{"id":7625,"date":"2017-07-06T09:50:45","date_gmt":"2017-07-06T07:50:45","guid":{"rendered":"https:\/\/blog.wika.com\/en\/?p=7625"},"modified":"2023-08-31T13:07:44","modified_gmt":"2023-08-31T11:07:44","slug":"pt100-in-2-3-or-4-wire-connection","status":"publish","type":"post","link":"https:\/\/blog.wika.com\/en\/knowhow\/pt100-in-2-3-or-4-wire-connection\/","title":{"rendered":"Pt100 in 2-, 3- or 4-wire connection?"},"content":{"rendered":"

Pt100, Pt1000<\/a> and NTC are the most-used measuring elements in resistance thermometers<\/a>. I would like to use this blog to consider the question of connection types in greater detail.<\/span><\/span><\/strong><\/p>\n

Resistance thermometers change their electrical resistance as a function of temperature. This physical effect makes it possible to measure the temperature of a process with a Pt100. The resistance is determined by electronics (e.g. temperature transmitter<\/a>) by using a constant current and measuring the voltage drop. According to Ohm’s law (R = U\/I), the resistance [R] and voltage [U] are proportional to each other at a constant current [I]. There are three possible ways to connect the Pt100 to the transmitter: in a 2-, 3- or 4-wire connection.<\/span><\/span><\/p>\n\n

\"PTC\/NTC

Fig.: Pt100 in 2-wire connection<\/p><\/div>\n \n

Pt100 in 2-wire connection<\/h2>\n

With a 2-wire connection, the resistance of the cable is added as an error in the measurement. For a copper cable with a cross-section of 0.22 mm2<\/sup>, the following guide value applies: 0.162 \u03a9\/m \u2192 0.42 \u00b0C\/m for Pt100. For a version with Pt1000 the influence of the supply line (at 0.04 \u00b0C\/m) is smaller by a factor of 10 in relation to the basic resistance. The lead resistance becomes still less significant in relation to the basic resistance R25 with an NTC measuring element (e.g. R25 = 10k). Due to the sloping characteristic curve of the NTC, the influence at higher temperatures increases disproportionately.<\/span><\/span><\/p>\n\n

\"PTC\/NTC

Fig.: Pt100 in 3-wire connection<\/p><\/div>\n \n

Pt100 in 3-wire connection<\/h2>\n

The influence of the lead resistance is compensated to the greatest possible extent with a 3-wire connection. The requirement for this is that the lead resistances are the same, as can be assumed with a 3-wire connection. The maximum length of the connection lead depends on the conductor cross-section and the compensation options of the evaluation electronics (transmitter, display, controller or process control system).\u00a0<\/span><\/span><\/p>\n\n

\"NTC\/PTC

Fig.: Pt100 in 4-wire connection<\/p><\/div>\n \n

Pt100 in 4-wire connection<\/h2>\n

The 4-wire connection completely eliminates the influence of the connection lead on the measuring result since any possible asymmetries in the lead resistance of the connection lead are also compensated.<\/p>\n

<\/span><\/span><\/p>\n

Alternative measures<\/h2>\n\n

Example: Measuring error at 150 \u00b0C, cable length 10 m, conductor cross-section 0.22 mm2<\/p><\/div>\n \n

A further possibility to substantially decrease the influence of the cabling is to increase the conductor cross-section. With a cross-section of 0.5 mm2<\/span><\/sup><\/span> the line resistance is only 0.036 \u03a9\/m or 0.1 \u00b0C\/m. Both options (3\/4-wire connection or increasing the cross-section) lead to a higher cost in the cabling, which can be problematic, especially in cost-sensitive markets such as machine building. As a compromise between cost and accuracy, for smaller cable lengths, a class A, 2-wire connection Pt1000 measuring element can be offered.<\/span><\/span><\/p>\n

Conclusion<\/h2>\n