Monday, March 17, 2014

Exposed thermocouple junction | tip insulation

As we know, there are three types of thermo-couples according to junction type:

- Grounded junction
- Under-grounded junction
- Exposed junction

The following are some methods to electrical insulate exposed junction:

Using hot shrink tube:
A shrink tube of the correct size may be used to electrically insulate the exposed junction of thermocouple.

Using PVC insulation tape:
This is a very simple method but the PVC tape has low thermal conductivity.
Using beryllium oxide:

The most thermally conductive electrical insulator is beryllium oxide.  It has the thermal conductivity of metallic aluminum and the electrical conductivity of aluminum oxide.  Insulating washers of this material are available to fit most bolt-down  transistor and diode cases, and this is the only practical way to use it, since BeO is a hard and very refractory ceramic.  Note that BeO dust is carcinogenic.  It's not hazardous in pieces too large to accidentally ingest, but don't even think of grinding a notch in a BeO insulator to fit an odd corner.

Thermal conductivity [W/m.K]

Dielectric strength [kV/mm]
Water resistance
Wear resistance
Maximum temperature (melting point)   [°C]

Using motor | transformer winding enamel | insulation

Key words: wire enamel, insulation varnish

Thermal conductivity [W/m.K]

Dielectric strength [kV/mm]
Water resistance
Wear resistance
Maximum temperature [°C]

Using thermal conductive silicon pads

Using mica

Using diamond:

Diamond has the highest thermal conductivity on earth. Also, it has a very high hardness, so it is wear resisting. It can withstand high temperatures. For sure, diamond does not seem to be an economic solution in most cases.

Thermal conductivity [W/m.K]
Dielectric strength [kV/mm]
Water resistance
Wear resistance
Maximum temperature [°C]
 Min. 3500

Using thermal conductive epoxy:

This method seems to be the most feasible and simple preparation method for relatively low temperature range. Thermal conductive epoxy is used in electronics field to seal components that are sensitive to water for water proofing like and in solid state electronic components like relays, diode bridges, and thermo-switches.

Epoxy pack is composed of two tubes (materials): one contains the resin and the other contains the catalyst.

The method is already used by Omega engineering for thermocouple types J, K, T, E (check the link below):

The highest thermal conductivity Omega provides provide is 0.712 W/m.K [model OT201]

3M adhesives (one of the leading companies in adhesives) also produces thermal-conductive epoxy with two options:
* 1-1.4 W/m.K [code TC2801]
* 0.72 W/m.K [code TC7207]

Thermal conductivity [W/m.K]
Dielectric strength [kV/mm]

Water resistance
Wear resistance
Maximum temperature [°C]

Sunday, March 9, 2014

Refrigerator star rating

Each domestic refrigerator-freezer has what we call "Star rating". Star rating defines what is the storage temperature of the freezer compartment is. The lower the storage temperature temperature in freezer, the longest frozen food life is as the activity of micro-organisms is reduced. 

One star refrigerator-freezer has a storage temperature lower than or equal -6 C.

Two stars refrigerator-freezer has a storage temperature lower than or equal -12 C.

Three stars refrigerator-freezer has a storage temperature lower than or equal -18 C.

Four stars refrigerator-freezer has a storage temperature lower than or equal -18 C, but has a freezing capacity higher than 4.5 kg/100 L of the freezer's volume.

A question is: why freezers are rated using stars?!!!!

The answer simply is: it is inspired by the snow | ice flake shape under microscope which in most cases have the shape of the star.

Saturday, March 1, 2014

Some funny facts about refrigerators

Nowadays, each house has one or more of these appliances:

- Food preservation appliances: refrigerators, refrigerator-freezers, chest freezers, and upright freezers
- Food preparation appliances: gas cookers, electric cookers, induction cook-tops, microwaves, and friers 
- Fabric care appliances: automatic washing machines, half-automatic washing machines, manual washing machines, driers, and electric iron
- Water care appliances: electric water heaters, gas water heaters, solar water heaters, water dispensers, water boilers
- Air care: air-conditioners, fans, and heaters
- Other

The following funny facts apply on refrigerators:

- Refrigerators are the most basic and high-priority appliances in each home
- Refrigerator is the only appliance that is connected to power source during the whole year
- The only appliance that is running for the whole year
- Refrigerators have wide variety of structures and layouts
- The most difficult appliance to dispose
- Normally, the longest appliance in home
- Also, the heaviest appliance in home
- The most difficult appliance to transfer.
- The most difficult appliance to repair with the available hand tools at home
- The most user-interacting appliance at home, this is why memos are stick to refrigerator doors!!!
- The only appliance you must run even if it is empty
- Refrigerator doors are the most mechanically-reliable doors in all appliances
- The only appliance you care about when electricity is cut off
- Number one appliance that can be used for built-in layout with door-on-door layout.
- Number one appliance that needs no utilities except for electricity.
- The most appliance that has been integrated with touch screens, and remote-control
- The only appliance that you can place large amount of staff on top of it!!!
- The first appliance to have lights inside it
- One of the most needed appliances for singles!!!