ID field is blank.
   
2. Scan the core label using a scanner, then click Verify Sample.
   
3. If login is requested, enter UserName and Password and then click OK.
   
4. The folder path is shown on the screen. Do not close this window during measurement. 
   
5. Run TK04 program and choose Measuring > Configuration. 
   
6. Set configuration parameters as follows (see figure below):
   1. Probe Number: serial number of probe to be used in the measurement (Note: results may be wrong by several percent if the wrong serial number is entered or by a factor of ~2 if the wrong type of probe is entered),
   2. Root Name: six characters or less; suggest Core-Type-Section (no special characters in the root name).
   3. Serial Number: number of repeat measurements at each measurement point (1–99 single measurements).
   4. Folder: path for saving data results.
   5. Heating Power: for the VLQ (needle probe), set to twice the estimated thermal conductivity value of measured sediment. For example, 2–3 is good for sediment. (See the Appendix: TK04 Recommended Heating Power for power guidance.)
   6. Measuring Time: set to at least 80 s, or for mini HLQ 60 s.
   7. Click Expert Options to configure Drift Control and Pause in Minutes (see Step 7).
   8. Enter comments.

To configure Drift Control and Pause in Minutes in Expert Options:

• Drift Control (DCL): limit for the range of temperature drift allowed prior to heating and measuring. A larger number allows quicker but less accurate measurement. Default (unchecked) DCL = 10; Recommended DCL = 40.
• Pause in Minutes: insert a pause between single measurements; recommended = 10 min. This parameter does not apply when conducting a single measurement on each core or each core section.

Measuring Samples

1. Confirm configuration settings shown in the lower part of the TK04 screen, insert the probe into the hole drilled into the sample, and click Start Measuring. 
   
2. Drift control (DCL) repeats until the criterion for drift (set in the Expert Options window) is met. Each drift control measurement takes 0.5 min. For DCL = 40, drift control is <10 series (~5 min). For DCL = 10, drift control is <30 (~15 min).
   1. Counter: remaining number of measurements in the current drift series
   2. Recording: currently recorded time and temperature and drift series being recorded
   3. Drift: drift value from the last drift series; indicated in blue on the slider scale when acceptable
   4. Start signal light:
      1. Yellow: drift has reached the threshold; approximately half the drift time has passed
      2. Green: drift limit has been reached and measurement can begin
         
3. After satisfying drift control, sample heating and measuring begins, and temperature values are corrected automatically for the drift effect predicted from the last drift series. Elapsed measurement time is controlled by the value entered in Configuring Measurement Program > Step 6F.
   1. Recording: displays currently recorded time and temperature
   2. Data: lists recorded time/temperature values from the heating curve
   3. Heating diagram: continuously updated; can display as linear or logarithmic scale
      
4. Solutions calculated by the algorithm are shown on the screen. Note the result on the handwritten log.
   1. TC: thermal conductivity
   2. LET: logarithm of the extreme time (lower limit = 4); the measurement with the largest LET is used to calculate thermal conductivity
   3. CV: contact value
   4. PC: power control is shown on the plot in the lower part of the screen (recommended PC = 2–3. If PC is out of range, adjust the Heating Power (HP) in Configuring Measurement Program > Step 6E as follows: if PC > 3, decrease HP; if PC < 2, increase HP).
   5. Mean: mean thermal conductivity
   6. Count: number of measurements used to calculate thermal conductivity

Uploading Data to LIMS

1. To upload results to LIMS, click Upload to LIMS. If upload is successful, a message like "Logged results for sample …" is shown. Close the ThermCon program.

Verifying Data in LIMS

Verifying Data in LIVE (LIMS Viewer)

Run LIVE from the ship application page to check thermal conductivity data:

1. Select the PHYS_PROPS_Summary template.
2. Select the Site/Hole/Interval to be queried.
3. Click View Data.

Retrieving Data from LIMS REPORTs

1. Go to LIMS Reports at http://webserv.ship.iodp.tamu.edu:8080/UWQ/.
2. Under Select Report, choose Physical Properties > Thermal Conductivity (TCON).

Note: The "expanded" report shows all of the database parameters and may be confusing to a general user; use the "standard" report.

3. Under Select Sample Range, specify Expedition, Site, Hole, and Section image(s) to retrieve.

4. Click View data or Download data file to view results or download a CSV file.

After Verifying Data Upload

1. Once uploaded data are confirmed, clean the needle probe and place it in its styrofoam storage container.
2. Repeat sample measurement process with a new sample.

Data Management

Once all sections for the Expedition have been sent through the track, all data needs to be placed in the appropriate folders on data1 (S:\data1).
1. Copy files from archive and place them in the 5.1 Petrophysics TCON thermcon folder. Confirm relocation. Delete all files off the local drive.

LIMS Component descriptions

Troubleshooting

Drift phase takes too long:

• Ambient temperature is not stable: allow sample more time to equilibrate
• Probe and sample are not in equilibrium with ambient temperature: place sample/probe in insulated case
• If necessary, force measurements by choosing a weaker drift limit

Variation of measurement series is too high:

• Ambient temperature is not stable enough
• Heating power too low to produce sufficient temperature increase: increase heating power
• Start time maximum value is too high: set to 40 s
• Interval length minimum value is too low: set to >25–30 s
• Contact values vary strongly: omit outliers from evaluation

Evaluation returns few or no solutions:

• Wet sample may cause convection effects: reduce heating power
• Poor contact between probe and sample: use contact fluid
• Interval length minimum value too high: set <30 s
• Start time maximum value too low: set to 40 s
• Variations in ambient temperature: insulate sample and/or probe
• Heat transport into the sample is not distributed: smooth sample surface and apply contact fluid
• Issue with the probe: Run test on Macor Standard to see if you get expected results

LET values too low:

• Poor contact between probe and sample
• Unstable ambient temperature
• Interval length or start time minimum values too high

Evaluation intervals start later than ~35 s:

• Poor contact between probe and sample
• Heating power too high
• Boundary effects caused by finite probe length

Descending trend in thermal conductivity values:

• Water-saturated samples drying out: keep sample wet during analysis

Appendix: TK04 Recommended Heating Power

Note: for loose sediments, use a lower heating power to avoid convective heat transport of pore fluids.