higher), and has moderate accuracy (0.05°C to 1°C). This type uses the
Callendar-Van Dusen equation to calculate temperature from resistance:
rt C
Rt
t
t
R
([ ])°=
+− −
⎛
⎝
⎜
⎞
⎠
⎟
⎡
⎣
⎢
⎤
⎦
⎥
⎧
⎨
⎩
⎫
⎬
⎭
≥
0
0
1
1
100 100
10αδ
1
100 100
1
100
1
100
3
+− −
⎛
⎝
⎜
⎞
⎠
⎟
−−
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
⎡
⎣
⎢
αδ βt
tt t t
⎢
⎤
⎦
⎥
⎥
⎧
⎨
⎩
⎪
⎫
⎬
⎭
<
⎧
⎨
⎪
⎪
⎩
⎪
⎪
t 0
The coefficients R0, α, δ,andβ can be set by the user using the functions
“R0", ”Alpha", “Delta”, and “Beta” respectively. When editing “Alpha”, there
is an assumed exponent of 10
–3
(i.e. 3.8500 is actually 0.0038500).
7.8.12 YSI-400
The YSI-400 type is for 2252Ω thermistors 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 to 150°C), good accuracy (0.1°C to 0.4°C), and
excellent resolution (0.002°C). There are no user programmable coefficients as-
sociated with this probe type. Temperature is converted from resistance based
upon a standard YSI-400 table resident in the instrument.
7.8.13 THERM
The THERM thermistor type is for thermistors that have unique calibration co-
efficients or do not match the YSI-400 specification. This type of probe gener-
ally has moderate cost, 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 the Steinhart-Hart equation in the following form:
RTK b bT bT bT( [ ])[ ] exp[ ]Ω= +++
−−−
01
1
2
2
3
3
The coefficients b0 through b3 are usually found on the calibration certificate
of the probe. These coefficients are set with the functions “b0", ”b1", “b2", and
”b3". In some cases the equation is used without the b2 term. For this case, the
“b2" coefficient should be set to 0. Consider the following examples.
Example 1:
A thermistor has coefficients for the equation ln(R) as a function of T given as
a, b, c, and d. Set the 1521 coefficients to the certificate values as follows:
1521 Coefficients Certificate Value
b0 a
b1 b
b2 c
b3 d
1521 Handheld Thermometer Readout
User’s Guide
34
