UM-0085-B09
DT80 Range User Manual
Page 317
RG
A voltage channel is then used to measure the offset voltage and the
TZ
channel option is specified. This will cause all
subsequent thermocouple channels in the same schedule to use this value as their zero, rather then the DT80's internal
zero. For example:
RA10S 5AD590(TR) 6V(TZ) 1..4TT
Note:
If a
TR/TZ
channel is used, it must precede the thermocouple channel(s) to which it applies, and be in the same sampling
schedule. If a statistical option (e.g.
AV
,
MX
) is applied to a thermocouple channel then the sampling schedule is the statistical schedule,
so the reference channel must be sampled in the statistical schedule too. This can be achieved by applying a statistical option to the
reference channel, e.g
.
RS1S
RA1M 5PT385(TR,AV,W) 1..2TE(AV) 3TK
In this example, the PT385 will be read once a second, and will be used as the cold junction reference for the two
subsequent TE thermocouple measurements. The average thermocouple readings will then be reported once a minute.
The third thermocouple (
3TK
) is not sampled in the statistical schedule, and will use the default internal temperature
reference.
Accuracy — Thermocouple Techniques
The accuracy of temperature measurement with thermocouples depends on
•
the reference junction isothermal characteristics
•
the reference temperature sensor accuracy
•
induced electrical noise
•
the quality of the thermocouple wire
•
drift in the wire characteristics, especially at high temperatures
•
the basic measurement accuracy of the DT80
•
the linearization accuracy of the DT80
Reference Junction Error
The most significant source of error is the reference junction. The DT80 must not be exposed to non-uniform heating
because a single reference temperature sensor is used to measure the temperature of the terminals of all channels. If a
temperature gradient occurs along the terminals, errors of the magnitude of the temperature difference occur.
For the DT80/81, the reference temperature sensor is positioned behind analog channels 2 and 4, and behind channel
10 on the DT85. Therefore, when precise temperature measurements are required, attach thermocouples here for the
least temperature differential from the DT80’s reference temperature.
Linearization Error
The DT80's linearization errors are much lower (< 0.1°C over the full range) than other error sources.
Temperature – Thermistors
Thermistors are devices that change their electrical resistance with temperature. They measure temperatures from –
80°C up to 250°C, and are sensitive but highly nonlinear.
Thermistors may be connected using any of the resistance wiring configurations, i.e. in 2-wire, 3-wire or 4-wire
configuration. See
.
Channel Types
The DT80 has channel types for many 2-wire YSI (Yellow Springs Instruments) thermistors and, for other thermistor
types, the DT80 supports thermistor scaling — see
.
Channel Type
R (Ω) at 25°C
YSI Thermistor
Max. Temp °C Min. Temp °C (without Rp)
YS01
100Ω
44001A, 44101A
100
–75
YS02
300Ω
44002A, 44102A
100
–50
YS03
1kΩ
44003A, 44103A
100
–25
44035
100
YS04
2252Ω
44004, 44104
150
–5
44033
75
45004, 46004, 46033, 46043
200
44901
90
44902
70
YS05
3kΩ
44005, 44105
150
0
44030
75
