Cleaning methods for platinum rhodium thermocouples

1, Cleaning: Precious metal thermocouples must stop cleaning before calibration to eliminate nominal impurities, inorganic substances, and oxides on the hot electrode, thereby increasing contamination on the hot electrode, stabilizing the thermoelectric characteristics, and extending the service life. Low cost metal thermocouples do not need to be cleaned on weekdays. When the electrodes are dirty, a wire brush can be used to remove dirt and oxide scale. Armored thermocouples do not require cleaning.

2, Acid washing

Influence is the removal of inorganic substances and departmental oxides in the name of thermoelectric electrodes.

1. The acid washing method for platinum rhodium series thermocouples is as follows: the thermocouple is coiled into a circle with a diameter of not less than 80mm and immersed in a hydrochloric acid solution with a concentration of about 30% chemical purity for 1 hour or boiled for 15 minutes. After taking it out, it is boiled several times with distilled water until the acidity is eliminated.

2. The pickling method for platinum rhodium series thermocouples for short tasks: coil the thermocouple into a circle with a diameter of not less than 60mm, immerse it in hydrochloric acid solution with a concentration of 30%~50% chemical purity for 1 hour or boil for 15 minutes, take it out and boil it several times with distilled water to eliminate acidity.

3. Acid washing method for gold platinum thermocouples: Before cleaning, remove the firm clamp of the thermocouple and cut off the thermocouple measuring head. Stop acid washing to distinguish between platinum wire and gold wire. Soak the platinum wire in a 30% solution of your chemically pure hydrochloric acid or nitric acid for 1 hour, then take it out and boil it several times with distilled water to eliminate acidity. Immerse the gold wire in a hot solution composed of 100g of water, 15g of bleach powder, 15g of sodium bicarbonate, and 5g of salt for 2 hours, and then rinse and clean with a solution of 1kg of hot water and 25g of sodium bicarbonate.

Attention should be paid when stopping acid washing of thermocouples:

1. The concentration of 30% -50% acid refers to the percentage of solvent;

2. To manipulate the safety, distilled water should be poured into a beaker before adding acid during preparation.

3, Borax washing:

Borax is a colorless and transparent crystal that dehydrates to form sodium tetraborate when heated to about 400 ℃. The melting point of sodium tetraborate is very low, melting into a glassy state at 878 ℃. Its identity is sodium borate and boron gangue.

Molten borax can melt metal oxides and form a glass like compound that is naturally stable with the oxide, allowing for the removal of the metal oxide from the thermoelectric electrode. Borax is a good pollutant for metals. This method is to use borax, which has the ability to melt metal oxides at low temperatures, to nominally stop scrubbing precious metal thermocouples.

Used thermocouples need to stop borax scrubbing after acid washing, while newly made thermocouples do not need to stop borax scrubbing.

When cleaning, the thermoelectric electrode should be extended and the reference end should be hung on the cleaning bracket with a platinum hook. The angle between the two electrodes should be kept small.

Slowly adjust the input current, apply a current of 10 to the platinum rhodium 10 platinum thermocouple and platinum rhodium 13 platinum thermocouple, and apply a current of 11A to the platinum rhodium 30 platinum rhodium 6 thermocouple to heat the hot electrode to (1100~1150) ℃. Use a chemically pure borax block near the reference end of the thermoelectric electrode to melt the borax into droplets, which flow down the electrode. After several washings, the thermoelectric electrode will exhibit a metallic glow. If there is still a severe blackening phenomenon on the thermoelectric electrode after repeated cleaning (more than 3 times), it is considered that the surface is uneven and will not be tested for indication. After washing with borax, the current should be slowly reduced and the power supply should be blocked. The thermocouple should be removed and coiled into a corresponding circle. It should be boiled in distilled water until all the glassy material on the electrode is melted.

The platinum rhodium thermocouple only needs to stop the borax scrubbing of the platinum wire according to the above method, and the current applied is 10. There is no need to stop washing the rhodium wire with borax.

Precautions:

1. Thermoelectric electrodes and platinum hooks should have excellent combat skills;

2. When hanging the hot electrode, the angle should be kept small, with the goal of increasing the stress on the hot electrode and making it easy for melted borax to flow down the electrode;

3. The heating temperature should be strictly controlled. If the temperature is too low, the melted borax is prone to solidification, which affects the cleaning process; If the temperature is too high, the thermoelectric grains will stay overnight for many years, resulting in a decrease in their lifespan.