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LM1042 データシート(PDF) 5 Page - National Semiconductor (TI) |
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LM1042 データシート(HTML) 5 Page - National Semiconductor (TI) |
5 / 8 page Application Notes THERMO-RESISTIVE PROBES OPERATION AND CONSTRUCTION These probes work on the principle that when power is dis- sipated within the probe the rise in probe temperature is dependent on the thermal resistance of the surrounding ma- terial and as air and other gases are much less efficient conductors of heat than liquids such as water and oil it is possible to obtain a measurement of the depth of immersion of such a probe in a liquid medium This principle is illustrat- ed in Figure 1 TLH8709 – 4 FIGURE 1 During the measurement period a constant current drive I is applied to the probe and the voltage across the probe is sampled both at the start and just before the end of the measurement period to give DV RTH Air and RTH Oil repre- sent the different thermal resistances from probe to ambient in air or oil giving rise fo temperature changes DT1 and DT2 respectively As a result of these temperature changes the probe resistance will change by DR1 or DR2 and give corre- sponding voltage changes DV1 or DV2 per unit length Hence D V e LA L D V1 a (L b LA) L D V2 and for DV1 l DV2 RTH Air l RTH Oil DV will increase as the probe length in air increases For best results the probe needs to have a high temperature coefficient and low ther- mal time constant One way to achieve this is to make use of resistance wires held in a suitable support frame allowing free liquid access Nickel cobalt iron alloy resistance wires are available with resistivity 50 mXcm and 3300 ppm tem- perature coefficient which when made up into a probe with 4 c 2 cm 008 mm diameter strands between supports (10 cm total) can give the voltage vs time curve shown in Figure 2 for 200 mA probe current The effect of varying the probe current is shown in Figure 3 To avoid triggering the probe failure detection circuits the probe voltage must be between 07V and 53V (VREG b 6V) hence for 200 mA the permis- sible probe resistance range is from 35X to 24X The ex- ample given has a resistance at room temperature of 9X which leaves plenty of room for increase during measure- ments and changes in ambient temperature Various arrangements of probe wire are possible for any given wire gauge and probe current to suit the measurement range required some examples are illustrated schematically in Figure 4 Naturally it is necessary to reduce the probe TLH8709 – 5 FIGURE 2 current with very fine wires to avoid excessive heating and this current may be optimized to suit a particular type of wire The temperature changes involved will give rise to no- ticeable length changes in the wire used and more sophisti- cated holders with tensioning devices may be devised to allow for this TLH8709 – 6 FIGURE 3 Probes need not be limited to resistance wire types as any device with a positive temperature coefficient and sufficient- ly low thermal resistance to the encapsulation so as not to mask the change due to the different surrounding mediums could be used Positive temperature coefficient thermistors are a possibility and while their thermal time constant is like- ly to be longer than wire the measurement time may be increased by changing CT to suit TLH8709 – 7 FIGURE 4 5 |
同様の部品番号 - LM1042 |
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同様の説明 - LM1042 |
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