Water Level Probes

Mostly temperature sensors are used for following purposes in vending machines.

• To measure the temperature of hot water / steam in a beverage machine
• To measure the ambient in the storage space in a refrigerated vending machine

For ambient measurement a thermister would be ideal. Mounting it with a spacer (to separate it from the wall) on a wall is the normal method

For steam / hot water temperature measurement, the sensor can be mounted in a variety of ways.

• Inserting inside the boiler with direct contact with water / steam: This will need leak proof fixing method – mostly a quarter inch BSP thread with a seal may be required.
• Inserting inside the boiler but NOT in direct touch – this basically means a tube to be welded to the side of the boiler and the sensor is slide fitted. This avoids the leakage issue, but the sensing is somewhat compromise because of the slide fit and the possibility of accumulation of foreign matter in the annular space making it get worse with time.
• Fixing the sensor externally on the surface of the boiler. This makes the cheapest solution in terms of the sensor cost as well as fitment cost. But here the issue is that the sensor may lose heat due to its exposed surface there by resulting in lower reading than actual. This can be arrested easily by appling an insulating material on top of the sensor

The the mounting methods 2 & 3 we also face an additional problem. In case you use water with some hardness, then the salt deposition on the insides of the boiler will reduce the resposiveness of the sensor – the sensor will sense a lower temperature than what is actual. This is possibly the price we pay to use a lower cost mounting solution!!!

The most common method of mounting sensors in low cost premix based hot beverage machines is the 3rd method above.

To acheive maximum performance in this method, the following precautions need to be taken.

• The screw should be welded from inside the boiler.
• Weld material should NOT flow out and creat small bubbles on the exterior – this will disturb proper mounting of the sensor
• The outer wall should be throughly scrubbed with emery sheets before mounting the sensor.
• The sensor should sit FLAT on the boiler surface and then a nut should be threaded to tghtly fix the sensor
• Insulating cover on top of the sensor and mounting screw will be important in cold climate.