Block Digesters (DK 6/48, DK 20/26 and DK 42/26) - Specifications

Available as a PDF (2.11 MB)

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Block Digesters (Ai Scientific's AIM600)

Available as a PDF (1.6MB)

I'm pleased to announce we have released a new block digestion system,

It offers: faster digestions elevated standby temperatures increased methods capacity more programming functions

The AIM600 introduces a number of design improvements and has some fantastic performance specifications.

Things to note:

• The AIM600 is replacing the AIM500 and first units will be available October 2006
• The AIM600 uses the same accessories as the AIM500 - same tube rack, same cooling stand, some digestion tubes options, etc.
• The AIM600 is CE complaint and meets ROHS guidelines.
• The AIM500 block and controller are not compatible with the AIM600 block or controller.

The AIM600 requires two power cords. One for the block and one for the controller. The block requires a 220-250VAC power source, however, the controller can now operate on a standard 110VAC power source.

Manatee County is looking for a 50-pl "basic" system #DG-U-A014 with an extra cooling stand. They have an AIM500 and can use their tube racks and glassware.

Heating Rate - AIM600 versus AIM500

The graph above shows the heating rates of the AIM600 against AIM500. Starting from ambient temperature, the AIM600 takes about 48 minutes to reach 400° C, 9 minutes faster than the AIM500.

The actual improvement in heating time depends on several factors including initial and final temperatures, supply voltage and ambient temperature.

The improvement in heating performance was supported by a user who evaluated the AIM600 against the AIM500 for digestions of carrying out Total Kjeldahl Nitrogen of water and waste water samples in August 2006 using the following programmed method:

1. Step to 200 deg C
2. Hold for 60 mins
3. Ramp at 5 deg C/min to 380 deg C
4. Hold at 380 deg C for 120 mins

The user reported the comparative time to reach temperature in each of the method steps as follows:

"In both instances the AIM600 was about 10 min faster to attain the requisite ramp cycle temperatures."

Feature Comparison Table - AIM500 vs AIM600

Temperature Accuracy and Precision of the AIM600

The following data shows the accuracy and precision of performance of the AIM600 across its operating temperature range (ambient to 450°C). The temperature specification for the AIM600 block is to maintain a consistent temperature across the block (within +/- 2°C). Note the performance of the block has been optimised for the situation when it is fully loaded with a full set of digestion tubes and samples. It will perform slightly differently when not loaded with any digestion tubes due to the difference in thermal mass.

1. Temperature Precision at Different Temperatures

The above graphs shows heating a fully loaded block to 80°C. The block very quickly heats to 79.5°C and then takes a few minutes to reach final temperature. Once the block reaches the setpoint temperature, it maintains it within +/- 0.2°C.	Heating an unloaded block to 450°C shows a similar performance though in this case there is an overshoot in the setpoint temperature of the block caused by reduced thermal mass of not having digestion tubes in place. As above, once the block reaches the setpoint temperature, it maintains the setpoint temperature within +/-0.2°C .

2. Precision of Temperature Ramping

Positive Ramp at 5 degrees per minute.
Negative Ramp at 1 degree per minute.

The results from testing show that the control system controls the block temperature to within +/- 0.2°C . This is a full order of magnitude better than the specified +/- 2°C. Rise time is as fast as allowed by the thermal mass of the block, and is much faster than the AIM500 Block Digestion System, which used a gain scheduling approach to reduce power as the block reaches a set point. There was no significant phase-delay measured in the ramp rates.

3. Accuracy of Block Temperature

3.1 Calibration Accuracy
Using the thermocouple simulator the block was calibrated for 2 points (user calibration) and 5 points (factory calibration).

The calibration values were:

After calibration, the full range of temperature from 0 to 400°C was reported correctly to +/- 1°C, regardless of the number of calibration points. The readings did not drift over a test period of 24 hours.

3.2 Absolute Accuracy
In section 1, it has been shown that the control accuracy is to within 0.2°C. However this assumes the thermocouple is measuring the actual block temperature. To check the thermocouple reading against the actual block temperature, several vial reservoirs were coated in heat-sink compound, and a welded tip thermocouple was inserted into the heat-sink. The block was heated to certain temperatures according to the thermocouple and soaked for 20 minutes. The actual block temperature was measured as an average across 5 tubes spread across the block surface.

These readings show the measured temperature matches the actual block temperature to within 1° C. It should be noted that these measurements are an average of 5 vial positions. All positions were within 2° C of each other. Also the measurements were made near the bottom of the vials (1/3rd from the bottom), the temperatures at the top of the vials are typically 1-2° C cooler.

Therefore the specified +/- 2° C has been achieved, but will depend greatly on where and how the block is measured.

4. Sample Temperature

The AIM600 was fully loaded with 50 digestion tubes of water. The tubes were surrounded with a draft shield and the block was heated to 80° C. When the block reached 80° C, the water samples had only reached a temperature of 41° C, indicating the delay in heat transfer from block to sample.

The AIM600 block was allowed to soak at 80° C degrees for 1 hour. After this time, the temperature of the samples had essentially equilibrated. The average water temperature was 71.7° C.

The hottest measurement was 74.6° C and the lowest was 68.5° C with a std dev of 1.25° C. The distribution of temperature is shown on the surface plot below…

This plot shows the temperature of each sample versus its position in the block and shows there are no localised hot or cold spots.

5. Temperature Linearity

The design of the AIM600 includes a thermocouple transmitter that performs some electrical linearization on the thermocouple signal. The firmware can then scale and shift this linear signal into an accurate temperature reading.

The graph shows the actual block temperature versus temperature reported by the controller.

The results show the output of the transmitter is very linear across the operating range of the block and requires zero/gain compensation (linear).

6. Summary

The AIM600 controller is capable of controlling the temperature of the AIM600 block to within 0.2° C.

The variation in temperature across the AIM600 block at a specific temperature is within +/- 1° C. The AIM600 block is linear in temperature acorss the range of ambient to 450° C.

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