Measurement: automatic temperature measurement
ALGs usually include a device for measuring the average temperature of the liquid within the tank. There are also separate devices for measuring the liquid temperature of tank contents as a whole or at fixed or adjustable “spot” points within the tank. Except for a few spot measuring devices, which use thermistors or thermo-couples, all automatic temperature measuring devices in general use, depend on the effect of the temperature of the liquid upon the electrical resistance of a length of sensitive wire immersed in the liquid. Apparatus connected to the sensitive wire measures its resistance and deduces the temperature of the liquid.
The main types of temperature measuring equipment are:
- The averaging “bulb” type. Five or more sensitive wires of varying lengths, electrically insulated from one another, extend vertically upwards from the tank bottom. The longest wire which is totally immersed in the liquid is selected manually or automatically.
- The averaging spiral type. A single length of sensitive wire is enclosed in a spiral nylon tube connected to a plate near the tank bottom and a float on the surface. It is maintained in position by vertical cables. The tube is loaded so as to be virtually weightless and is extended as the product level rises to form an even spiral throughout the depth of liquid.
- The averaging beam type. A single length of sensitive wire is laid along the length of a beam. The beam is pivoted at the bottom of the tank and supported by a float on the liquid surface. The sensitive wire is thus wholly immersed in the liquid and lies diagonally in the tank between the beam pivot and the float.
- The spot type. A length of sensitive wire is supported horizontally within the tank at a fixed or adjustable depth below the tank roof or below a float on the liquid surface.
- The portable battery-operated type. A sensitive bulb is attached to a cable about 30 metres in length. This terminates in a small easily carried instrument with a digital or moving coil temperature indicator. The cable is marked at intervals to indicate the depth at which the temperature is being recorded.
Temperature measurement instruments and accuracy
The Energy Institute standard for temperature measurement instruments in storage tanks requires that they should be accurate to within 0.5ºC. Different types and makes of apparatus require different times to respond accurately from the time the electric circuit is switched on. Spiral and beam types can give an accurate indication within 15 seconds. All averaging devices are subject to errors caused by temperature variations which are not in uniform horizontal layers. In tanks whose diameter is greater than their height, averaging beams can sometimes be laid across the line of vertical layering and are more accurate than bulb or spiral types.