higher), and has moderate accuracy(0.05°C to 1°C). This type uses the
Callendar-VanDusen equation to calculate temperature from resistance:
rt C
Rt tt
R
([ ])°=
+− −
⎛
⎝
⎜⎞
⎠
⎟
⎡
⎣
⎢⎤
⎦
⎥
⎧
⎨
⎩
⎫
⎬
⎭≥
0
0
11
100 100 10αδ
1100 100 1100 1100
3
+− −
⎛
⎝
⎜⎞
⎠
⎟−−
⎛
⎝
⎜⎞
⎠
⎟⎛
⎝
⎜⎞
⎠
⎟
⎡
⎣
⎢
αδ βttt t t
⎢
⎤
⎦
⎥
⎥
⎧
⎨
⎩
⎪
⎫
⎬
⎭
<
⎧
⎨
⎪
⎪
⎩
⎪
⎪t0
The coefficientsR0, α,δ,andβcan be set by the user using the functions
“R0", ”Alpha", “Delta”, and “Beta” respectively.When editing “Alpha”, there
is an assumed exponentof 10–3 (i.e. 3.8500 is actually 0.0038500).
7.8.12 YSI-400
The YSI-400 type isfor 2252Ωthermistors matching the characteristics of the
YSI-400 series of thermistors.This type of probe generally is low cost, has a
limited operating range (–40°C to150°C), good accuracy (0.1°C to 0.4°C), and
excellentresolution (0.002°C). There are no user programmable coefficients as-
sociated with this probe type. Temperatureis converted from resistance based
upon a standardYSI-400 table resident in the instrument.
7.8.13 THERM
The THERM thermistor type is for thermistors that have unique calibration co-
efficientsor do not match the YSI-400 specification. This type of probe gener-
ally has moderatecost, a limited operating range (–40°C to 100°C), excellent
accuracy(0.005°C to 0.02°C) and excellent resolution (0.002°C). This type
uses theSteinhart-Hart equation in the following form:
RTK b bT bT bT( [ ])[ ] exp[ ]Ω= +++
−−−
01
1
2
2
3
3
The coefficientsb0 through b3 are usually found on the calibration certificate
of the probe. These coefficientsare set with the functions “b0", ”b1", “b2", and
”b3". In some cases theequation is used without the b2 term. For this case, the
“b2" coefficientshould be set to 0. Consider the following examples.
Example 1:
A thermistorhas coefficients for the equation ln(R) as a function of T given as
a, b, c, and d. Set the 1521 coefficientsto the certificate values as follows:
1521Coefficients CertificateValue
b0 a
b1 b
b2 c
b3 d
1521 Handheld Thermometer Readout
User’s Guide
34