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**Sensor:** Surface Air Temperature

**Manuf/Model:** Thermometrics Corporation

**Description:** The Platinum Resistance Thermometer (PRT) is a
precision temperature sensor with the following specifications:

Type: Platinum 1000 ohm ± 0.04% at 0°C (per IEC-751, Class A accuracy)

Temperature Coefficient: 0.00385 (per IEC-751 and ASTM E-1137)

Range: -60°C to +300°C

Sheath: Type 316 stainless steel, 0.125 inches diameter ± 0.010 inches
(per ASTM A632) by 4 inches long, filled with MgO powder and
vibro-compacted; sealed with epoxy at the leads.

Leads: Three-wire type, 22 AWG silver-coated copper wires with Teflon
insulation, with

aluminized Mylar shield and drain wire

Each probe etched with lot number for traceability

Repeatability and Stability: Better than ± 0.01°C per year

Accuracy: ±0.04% over full range

Time Constant: 63% of thermal response in 13 sec when immersed from 20°C
air into 50°C

water flowing at 0.2 m/s

**How is it installed?**

Three completely independent PRTs are used. Each PRT probe is installed
in a pair of clips inside its own mechanically (fan) aspirated radiation
shield (Met One model 076B/7308). Each shield is mounted vertically
downward from its own 3 m long cross-member arm, about 1.5 m above the
surface of the ground. Each shield is leveled and held in position by
set screws.

**Why measure this parameter?**

Surface Air Temperature is one of the primary measurements for USCRN.

**How are data recorded?**

The three PRT probes are polled every two seconds by a Campbell
Scientific CSI 23X data logger using the P7 command to determine the
ratio of the sensor resistance to a known resistance hour (refer to
'CR23X Micrologger Operator's Manual', revised 3/00). A three-wire
half-bridge circuit with a 1000 ohm ± 0.01% precision resistor is used;
a second voltage-sensing wire compensates for the effects of lead
resistance. The measurement sequence is to apply an excitation voltage,
make a single-ended measurement on the first channel, and then repeat
the process using an excitation voltage of opposite polarity. The same
sequence is then applied to the second single-ended channel. This
sequence is used to remove any offset voltages.

**How are data computed?**

The ratio obtained via the P7 command is applied in an equation obtained
from a calibration performed in a precision temperature bath. The
equation is a second-order polynomial. At the end of every five minutes
the average temperature (command P71) is determined for the five-minute
period. At the end of the hour, the average (P71), standard deviation
(P82), minimum (P74), and maximum (P73) temperatures are determined for
the twelve five-minute averages for the hour (refer to 'CR23X
Micrologger Operator's Manual', revised 3/00).

**What data/information are stored on the data logger?**

Values for [each of the three sensors]{.underline}: The average hourly
temperature (command P71, computed from the 12 five minute averages ),
the minimum (P74) and maximum (P73) temperatures for the hour
(determined from the twelve five-minute averages for the hour), the time
of occurrence of the maximum and minimum temperature values, and the
standard deviation (P82, computed from the twelve five-minute periods,
reference page 11-7 of the 'CR23X Micrologger Operator's Manual' revised
3/00)), are stored in the data logger (refer to 'CR23X Micrologger
Operator's Manual' revised 3/00). In addition, the five-minute average
temperature for the last five-minute period of the hour is stored in the
data logger (per NWS request).

**What values are transmitted?**

Seven values [for each of the three temperature sensors]{.underline} (21
total values) are in the data stream.

\- The hourly average temperature.

\- The hourly maximum and minimum temperatures and the time of their
occurrence.

\- The standard deviation.

\- The five-minute average temperature for the last five-minute period
in the hour (NWS request).