Catalogues - sensors-instruments-solutions-guide

Catalogue Sensors & Instruments Solutions Guide

www.minco.com

Page

/ 164

Contact the
Manufacturer

Where to buy
this product ?

Request
a Quote

/ 164

See other catalogues for MINCO

Text version of the page

RTD Connections: 2-Wire, 3-Wire, 4-Wire?

Because an RTD is a resistance type sensor, resistance introduced by connecting copper extension wires between the RTD and control instrument will add to readings. Furthermore, this additional resistance is not constant but increases with ambient temperature. To estimate leadwire error in 2-wire circuits, multiply the total length of the extension leads times the resistance per foot in the table below. Then divide by the sensitivity of the RTD, given in the next two pages, to obtain an error figure in °C. For example, assume you have connected 100 feet of AWG 22 wires to a 100 Q platinum RTD (PD element). Lead resistance is:

R = (200 ft.) x (0.0165 Q / ft.) = 3.3 Q

Approximate error is:

2-wire circuit

Shown above is a 2-wire RTD connected to a typical Wheat-stone bridge circuit. E_S is the supply voltage; E_O is the output voltage; R₁,R₂, and R₃ are fixed resistors; and R_T is the RTD. In this uncompensated circuit, lead resistances L₁ and L₂ add directly to Rt.

Es"

_r 3.3 n

= 8.6°C

0.385 Q/°C


	Copper Leadwire AWG	Ohms/ft. at25°C
	12	0.0016
	14	0.0026
	16	0.0041
	18	0.0065
	20	0.0103
	22	0.0165
	24	0.0262
	26	0.0418
	28	0.0666
	30	0.1058

3-wire circuit

In this circuit there are three leads coming from the RTD instead of two. L, and L₃ carry the measuring current while L₂ acts only as a potential lead. No current flows through it while the bridge is in balance. Since L, and L₃ are in separate arms of the bridge, resistance is canceled. This circuit assumes high impedance at E_O and close matching of resistance between wires L, and L₃. Minco matches RTD leads within 5%.

^L,

^L2

-vAA^-

You can reduce leadwire error by:

• Using larger gauge extension wires.

• Specifying an RTD with greater sensitivity; 1000 Q instead of 100 Q, for example.

• Employinga3or4-wireresistance canceling circuit as shown at right. Common leads, connected to the same end of the sensing element, are the same color.

• Using a 2-wire current transmitter. Its linearized signal is immune to electrical noise as well as resistance and can maintain accuracy over runs of several thousand feet.

See Section 5 for more information on industrial temperature transmitters.

4-wire circuit

4-wire RTD circuits not only cancel leadwires but remove the effects ofmismatched resistances such as contact points. A common version is the constant current circuit shown above. I_Sdrives a precise measuring current through L, and L₄.L₂ and L₃measure the voltage drop across the RTD element. E_O must have high impedance to prevent current flow in the potential leads. 4-wire circuits may be usable over longer distances than 3-wire, but you should consider using a transmitter in electrically noisy environments.


	Extension leads	ID- RTD leads
	1
	Extension leads	T RTD leads \-

If necessary you can connect a 2-wire RTD to a 3-wire circuit or 4-wire circuit, as shown to the right. As long as the

junctions are near the RTD, as in a connection head, errors are negligible.

Access: Minco

Sales and Support

Americas: 763.571.3121 | Europe: (33) 5 61 03 24 01 | Asia Pacific: (65) 6511 3388

Page 1-12