The accuracy
of a platinum resistance thermometer is dependent upon two factors:
- The offset
of its actual resistance from the nominal value (typically 100
ohms) at some reference temperature
(usually 0°C).
- Variation
from a nominal value of the effective resistance temperature
coefficient over a given temperature interval, giving rise to
an additional error component that is proportional to the temperature.
Both of these
factors are variable from sensor to sensor, but their magnitude
limits are implicit for a given tolerance 'class'.
The designation
of a PRT tolerance class is based on the percent allowable variation,
in ohms, of the nominal resistance value at the reference temperature.
However, for convenience, this ohmic tolerance is often expressed
as an equivalent °C temperature variation. To this base uncertainty
must be added the allowable proportional error for the class,
which is stated as a percentage of measured temperature. This
percentage applies when temperatures are expressed in degrees
Celsius. A tolerance in degrees Fahrenheit is obtained by multiplying
the Celsius equivalent sum temperature tolerance by 9/5.
Tolerance
= Offset Error + Proportional Error
| Marlin
Code |
Tolerance
Class |
Offset
Error |
Proportional
Error (°C) |
*Sum
of Errors (°C) |
| 0 |
0.05% |
0.15°C |
0.3%
(T) |
0.15
+ 0.003 (T) |
| 1 |
0.1% |
0.3°C |
0.5%
(T) |
0.3
+ 0.005 (T) |
| 2 |
0.5% |
1.3°C |
0.8%
(T) |
1.3
+ 0.008 (T) |
*(T)
is the temperature in °C without regard to sign.
PRT
CIRCUITRY
Resistance
bridge techniques are used with resistance thermometers for temperature
measurement. In these techniques the resistance change with temperature
of the PRT, which is the basis for a resistance thermometer, can
be affected by the lead resistance. marlin offers various circuits
to fulfill the requirements of your instrumentation.
The accuracy
of a PRT may be independent of the distances between the sensor
and the instrument whether it be an indicator, recorder, controller,
data logger or computer. Click
Here For More Information Regarding Marlin Instruments!
The distance may vary from a few inches to many miles. Copper
hook-up wire is generally used between the sensor and instrument.
The comparatively
high signal level of the PRT eliminates the need for high gain
amplifiers and generally reduces the susceptibility of the measuring
system to noise and signal interference.
| Marlin
Code |
Circuit
Type |
| 12 |
2-Wire
Single Element |
| 13 |
3-Wire
Single Element |
| 14 |
4-Wire
Single Element |
| 15 |
4-Wire
Single Element
Compensating Loop |
| 22 |
(2)
2-Wire Dual Element |
| 23 |
(2)
3-Wire Dual Element |
NOTES:
Dual element PRT's require 0.250" O.D. minimum sheath diameter.
For
more information regarding Platinum Resistance Thermometers
Please Consult your Marlin Catalog, Pages C-3 to C-19.
If
you do not have a Marlin Catalog - Click
Here!
