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| | | Solution: Minco specializes in fast response RTDs. Most models in this guide have a time constant of 2 seconds or less. Certain custom-designed models are faster yet. Time constant is defined as the time it takes a sensor to reflect 63% of a step temperature change: | | |
| | | Lamination Minco's winding and lamination technology enables manufacture of flat, flexible sensors in any size or shape. The custom Thermal-Ribbon™ below has a wire element to average temperatures over its entire area. | | |
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| | | Testing Minco has complete in-house testing and metrology equipment to meet stringent quality requirements: • NIST traceable calibrations • Helium leak testing • Automated resistance measurement • Humidity testing | | |
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| | | Conduction errors: Heat conducted into sensors from ambient air alters the temperature of the sensing tip. Solution: Use smaller sensors or tip-sensitive probes, and be sure they are sufficiently immersed or embedded in the sensed medium. | | |
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| | | Point sensing errors: In places where temperatures are stratified or gradients are large, the temperature at a single point may be unrepresentative or misleading. Solution: Use temperature averaging probes orThermal-Ribbons. | | |
| | | Designing for accuracy How accurate is a temperature sensor manufacturer? To many, the answer to this question is the sensor's interchangeability specification. For example, 100 Q platinum RTDs are typically interchangeable within 0.1 Q (0.3°C) at0°C. But interchangeability only tells how closely the electrical characteristics of a sensor conform to its published tables. What you really want to know is how much the temperature seen at your readout or controller deviates from the actual temperature of the material you are sensing. Interchangeability is only one of the potential sources of error in the system, and it is usually not the largest. Following are some other error modes along with suggested solutions. | | |
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| | | Leadwire resistance: Resistance in the leads between RTDs and control points elevates apparent readings. Solutions: • Specify sensors with higher resistances. • Use 3 or4-wire compensating circuits (see page 1-12). • Eliminate leadwire effects witha4to20mAtransmitter. | | |
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| | | Repeatability/stability: Repeatability tells how well the sensor repeats subsequent readings at the same temperature. Stability is the absence of long term drift. In many cases, the user is less concerned with absolute accuracy than with the ability of a sensor to maintain a process at the same point once properly set. Solution: Platinum RTDs are the most stable sensor in common use and are used to interpolate over the standard temperature scale from -260 to 962°C. Ordinary industrial models will drift less than 0.1°C per year in normal use. | | |
| | | Self-heating: The measuring current through an RTD can raise its temperature above the true value. Solution: As a general rule, limit current to 5 mA for industrial applications. Most Minco RTDs, and especially Thermal-Ribbons, have a large surface area to dissipate heat and reduce self-heating effects. Custom designs Ifyou have special requirements - or an OEM design - Minco can typically manufacture a custom sensor solution to improve accuracy and reduce cost at the same time. Contact Access: Minco Sales and Support today to discuss your application. | | |
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| | | Time lag: When temperatures change rapidly, sensors may not keep up. | | |
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| | | Sensors & Instruments Solutions Guide I www.minco.com Page 1-7 | | |
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