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Section 3. CR800 Measurement Details

3-9

junction, which becomes the reference junction, is formed where the other

ends of the wires are connected to the measuring device. (With the connectors

at the same temperature, the chemical dissimilarity between the thermocouple

wire and the connector does not induce any voltage.) When the temperature of

the reference junction is known, the temperature of the measuring junction can

be determined by measuring the thermocouple voltage and adding the

corresponding temperature difference to the reference temperature.

The CR800 determines thermocouple temperatures using the following

sequence. First the temperature of the reference junction is measured and

stored in °C. If the reference junction is the CR800 analog input terminals, the

temperature is measured with the built in thermistor (PanelTemp instruction).

The thermocouple measurement instruction measures the thermocouple voltage

(TCDiff or TCSE). The thermocouple instruction calculates the voltage that a

thermocouple of the type specified would output at the reference junction

temperature if its reference junction were at 0

C, and adds this voltage to the

thermocouple voltage. The temperature of the measuring junction is then

calculated from a polynomial approximation of the NIST TC calibrations

3.4.1 Error Analysis

The error in the measurement of a thermocouple temperature is the sum of the

errors in the reference junction temperature, the thermocouple output

(deviation from standards published in NIST Monograph 175), the

thermocouple voltage measurement, and the linearization error (difference

between NIST standard and CR800 polynomial approximations). The

discussion of errors which follows is limited to these errors in calibration and

measurement and does not include errors in installation or matching the sensor

to the environment being measured.

Panel Temperature

The panel temperature thermistor is just under the panel in the center of the

two rows of analog input terminals.

The thermistor (Betatherm 10K3A1A) has an interchangeability specification

of 0.1 °C for temperatures between 0 and 70 °C. Below freezing and at higher

temperatures this specification is degraded. Combined with possible errors in

the completion resistor measurement, and the Steinhart and Hart equation used

to calculate the temperature from resistance, the accuracy of panel temperature

is ±.0.3°C @ -25 to 50°C and ±.0.8°C @ -55 to 85°C.

The error in the reference temperature measurement is a combination of the

error in the thermistor temperature and the difference in temperature between

the panel thermistor and the terminals the thermocouple is connected to. The

terminal strip cover should always be used when making thermocouple

measurements. It insulates the terminals from drafts and rapid fluctuations in

temperature as well as conducting heat to reduce temperature gradients. In a

typical installation where the CR800 is in a weather proof enclosure not

subject to violent swings in temperature or lopsided solar radiation loading, the

temperature difference between the terminals and the thermistor is likely to be

less than 0.2 °C.

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