A quick look on Isotech MilliK thermometry readout

Accurate temperature measurements are quite challenging and often require expensive platinum resistive sensors (PRT) or high stability thermistors. But even with high quality sensors one must use low noise high performance resistance measurement readout to digitize output from the sensor and apply all the complex math to determine temperature from resistance data. It’s possible to use benchtop DMM such as Keysight 3458A or Fluke 8508A for such a task but DMM method has number of drawbacks due to limited options for calibration adjustments, lack of advanced mathematical built-in capabilities to directly obtain readings in temperature units. Many DMMs also don’t allow change of test current to the sensor, making it difficult to establish and account for self-heating errors in small sensors.

To simplify and workaround these challenges dedicated temperature readout instruments are designed and sold by recognized manufacturers. At xDevs.com lab we use number of them, for example Fluke 1529 Chub-E4 which supports up to 4 sensors. But in this article we’ll look at much newer instrument from Isotech, called MilliK. It is a current product available from the manufacturer at price point $6607 USD.

For this money user obtain capability for temperature measurements with uncertainty up to 0.004 °C with SPRTs and PRTs over a year and also enables high accuracy measurement from thermistors, thermocouples and even 4-20 mA process inputs. It can be also expanded with up to four 8-channel millisKanner multiplexers.

On the back we can find external DC in barrel power input, communication interfaces and current loop port. Instrument supplied with external power brick.

LCD shows some information on the boot. It is not touch sensitive, all interface control done with membrane buttons.

Whole setup consumes just 3 W from 120V 60Hz source. It can be also operated from 4 x AA batteries for portable use cases or sensitive measurements.

Interesting to note that if resistance at the input is higher than actual range (115 Ω in this case, with 190 Ω attached) milliK will show incorrect readout without any information to user that this reading is wrong and limited by range. It’s a bit strange to see in a metrology-type thermometer, but perhaps something Isotech could easily address in firmware update, if they add a detection of value to close to the range limit. When such overrange value detected instrument should display “OVER Range” or similar error message to ensure that operator does not use wrong value.

Connecting 1 MΩ resistance on 500 kΩ thermistor range also have same firmware design flaw as with PRT case.

Internal photo

Here’s the photo for the key components in milliK. We can see hermetic Analog Devices LT1021BCH-5 5V reference with cute stress-relief cutout pocket. Reference IC date code reads as 34 week 2011. TSSOP chip Analog Devices LTC2442CG 24-bit SD-ADC used as a main digitizer. There is fancy VHP101 1.75 kΩ on a socket and some other Vishay resistor networks in wide SOIC16 packages. PCB manufactured with multilayer stack with copper over vias to allow via in the pad technology. Exposed pads have ENIG plating.

Internal connections in millisKanner.

Author: Ilya Tsemenko
Created: May 28, 2024, 4:11 a.m.
Modified: June 2, 2024, 5:07 p.m.