Thermometers Glass Tube Thermometers Description and construction There are a wide variety of thermometers available on the market today. Some highly precise measurements are still done with glass thermometers. Since the properties of fluids, and in particular, mercury are well known, the only limitation to accuracy and resolution come in the form of how well you can manufacture a glass tube with a precision bore. Some manufacturers have made thermometers that have variable scales for specific uses. One such use is a process called wet viscosity. In this process it is important to know the precise temperature of the water bath. The glass thermometer is still used because of it extreme repeat ability. These specialized thermometers have a bore that narrows at a particular point. In this way it can expand a two degree temperature range in the middle of its scale to approximately two inches long, allowing readings down to a fraction of a tenth of a degree C.
Many of today’s thermometers use fluids other than mercury due to the hazards of spilled mercury. These newer devices use other fluids that have been engineered to have specific rates of expansion. The draw back to these fluids is that they typically do not have the high temperature capabilities that mercury does. One major drawback of the glass thermometer is the limited pressure capacity of the glass. Also inserting the glass bulb into a pressurized fluid or chamber caused the accuracy of the thermometer to suffer. This led to the use of A thermowell is a closed end metal tube that sticks into the chamber or fluid, and the thermometer sits in this well, making contact with its sides. Ranges and accuracy The range of a thermometer and it reading accuracy is dependent on the size of the hole, the length of the tube and the fluid in the thermometer. Typically the smaller the reading increment, the less range it will have. As an example, a 0.1° C accuracy mercury thermometer with a range of 100°C will typically be about 600 mm long. The restrictions rest with how well the maker can fabricate a readable scale. To increase readability some manufacturers have moved to non-round thermometer bodies, The rounded corner on the reading side acts as a magnifying glass, making the liquid column show up wider, and easier to read. The round thermometer is still the standard and there are a variety of holders and seals to fit them. There are also armored sleeves to put them in that allow them to be used, but reduce the chance of breakage. The chart below lists some thermometers commercially available. These are clearly not all the thermometers available, but a limited selection to give you some idea of what some more standard sizes and ranges are. The accuracy of a thermometer is greatly dependent on the manufacturing process, but also can be affected by usage. As stated earlier, the pressure exerted on the thermometer bulb can affect the reading to a certain degree. Even more so the amount of immersion in the fluid will have a drastic effect on the accuracy. Most commercial thermometers have lines etched in them to show you the calibrated depth of immersion. Failure to immerse the thermometer in deep enough will cause low readings, while putting it in too deeply will cause the readings to be artificially high. Thermometers are not designed to be totally immersed in the fluid they are measuring.
Bimetal Thermometers Description and construction The Bimetal thermometer was designed to be a less accurate, but more rugged measuring device than the glass thermometer. In many industrial applications there are still locations where it is desirable to know what the temperature of a fluid or device is, but it is not worth the cost of a more expensive probe and readout. Some examples of this are cooling water loops, gas grills, furnaces and ovens. In general the user would like a quick check to see what the approximate temperature is, but don’t need to know to the tenth of a degree. Probably within a few degrees is more than enough for most of the applications. Bimetal thermometers are constructed of a metal sensing rod, which conducts the temperature to the thermal element, the thermal element and a scale. The bimetal sensing element consists of a metal element shaped like a flat spring. This element is two different metallic materials sandwiched together. When a temperature is sensed by the element, the metallic components want to expand. Since they are different materials and expand at different rates, a stress in generated in the coil of material. This stress causes the element to try to wrap around itself. The indicator needle is attached to the end of this either directly or by mechanism. The motion of the spring shaped material moves the indicator. Prior to the advent of electrical thermostats, the most common use of these thermometers was in home heating systems. The thermostat consisted of a bimetallic spring such as used in the gauge type thermometer and a switch, usually a mercury level switch. As the spring wound and unwound with temperature change, the angle of the mercury switch would change, closing or opening the contacts. These are still used in many homes today. Another typical location that you may find this type of thermometer is your home grill, or if you have purchased an in-oven thermometer. Many of these have exposed elements such that you can look and see how they are constructed.
Temperature Transmitters A temperature transmitter is an electrical instrument that interfaces a temperature sensor (e.g. thermocouple, RTD, or thermistor) to a measurement or control device (e.g. PLC, DCS, PC, loop controller, data logger, display, recorder, etc.). Typically, temperature transmitters isolate, amplify, filter noise, linearize, and convert the input signal from the sensor then send (transmit) a standardized output signal to the control device. Common electrical output signals used in manufacturing plants are 4-20mA or 0-10V DC ranges. For example, 4mA could represent 0°C and 20mA means 100°C. Temperature Transmitter Inputs A thermocouple sensor is a pair of dissimilar metal wires joined at one end. The junction produces a low level voltage proportional to the difference in temperature between the open and closed ends. An RTD or Resistance Temperature Detector is a passive circuit element whose resistance increases with increasing temperature in a predictable manner.